DATAMONGO-1880 - Add support for ClientSession.

We now support ClientSession via MongoOperations and ReactiveMongoOperations. Client sessions introduce causal consistency and retryable writes. A client Session can be either provided by application code or managed by specifying ClientSessionOptions. Binding a ClientSession via MongoOperations.withSession(…) provides access to a Session-bound MongoOperations instance that associates the session with each MongoDB operation.

ClientSession support applies only to MongoOperations and ReactiveMongoOperations and is not yet available via repositories.

ClientSession session = client.startSession(ClientSessionOptions.builder().causallyConsistent(true).build());

Person person = template.withSession(() -> session)
        .execute(action -> {

          action.insert(new Person("wohoo"));
          return action.findOne(query(where("id").is("wohoo")), Person.class);
        });

session.close();

Original pull request: #536.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/ClientSessionException.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb;
+
+import org.springframework.dao.NonTransientDataAccessException;
+import org.springframework.lang.Nullable;
+
+/**
+ * {@link NonTransientDataAccessException} specific to MongoDB {@link com.mongodb.session.ClientSession} related data
+ * access failures such as reading data using an already closed session.
+ *
+ * @author Christoph Strobl
+ * @since 2.1
+ */
+public class ClientSessionException extends NonTransientDataAccessException {
+
+	/**
+	 * Constructor for {@link ClientSessionException}.
+	 *
+	 * @param msg the detail message. Must not be {@literal null}.
+	 */
+	public ClientSessionException(String msg) {
+		super(msg);
+	}
+
+	/**
+	 * Constructor for {@link ClientSessionException}.
+	 *
+	 * @param msg the detail message. Can be {@literal null}.
+	 * @param cause the root cause. Can be {@literal null}.
+	 */
+	public ClientSessionException(@Nullable String msg, @Nullable Throwable cause) {
+		super(msg, cause);
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoDbFactory.java

@@ -20,8 +20,10 @@ import org.springframework.dao.DataAccessException;
 import org.springframework.dao.support.PersistenceExceptionTranslator;
 import org.springframework.data.mongodb.core.MongoExceptionTranslator;
 
+import com.mongodb.ClientSessionOptions;
 import com.mongodb.DB;
 import com.mongodb.client.MongoDatabase;
+import com.mongodb.session.ClientSession;
 
 /**
  * Interface for factories creating {@link DB} instances.
@@ -67,4 +69,35 @@ public interface MongoDbFactory extends CodecRegistryProvider {
 	default CodecRegistry getCodecRegistry() {
 		return getDb().getCodecRegistry();
 	}
+
+	/**
+	 * Obtain a {@link ClientSession} for given ClientSessionOptions.
+	 *
+	 * @param options must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 2.1
+	 */
+	ClientSession getSession(ClientSessionOptions options);
+
+	/**
+	 * Obtain a {@link ClientSession} bound instance of {@link MongoDbFactory} returning {@link MongoDatabase} instances
+	 * that are aware and bound to a new session with given {@link ClientSessionOptions options}.
+	 *
+	 * @param options must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 2.1
+	 */
+	default MongoDbFactory withSession(ClientSessionOptions options) {
+		return withSession(getSession(options));
+	}
+
+	/**
+	 * Obtain a {@link ClientSession} bound instance of {@link MongoDbFactory} returning {@link MongoDatabase} instances
+	 * that are aware and bound to the given session.
+	 *
+	 * @param options must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 2.1
+	 */
+	MongoDbFactory withSession(ClientSession session);
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/ReactiveMongoDatabaseFactory.java

@@ -16,12 +16,16 @@
 
 package org.springframework.data.mongodb;
 
+import reactor.core.publisher.Mono;
+
 import org.bson.codecs.configuration.CodecRegistry;
 import org.springframework.dao.DataAccessException;
 import org.springframework.dao.support.PersistenceExceptionTranslator;
 import org.springframework.data.mongodb.core.MongoExceptionTranslator;
 
+import com.mongodb.ClientSessionOptions;
 import com.mongodb.reactivestreams.client.MongoDatabase;
+import com.mongodb.session.ClientSession;
 
 /**
  * Interface for factories creating reactive {@link MongoDatabase} instances.
@@ -65,4 +69,24 @@ public interface ReactiveMongoDatabaseFactory extends CodecRegistryProvider {
 	default CodecRegistry getCodecRegistry() {
 		return getMongoDatabase().getCodecRegistry();
 	}
+
+	/**
+	 * Obtain a {@link Mono} emitting a {@link ClientSession} for given {@link ClientSessionOptions options}.
+	 *
+	 * @param options must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 2.1
+	 */
+	Mono<ClientSession> getSession(ClientSessionOptions options);
+
+	/**
+	 * Obtain a {@link ClientSession} bound instance of {@link ReactiveMongoDatabaseFactory} returning
+	 * {@link MongoDatabase} instances that are aware and bound to the given session.
+	 *
+	 * @param options must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 2.1
+	 */
+	ReactiveMongoDatabaseFactory withSession(ClientSession session);
+
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/SessionAwareMethodInterceptor.java

@@ -0,0 +1,173 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb;
+
+import java.lang.reflect.Method;
+import java.lang.reflect.Proxy;
+import java.util.Optional;
+import java.util.function.BiFunction;
+
+import org.aopalliance.intercept.MethodInterceptor;
+import org.aopalliance.intercept.MethodInvocation;
+import org.springframework.core.MethodClassKey;
+import org.springframework.lang.Nullable;
+import org.springframework.util.ClassUtils;
+import org.springframework.util.ConcurrentReferenceHashMap;
+import org.springframework.util.ObjectUtils;
+import org.springframework.util.ReflectionUtils;
+
+import com.mongodb.WriteConcern;
+import com.mongodb.session.ClientSession;
+
+/**
+ * {@link MethodInterceptor} implementation looking up and invoking an alternative target method having
+ * {@link ClientSession} as its first argument. This allows seamless integration with the existing code base.
+ * <p />
+ * The {@link MethodInterceptor} is aware of methods on {@code MongoCollection} that my return new instances of itself
+ * like (eg. {@link com.mongodb.reactivestreams.client.MongoCollection#withWriteConcern(WriteConcern)} and decorate them
+ * if not already proxied.
+ *
+ * @since 2.1
+ */
+public class SessionAwareMethodInterceptor<D, C> implements MethodInterceptor {
+
+	private static final MethodCache METHOD_CACHE = new MethodCache();
+
+	private final ClientSession session;
+	private final BiFunction collectionDecorator;
+	private final BiFunction databaseDecorator;
+	private final Object target;
+	private final Class<?> targetType;
+	private final Class<?> collectionType;
+	private final Class<?> databaseType;
+
+	/**
+	 * Create a new SessionAwareMethodInterceptor for given target.
+	 * 
+	 * @param session the {@link ClientSession} to be used on invocation.
+	 * @param target the original target object.
+	 * @param databaseType the MongoDB database type
+	 * @param databaseDecorator a {@link BiFunction} used to create the proxy for an imperative / reactive
+	 *          {@code MongoDatabase}.
+	 * @param collectionType the MongoDB collection type.
+	 * @param collectionCallback a {@link BiFunction} used to create the proxy for an imperative / reactive
+	 *          {@code MongoCollection}.
+	 * @param <T>
+	 */
+	public <T> SessionAwareMethodInterceptor(ClientSession session, T target, Class<D> databaseType,
+			BiFunction<ClientSession, D, D> databaseDecorator, Class<C> collectionType,
+			BiFunction<ClientSession, C, C> collectionDecorator) {
+
+		this.session = session;
+		this.target = target;
+		this.databaseType = ClassUtils.getUserClass(databaseType);
+		this.collectionType = ClassUtils.getUserClass(collectionType);
+		this.collectionDecorator = collectionDecorator;
+		this.databaseDecorator = databaseDecorator;
+
+		this.targetType = ClassUtils.isAssignable(databaseType, target.getClass()) ? databaseType : collectionType;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.aopalliance.intercept.MethodInterceptor(org.aopalliance.intercept.MethodInvocation)
+	 */
+	@Override
+	public Object invoke(MethodInvocation methodInvocation) throws Throwable {
+
+		if (requiresDecoration(methodInvocation)) {
+
+			Object target = methodInvocation.proceed();
+			if (target instanceof Proxy) {
+				return target;
+			}
+
+			return decorate(target);
+		}
+
+		if (!requiresSession(methodInvocation)) {
+			return methodInvocation.proceed();
+		}
+
+		Optional<Method> targetMethod = METHOD_CACHE.lookup(methodInvocation.getMethod(), targetType);
+
+		return !targetMethod.isPresent() ? methodInvocation.proceed()
+				: ReflectionUtils.invokeMethod(targetMethod.get(), target, prependSessionToArguments(methodInvocation));
+	}
+
+	private boolean requiresDecoration(MethodInvocation methodInvocation) {
+
+		return ClassUtils.isAssignable(databaseType, methodInvocation.getMethod().getReturnType())
+				|| ClassUtils.isAssignable(collectionType, methodInvocation.getMethod().getReturnType());
+	}
+
+	protected Object decorate(Object target) {
+
+		return ClassUtils.isAssignable(databaseType, target.getClass()) ? databaseDecorator.apply(session, target)
+				: collectionDecorator.apply(session, target);
+	}
+
+	private boolean requiresSession(MethodInvocation methodInvocation) {
+
+		if (ObjectUtils.isEmpty(methodInvocation.getMethod().getParameterTypes())
+				|| !ClassUtils.isAssignable(ClientSession.class, methodInvocation.getMethod().getParameterTypes()[0])) {
+			return true;
+		}
+
+		return false;
+	}
+
+	private Object[] prependSessionToArguments(MethodInvocation invocation) {
+
+		Object[] args = new Object[invocation.getArguments().length + 1];
+		args[0] = session;
+		System.arraycopy(invocation.getArguments(), 0, args, 1, invocation.getArguments().length);
+		return args;
+	}
+
+	/**
+	 * Simple {@link Method} to {@link Method} caching facility for {@link ClientSession} overloaded targets.
+	 *
+	 * @since 2.1
+	 * @author Christoph Strobl
+	 */
+	static class MethodCache {
+
+		private final ConcurrentReferenceHashMap<MethodClassKey, Optional<Method>> cache = new ConcurrentReferenceHashMap<>();
+
+		Optional<Method> lookup(Method method, Class<?> targetClass) {
+
+			return cache.computeIfAbsent(new MethodClassKey(method, targetClass),
+					val -> Optional.ofNullable(findTargetWithSession(method, targetClass)));
+		}
+
+		@Nullable
+		private Method findTargetWithSession(Method sourceMethod, Class<?> targetType) {
+
+			Class<?>[] argTypes = sourceMethod.getParameterTypes();
+			Class<?>[] args = new Class<?>[argTypes.length + 1];
+			args[0] = ClientSession.class;
+			System.arraycopy(argTypes, 0, args, 1, argTypes.length);
+
+			return ReflectionUtils.findMethod(targetType, sourceMethod.getName(), args);
+		}
+
+		boolean contains(Method method, Class<?> targetClass) {
+			return cache.containsKey(new MethodClassKey(method, targetClass));
+		}
+	}
+
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/DefaultBulkOperations.java

@@ -275,18 +275,9 @@ class DefaultBulkOperations implements BulkOperations {
 
 		try {
 			
-			MongoCollection<Document> collection = mongoOperations.getCollection(collectionName);
+			return mongoOperations.execute(collectionName, collection -> {
-			if (defaultWriteConcern != null) {
+				return collection.bulkWrite(models.stream().map(this::mapWriteModel).collect(Collectors.toList()), bulkOptions);
-				collection = collection.withWriteConcern(defaultWriteConcern);
+			});
-			}
-
-			return collection.bulkWrite(models.stream().map(this::mapWriteModel).collect(Collectors.toList()), bulkOptions);
-
-		} catch (BulkWriteException o_O) {
-
-			DataAccessException toThrow = exceptionTranslator.translateExceptionIfPossible(o_O);
-			throw toThrow == null ? o_O : toThrow;
-
 		} finally {
 			this.bulkOptions = getBulkWriteOptions(bulkOperationContext.getBulkMode());
 		}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/DefaultIndexOperations.java

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import static org.springframework.data.mongodb.core.MongoTemplate.*;
-
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
@@ -50,18 +48,22 @@ public class DefaultIndexOperations implements IndexOperations {
 
 	private static final String PARTIAL_FILTER_EXPRESSION_KEY = "partialFilterExpression";
 
-	private final MongoDbFactory mongoDbFactory;
 	private final String collectionName;
 	private final QueryMapper mapper;
 	private final @Nullable Class<?> type;
 
+	private MongoOperations mongoOperations;
+
 	/**
 	 * Creates a new {@link DefaultIndexOperations}.
 	 *
 	 * @param mongoDbFactory must not be {@literal null}.
 	 * @param collectionName must not be {@literal null}.
 	 * @param queryMapper must not be {@literal null}.
+	 * @deprecated since 2.1. Please use
+	 *             {@link DefaultIndexOperations#DefaultIndexOperations(MongoOperations, String, Class)}.
 	 */
+	@Deprecated
 	public DefaultIndexOperations(MongoDbFactory mongoDbFactory, String collectionName, QueryMapper queryMapper) {
 		this(mongoDbFactory, collectionName, queryMapper, null);
 	}
@@ -74,7 +76,10 @@ public class DefaultIndexOperations implements IndexOperations {
 	 * @param queryMapper must not be {@literal null}.
 	 * @param type Type used for mapping potential partial index filter expression. Can be {@literal null}.
 	 * @since 1.10
+	 * @deprecated since 2.1. Please use
+	 *             {@link DefaultIndexOperations#DefaultIndexOperations(MongoOperations, String, Class)}.
 	 */
+	@Deprecated
 	public DefaultIndexOperations(MongoDbFactory mongoDbFactory, String collectionName, QueryMapper queryMapper,
 			@Nullable Class<?> type) {
 
@@ -82,10 +87,26 @@ public class DefaultIndexOperations implements IndexOperations {
 		Assert.notNull(collectionName, "Collection name can not be null!");
 		Assert.notNull(queryMapper, "QueryMapper must not be null!");
 
-		this.mongoDbFactory = mongoDbFactory;
 		this.collectionName = collectionName;
 		this.mapper = queryMapper;
 		this.type = type;
+		this.mongoOperations = new MongoTemplate(mongoDbFactory);
+	}
+
+	/**
+	 * Creates a new {@link DefaultIndexOperations}.
+	 *
+	 * @param mongoOperations must not be {@literal null}.
+	 * @param collectionName must not be {@literal null}.
+	 * @param type can be {@literal null}.
+	 * @since 2.1
+	 */
+	public DefaultIndexOperations(MongoOperations mongoOperations, String collectionName, @Nullable Class<?> type) {
+
+		this.mongoOperations = mongoOperations;
+		this.mapper = new QueryMapper(mongoOperations.getConverter());
+		this.collectionName = collectionName;
+		this.type = type;
 	}
 
 	/*
@@ -187,11 +208,10 @@ public class DefaultIndexOperations implements IndexOperations {
 
 		Assert.notNull(callback, "CollectionCallback must not be null!");
 
-		try {
+		if (type != null) {
-			MongoCollection<Document> collection = mongoDbFactory.getDb().getCollection(collectionName);
+			return mongoOperations.execute(type, callback);
-			return callback.doInCollection(collection);
-		} catch (RuntimeException e) {
-			throw potentiallyConvertRuntimeException(e, mongoDbFactory.getExceptionTranslator());
 		}
+
+		return mongoOperations.execute(collectionName, callback);
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoExceptionTranslator.java

@@ -29,6 +29,7 @@ import org.springframework.dao.InvalidDataAccessResourceUsageException;
 import org.springframework.dao.PermissionDeniedDataAccessException;
 import org.springframework.dao.support.PersistenceExceptionTranslator;
 import org.springframework.data.mongodb.BulkOperationException;
+import org.springframework.data.mongodb.ClientSessionException;
 import org.springframework.data.mongodb.UncategorizedMongoDbException;
 import org.springframework.data.mongodb.util.MongoDbErrorCodes;
 import org.springframework.lang.Nullable;
@@ -119,18 +120,26 @@ public class MongoExceptionTranslator implements PersistenceExceptionTranslator
 			int code = ((MongoException) ex).getCode();
 
 			if (MongoDbErrorCodes.isDuplicateKeyCode(code)) {
-				throw new DuplicateKeyException(ex.getMessage(), ex);
+				return new DuplicateKeyException(ex.getMessage(), ex);
 			} else if (MongoDbErrorCodes.isDataAccessResourceFailureCode(code)) {
-				throw new DataAccessResourceFailureException(ex.getMessage(), ex);
+				return new DataAccessResourceFailureException(ex.getMessage(), ex);
 			} else if (MongoDbErrorCodes.isInvalidDataAccessApiUsageCode(code) || code == 10003 || code == 12001
 					|| code == 12010 || code == 12011 || code == 12012) {
-				throw new InvalidDataAccessApiUsageException(ex.getMessage(), ex);
+				return new InvalidDataAccessApiUsageException(ex.getMessage(), ex);
 			} else if (MongoDbErrorCodes.isPermissionDeniedCode(code)) {
-				throw new PermissionDeniedDataAccessException(ex.getMessage(), ex);
+				return new PermissionDeniedDataAccessException(ex.getMessage(), ex);
 			}
 			return new UncategorizedMongoDbException(ex.getMessage(), ex);
 		}
 
+		if (ex instanceof IllegalStateException) {
+			for (StackTraceElement elm : ex.getStackTrace()) {
+				if (elm.getClassName().contains("ClientSession")) {
+					return new ClientSessionException(ex.getMessage(), ex);
+				}
+			}
+		}
+
 		// If we get here, we have an exception that resulted from user code,
 		// rather than the persistence provider, so we return null to indicate
 		// that translation should not occur.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoOperations.java

@@ -18,6 +18,8 @@ package org.springframework.data.mongodb.core;
 import java.util.Collection;
 import java.util.List;
 import java.util.Set;
+import java.util.function.Consumer;
+import java.util.function.Supplier;
 
 import org.bson.Document;
 import org.springframework.data.geo.GeoResults;
@@ -40,11 +42,13 @@ import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.data.util.CloseableIterator;
 import org.springframework.lang.Nullable;
 
+import com.mongodb.ClientSessionOptions;
 import com.mongodb.Cursor;
 import com.mongodb.ReadPreference;
 import com.mongodb.client.MongoCollection;
 import com.mongodb.client.result.DeleteResult;
 import com.mongodb.client.result.UpdateResult;
+import com.mongodb.session.ClientSession;
 
 /**
  * Interface that specifies a basic set of MongoDB operations. Implemented by {@link MongoTemplate}. Not often used but
@@ -151,6 +155,62 @@ public interface MongoOperations extends FluentMongoOperations {
 	@Nullable
 	<T> T execute(String collectionName, CollectionCallback<T> action);
 
+	/**
+	 * Obtain a session bound instance of {@link SessionScoped} binding a new {@link ClientSession} with given
+	 * {@literal sessionOptions} to each and every command issued against MongoDB.
+	 *
+	 * @param sessionOptions must not be {@literal null}.
+	 * @return new instance of {@link SessionScoped}. Never {@literal null}.
+	 * @since 2.1
+	 */
+	SessionScoped withSession(ClientSessionOptions sessionOptions);
+
+	/**
+	 * Obtain a session bound instance of {@link SessionScoped} binding the {@link ClientSession} provided by the given
+	 * {@link Supplier} to each and every command issued against MongoDB.
+	 * <p />
+	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle.
+	 *
+	 * @param sessionProvider must not be {@literal null}.
+	 * @param onComplete a simple hook called when done .Must not be {@literal null}.
+	 * @since 2.1
+	 */
+	default SessionScoped withSession(Supplier<ClientSession> sessionProvider) {
+
+		return new SessionScoped() {
+
+			private final Object lock = new Object();
+			private @Nullable ClientSession session = null;
+
+			@Override
+			public <T> T execute(SessionCallback<T> action, Consumer<ClientSession> onComplete) {
+
+				synchronized (lock) {
+					if (session == null) {
+						session = sessionProvider.get();
+					}
+				}
+
+				try {
+					return action.doInSession(MongoOperations.this.withSession(session));
+				} finally {
+					onComplete.accept(session);
+				}
+			}
+		};
+	}
+
+	/**
+	 * Obtain a {@link ClientSession} bound instance of MongoOperations.
+	 * <p />
+	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle.
+	 *
+	 * @param session must not be {@literal null}.
+	 * @return {@link ClientSession} bound instance of {@link MongoOperations}.
+	 * @since 2.1
+	 */
+	MongoOperations withSession(ClientSession session);
+
 	/**
 	 * Executes the given {@link Query} on the entity collection of the specified {@code entityType} backed by a Mongo DB
 	 * {@link Cursor}.
@@ -769,8 +829,8 @@ public interface MongoOperations extends FluentMongoOperations {
 	}
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
-	 * <a/> to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query}.
+	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query}.
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification. Must not be {@literal null}.
@@ -782,8 +842,8 @@ public interface MongoOperations extends FluentMongoOperations {
 	<T> T findAndModify(Query query, Update update, Class<T> entityClass);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
-	 * <a/> to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query}.
+	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query}.
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
 	 *          fields specification. Must not be {@literal null}.
@@ -796,8 +856,8 @@ public interface MongoOperations extends FluentMongoOperations {
 	<T> T findAndModify(Query query, Update update, Class<T> entityClass, String collectionName);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
-	 * <a/> to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query} taking
+	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query} taking
 	 * {@link FindAndModifyOptions} into account.
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional
@@ -813,8 +873,8 @@ public interface MongoOperations extends FluentMongoOperations {
 	<T> T findAndModify(Query query, Update update, FindAndModifyOptions options, Class<T> entityClass);
 
 	/**
-	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify
+	 * Triggers <a href="https://docs.mongodb.org/manual/reference/method/db.collection.findAndModify/">findAndModify <a/>
-	 * <a/> to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query} taking
+	 * to apply provided {@link Update} on documents matching {@link Criteria} of given {@link Query} taking
 	 * {@link FindAndModifyOptions} into account.
 	 *
 	 * @param query the {@link Query} class that specifies the {@link Criteria} used to find a record and also an optional

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoTemplate.java

@@ -124,6 +124,7 @@ import org.springframework.util.ObjectUtils;
 import org.springframework.util.ResourceUtils;
 import org.springframework.util.StringUtils;
 
+import com.mongodb.ClientSessionOptions;
 import com.mongodb.Cursor;
 import com.mongodb.DBCollection;
 import com.mongodb.DBCursor;
@@ -152,6 +153,7 @@ import com.mongodb.client.model.ValidationAction;
 import com.mongodb.client.model.ValidationLevel;
 import com.mongodb.client.result.DeleteResult;
 import com.mongodb.client.result.UpdateResult;
+import com.mongodb.session.ClientSession;
 import com.mongodb.util.JSONParseException;
 
 /**
@@ -219,7 +221,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @param databaseName must not be {@literal null} or empty.
 	 */
 	public MongoTemplate(MongoClient mongoClient, String databaseName) {
-		this(new SimpleMongoDbFactory(mongoClient, databaseName), null);
+		this(new SimpleMongoDbFactory(mongoClient, databaseName), (MongoConverter) null);
 	}
 
 	/**
@@ -228,7 +230,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @param mongoDbFactory must not be {@literal null}.
 	 */
 	public MongoTemplate(MongoDbFactory mongoDbFactory) {
-		this(mongoDbFactory, null);
+		this(mongoDbFactory, (MongoConverter) null);
 	}
 
 	/**
@@ -261,6 +263,20 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		}
 	}
 
+	private MongoTemplate(MongoDbFactory dbFactory, MongoTemplate that) {
+
+		this.mongoDbFactory = dbFactory;
+		this.exceptionTranslator = that.exceptionTranslator;
+		this.mongoConverter = that.mongoConverter instanceof MappingMongoConverter ? getDefaultMongoConverter(dbFactory)
+				: that.mongoConverter;
+		this.queryMapper = that.queryMapper;
+		this.updateMapper = that.updateMapper;
+		this.schemaMapper = that.schemaMapper;
+		this.projectionFactory = that.projectionFactory;
+
+		this.mappingContext = that.mappingContext;
+	}
+
 	/**
 	 * Configures the {@link WriteResultChecking} to be used with the template. Setting {@literal null} will reset the
 	 * default of {@link #DEFAULT_WRITE_RESULT_CHECKING}.
@@ -394,7 +410,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 				Document mappedQuery = queryMapper.getMappedObject(query.getQueryObject(), persistentEntity);
 
 				FindIterable<Document> cursor = new QueryCursorPreparer(query, entityType)
-						.prepare(collection.find(mappedQuery).projection(mappedFields));
+						.prepare(collection.find(mappedQuery, Document.class).projection(mappedFields));
 
 				return new CloseableIterableCursorAdapter<T>(cursor, exceptionTranslator,
 						new ProjectingReadCallback<>(mongoConverter, entityType, returnType, collectionName));
@@ -509,7 +525,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		Assert.notNull(action, "DbCallbackmust not be null!");
 
 		try {
-			MongoDatabase db = this.getDb();
+			MongoDatabase db = prepareDatabase(this.getDbInternal());
 			return action.doInDB(db);
 		} catch (RuntimeException e) {
 			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
@@ -536,13 +552,31 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		Assert.notNull(callback, "CollectionCallback must not be null!");
 
 		try {
-			MongoCollection<Document> collection = getAndPrepareCollection(getDb(), collectionName);
+			MongoCollection<Document> collection = getAndPrepareCollection(getDbInternal(), collectionName);
 			return callback.doInCollection(collection);
 		} catch (RuntimeException e) {
 			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
 		}
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.MongoOperations#withSession(com.mongodb.ClientSessionOptions)
+	 */
+	@Override
+	public SessionScoped withSession(ClientSessionOptions options) {
+		return withSession(() -> mongoDbFactory.getSession(options));
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.MongoOperations#withSession(com.mongodb.session.ClientSession)
+	 */
+	@Override
+	public MongoTemplate withSession(ClientSession session) {
+		return new SessionBoundMongoTemplate(session, MongoTemplate.this);
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.MongoOperations#createCollection(java.lang.Class)
@@ -617,6 +651,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		return execute(new DbCallback<Boolean>() {
 			public Boolean doInDB(MongoDatabase db) throws MongoException, DataAccessException {
+
 				for (String name : db.listCollectionNames()) {
 					if (name.equals(collectionName)) {
 						return true;
@@ -659,7 +694,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @see org.springframework.data.mongodb.core.ExecutableInsertOperation#indexOps(java.lang.String)
 	 */
 	public IndexOperations indexOps(String collectionName) {
-		return new DefaultIndexOperations(getMongoDbFactory(), collectionName, queryMapper);
+		return new DefaultIndexOperations(this, collectionName, null);
 	}
 
 	/*
@@ -667,8 +702,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @see org.springframework.data.mongodb.core.ExecutableInsertOperation#indexOps(java.lang.Class)
 	 */
 	public IndexOperations indexOps(Class<?> entityClass) {
-		return new DefaultIndexOperations(getMongoDbFactory(), determineCollectionName(entityClass), queryMapper,
+		return new DefaultIndexOperations(this, determineCollectionName(entityClass), entityClass);
-				entityClass);
 	}
 
 	/*
@@ -847,10 +881,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		Class<T> mongoDriverCompatibleType = getMongoDbFactory().getCodecFor(resultClass).map(Codec::getEncoderClass)
 				.orElse((Class) BsonValue.class);
 
-		MongoIterable<?> result = execute((db) -> {
+		MongoIterable<?> result = execute(collectionName, (collection) -> {
 
-			DistinctIterable<T> iterable = db.getCollection(collectionName).distinct(mappedFieldName, mappedQuery,
+			DistinctIterable<T> iterable = collection.distinct(mappedFieldName, mappedQuery, mongoDriverCompatibleType);
-					mongoDriverCompatibleType);
 
 			return query.getCollation().map(Collation::toMongoCollation).map(iterable::collation).orElse(iterable);
 		});
@@ -1055,10 +1088,13 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		Assert.notNull(query, "Query must not be null!");
 		Assert.hasText(collectionName, "Collection name must not be null or empty!");
 
+		CountOptions options = new CountOptions();
+		query.getCollation().map(Collation::toMongoCollation).ifPresent(options::collation);
+
 		Document document = queryMapper.getMappedObject(query.getQueryObject(),
 				Optional.ofNullable(entityClass).map(it -> mappingContext.getPersistentEntity(entityClass)));
 
-		return execute(collectionName, collection -> collection.count(document));
+		return execute(collectionName, collection -> collection.count(document, options));
 	}
 
 	/*
@@ -1105,8 +1141,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	protected MongoCollection<Document> prepareCollection(MongoCollection<Document> collection) {
 
 		if (this.readPreference != null) {
-			return collection.withReadPreference(readPreference);
+			collection = collection.withReadPreference(readPreference);
 		}
+
 		return collection;
 	}
 
@@ -1372,6 +1409,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 				MongoAction mongoAction = new MongoAction(writeConcern, MongoActionOperation.INSERT, collectionName,
 						entityClass, document, null);
 				WriteConcern writeConcernToUse = prepareWriteConcern(mongoAction);
+
 				if (writeConcernToUse == null) {
 					collection.insertOne(document);
 				} else {
@@ -1781,10 +1819,10 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		String mapFunc = replaceWithResourceIfNecessary(mapFunction);
 		String reduceFunc = replaceWithResourceIfNecessary(reduceFunction);
-		MongoCollection<Document> inputCollection = getCollection(inputCollectionName);
+		MongoCollection<Document> inputCollection = getAndPrepareCollection(getDbInternal(), inputCollectionName);
 
 		// MapReduceOp
-		MapReduceIterable<Document> result = inputCollection.mapReduce(mapFunc, reduceFunc);
+		MapReduceIterable<Document> result = inputCollection.mapReduce(mapFunc, reduceFunc, Document.class);
 		if (query != null && result != null) {
 
 			if (query.getLimit() > 0 && mapReduceOptions.getLimit() == null) {
@@ -1867,13 +1905,13 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		}
 
 		if (document.containsKey("$reduce")) {
-			document.put("$reduce", replaceWithResourceIfNecessary(document.get("$reduce").toString()));
+			document.put("$reduce", replaceWithResourceIfNecessary(ObjectUtils.nullSafeToString(document.get("$reduce"))));
 		}
 		if (document.containsKey("$keyf")) {
-			document.put("$keyf", replaceWithResourceIfNecessary(document.get("$keyf").toString()));
+			document.put("$keyf", replaceWithResourceIfNecessary(ObjectUtils.nullSafeToString(document.get("$keyf"))));
 		}
 		if (document.containsKey("finalize")) {
-			document.put("finalize", replaceWithResourceIfNecessary(document.get("finalize").toString()));
+			document.put("finalize", replaceWithResourceIfNecessary(ObjectUtils.nullSafeToString(document.get("finalize"))));
 		}
 
 		Document commandObject = new Document("group", document);
@@ -1882,7 +1920,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			LOGGER.debug("Executing Group with Document [{}]", serializeToJsonSafely(commandObject));
 		}
 
-		Document commandResult = executeCommand(commandObject);
+		Document commandResult = executeCommand(commandObject, this.readPreference);
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug("Group command result = [{}]", commandResult);
@@ -2107,7 +2145,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		return execute(collectionName, (CollectionCallback<CloseableIterator<O>>) collection -> {
 
-			AggregateIterable<Document> cursor = collection.aggregate(pipeline) //
+			AggregateIterable<Document> cursor = collection.aggregate(pipeline, Document.class) //
 					.allowDiskUse(options.isAllowDiskUse()) //
 					.useCursor(true);
 
@@ -2214,9 +2252,17 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	}
 
 	public MongoDatabase getDb() {
+		return getDbInternal();
+	}
+
+	protected MongoDatabase getDbInternal() {
 		return mongoDbFactory.getDb();
 	}
 
+	protected MongoDatabase prepareDatabase(MongoDatabase database) {
+		return database;
+	}
+
 	protected <T> void maybeEmitEvent(MongoMappingEvent<T> event) {
 		if (null != eventPublisher) {
 			eventPublisher.publishEvent(event);
@@ -2559,7 +2605,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		try {
 			T result = objectCallback
-					.doWith(collectionCallback.doInCollection(getAndPrepareCollection(getDb(), collectionName)));
+					.doWith(collectionCallback.doInCollection(getAndPrepareCollection(getDbInternal(), collectionName)));
 			return result;
 		} catch (RuntimeException e) {
 			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
@@ -2594,7 +2640,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			try {
 
 				FindIterable<Document> iterable = collectionCallback
-						.doInCollection(getAndPrepareCollection(getDb(), collectionName));
+						.doInCollection(getAndPrepareCollection(getDbInternal(), collectionName));
 
 				if (preparer != null) {
 					iterable = preparer.prepare(iterable);
@@ -2630,7 +2676,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 			try {
 				FindIterable<Document> iterable = collectionCallback
-						.doInCollection(getAndPrepareCollection(getDb(), collectionName));
+						.doInCollection(getAndPrepareCollection(getDbInternal(), collectionName));
 
 				if (preparer != null) {
 					iterable = preparer.prepare(iterable);
@@ -2775,7 +2821,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		public Document doInCollection(MongoCollection<Document> collection) throws MongoException, DataAccessException {
 
-			FindIterable<Document> iterable = collection.find(query);
+			FindIterable<Document> iterable = collection.find(query, Document.class);
 
 			if (LOGGER.isDebugEnabled()) {
 
@@ -2820,7 +2866,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		public FindIterable<Document> doInCollection(MongoCollection<Document> collection)
 				throws MongoException, DataAccessException {
 
-			return collection.find(query).projection(fields);
+			return collection.find(query, Document.class).projection(fields);
 		}
 	}
 
@@ -3374,4 +3420,53 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			return ((Document) commandResult.get(CURSOR_FIELD)).get("id");
 		}
 	}
+
+	/**
+	 * {@link MongoTemplate} extension bound to a specific {@link ClientSession} that is applied when interacting with the
+	 * server through the driver API.
+	 * <p />
+	 * The prepare steps for {@link MongoDatabase} and {@link MongoCollection} proxy the target and invoke the desired
+	 * target method matching the actual arguments plus a {@link ClientSession}.
+	 *
+	 * @author Christoph Strobl
+	 * @since 2.1
+	 */
+	static class SessionBoundMongoTemplate extends MongoTemplate {
+
+		private final MongoTemplate delegate;
+
+		/**
+		 * @param session must not be {@literal null}.
+		 * @param mongoDbFactory must not be {@literal null}.
+		 * @param mongoConverter must not be {@literal null}.
+		 */
+		SessionBoundMongoTemplate(ClientSession session, MongoTemplate that) {
+
+			super(that.getMongoDbFactory().withSession(session), that);
+
+			this.delegate = that;
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.MongoTemplate#getCollection(java.lang.String)
+		 */
+		@Override
+		public MongoCollection<Document> getCollection(String collectionName) {
+
+			// native MongoDB objects that offer methods with ClientSession must not be proxied.
+			return delegate.getCollection(collectionName);
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.MongoTemplate#getDb()
+		 */
+		@Override
+		public MongoDatabase getDb() {
+
+			// native MongoDB objects that offer methods with ClientSession must not be proxied.
+			return delegate.getDb();
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveMongoOperations.java

@@ -20,6 +20,8 @@ import reactor.core.publisher.Mono;
 
 import java.util.Collection;
 import java.util.List;
+import java.util.function.Consumer;
+import java.util.function.Supplier;
 
 import org.bson.Document;
 import org.reactivestreams.Publisher;
@@ -37,11 +39,14 @@ import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 
+import com.mongodb.ClientSessionOptions;
 import com.mongodb.ReadPreference;
 import com.mongodb.client.result.DeleteResult;
 import com.mongodb.client.result.UpdateResult;
 import com.mongodb.reactivestreams.client.MongoCollection;
+import com.mongodb.session.ClientSession;
 
 /**
  * Interface that specifies a basic set of MongoDB operations executed in a reactive way.
@@ -141,6 +146,64 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 */
 	<T> Flux<T> execute(String collectionName, ReactiveCollectionCallback<T> action);
 
+	/**
+	 * Obtain a session bound instance of {@link SessionScoped} binding the {@link ClientSession} provided by the given
+	 * {@link Supplier} to each and every command issued against MongoDB.
+	 * <p />
+	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle. Use
+	 * {@link #withSession(Supplier, Consumer)} to provide a hook for processing the {@link ClientSession} when done.
+	 *
+	 * @param sessionProvider must not be {@literal null}.
+	 * @return new instance of {@link SessionScoped}. Never {@literal null}.
+	 * @since 2.1
+	 */
+	default ReactiveSessionScoped withSession(Supplier<ClientSession> sessionProvider) {
+
+		Assert.notNull(sessionProvider, "SessionProvider must not be null!");
+
+		return withSession(Mono.fromSupplier(sessionProvider));
+	}
+
+	/**
+	 * Obtain a session bound instance of {@link SessionScoped} binding a new {@link ClientSession} with given
+	 * {@literal sessionOptions} to each and every command issued against MongoDB.
+	 *
+	 * @param sessionOptions must not be {@literal null}.
+	 * @return new instance of {@link SessionScoped}. Never {@literal null}.
+	 * @since 2.1
+	 */
+	ReactiveSessionScoped withSession(ClientSessionOptions sessionOptions);
+
+	/**
+	 * Obtain a session bound instance of {@link SessionScoped} binding the {@link ClientSession} provided by the given
+	 * {@link Supplier} to each and every command issued against MongoDB.
+	 * <p />
+	 * <strong>Note:</strong> It is up to the caller to manage the {@link ClientSession} lifecycle. Use the
+	 * {@litera onComplete} hook to potentially close the {@link ClientSession}.
+	 *
+	 * @param sessionProvider must not be {@literal null}.
+	 * @return new instance of {@link SessionScoped}. Never {@literal null}.
+	 * @since 2.1
+	 */
+	default ReactiveSessionScoped withSession(Publisher<ClientSession> sessionProvider) {
+
+		return new ReactiveSessionScoped() {
+
+			private final Mono<ClientSession> cachedSession = Mono.from(sessionProvider).cache();
+
+			@Override
+			public <T> Flux<T> execute(ReactiveSessionCallback<T> action, Consumer<ClientSession> doFinally) {
+
+				return cachedSession.flatMapMany(session -> {
+					return Flux.from(action.doInSession(ReactiveMongoOperations.this.withSession(session)))
+							.doFinally((signalType) -> doFinally.accept(session));
+				});
+			}
+		};
+	}
+
+	ReactiveMongoOperations withSession(ClientSession session);
+
 	/**
 	 * Create an uncapped collection with a name based on the provided entity class.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveMongoTemplate.java

@@ -26,6 +26,7 @@ import reactor.util.function.Tuple2;
 
 import java.util.*;
 import java.util.Map.Entry;
+import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
@@ -105,6 +106,7 @@ import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 import com.mongodb.BasicDBObject;
+import com.mongodb.ClientSessionOptions;
 import com.mongodb.CursorType;
 import com.mongodb.DBCollection;
 import com.mongodb.DBCursor;
@@ -113,6 +115,7 @@ import com.mongodb.Mongo;
 import com.mongodb.MongoException;
 import com.mongodb.ReadPreference;
 import com.mongodb.WriteConcern;
+import com.mongodb.client.model.CountOptions;
 import com.mongodb.client.model.CreateCollectionOptions;
 import com.mongodb.client.model.DeleteOptions;
 import com.mongodb.client.model.Filters;
@@ -132,6 +135,7 @@ import com.mongodb.reactivestreams.client.MongoClient;
 import com.mongodb.reactivestreams.client.MongoCollection;
 import com.mongodb.reactivestreams.client.MongoDatabase;
 import com.mongodb.reactivestreams.client.Success;
+import com.mongodb.session.ClientSession;
 import com.mongodb.util.JSONParseException;
 
 /**
@@ -191,7 +195,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * @param databaseName must not be {@literal null} or empty.
 	 */
 	public ReactiveMongoTemplate(MongoClient mongoClient, String databaseName) {
-		this(new SimpleReactiveMongoDatabaseFactory(mongoClient, databaseName), null);
+		this(new SimpleReactiveMongoDatabaseFactory(mongoClient, databaseName), (MongoConverter) null);
 	}
 
 	/**
@@ -200,7 +204,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * @param mongoDatabaseFactory must not be {@literal null}.
 	 */
 	public ReactiveMongoTemplate(ReactiveMongoDatabaseFactory mongoDatabaseFactory) {
-		this(mongoDatabaseFactory, null);
+		this(mongoDatabaseFactory, (MongoConverter) null);
 	}
 
 	/**
@@ -236,6 +240,19 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		}
 	}
 
+	private ReactiveMongoTemplate(ReactiveMongoDatabaseFactory dbFactory, ReactiveMongoTemplate that) {
+
+		this.mongoDatabaseFactory = dbFactory;
+		this.exceptionTranslator = that.exceptionTranslator;
+		this.mongoConverter = that.mongoConverter;
+		this.queryMapper = that.queryMapper;
+		this.updateMapper = that.updateMapper;
+		this.schemaMapper = that.schemaMapper;
+		this.projectionFactory = that.projectionFactory;
+
+		this.mappingContext = that.mappingContext;
+	}
+
 	/**
 	 * Configures the {@link WriteResultChecking} to be used with the template. Setting {@literal null} will reset the
 	 * default of {@link ReactiveMongoTemplate#DEFAULT_WRITE_RESULT_CHECKING}.
@@ -406,6 +423,46 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		return createFlux(collectionName, callback);
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#withSession(org.reactivestreams.Publisher, java.util.function.Consumer)
+	 */
+	@Override
+	public ReactiveSessionScoped withSession(Publisher<ClientSession> sessionProvider) {
+
+		return new ReactiveSessionScoped() {
+
+			private final Mono<ClientSession> cachedSession = Mono.from(sessionProvider).cache();
+
+			@Override
+			public <T> Flux<T> execute(ReactiveSessionCallback<T> action, Consumer<ClientSession> doFinally) {
+
+				return cachedSession.flatMapMany(session -> {
+					return Flux
+							.from(action.doInSession(new ReactiveSessionBoundMongoTemplate(session, ReactiveMongoTemplate.this)))
+							.doFinally((signalType) -> doFinally.accept(session));
+				});
+			}
+		};
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#withSession(com.mongodb.session.ClientSession)
+	 */
+	public ReactiveMongoOperations withSession(ClientSession session) {
+		return new ReactiveSessionBoundMongoTemplate(session, ReactiveMongoTemplate.this);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#withSession(com.mongodb.ClientSessionOptions)
+	 */
+	@Override
+	public ReactiveSessionScoped withSession(ClientSessionOptions sessionOptions) {
+		return withSession(mongoDatabaseFactory.getSession(sessionOptions));
+	}
+
 	/**
 	 * Create a reusable Flux for a {@link ReactiveDatabaseCallback}. It's up to the developer to choose to obtain a new
 	 * {@link Flux} or to reuse the {@link Flux}.
@@ -417,7 +474,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		Assert.notNull(callback, "ReactiveDatabaseCallback must not be null!");
 
-		return Flux.defer(() -> callback.doInDB(getMongoDatabase())).onErrorMap(translateException());
+		return Flux.defer(() -> callback.doInDB(prepareDatabase(getDbInternal()))).onErrorMap(translateException());
 	}
 
 	/**
@@ -431,7 +488,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		Assert.notNull(callback, "ReactiveDatabaseCallback must not be null!");
 
-		return Mono.defer(() -> Mono.from(callback.doInDB(getMongoDatabase()))).onErrorMap(translateException());
+		return Mono.defer(() -> Mono.from(callback.doInDB(prepareDatabase(getDbInternal()))))
+				.onErrorMap(translateException());
 	}
 
 	/**
@@ -447,7 +505,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		Assert.notNull(callback, "ReactiveDatabaseCallback must not be null!");
 
 		Mono<MongoCollection<Document>> collectionPublisher = Mono
-				.fromCallable(() -> getAndPrepareCollection(getMongoDatabase(), collectionName));
+				.fromCallable(() -> getAndPrepareCollection(getDbInternal(), collectionName));
 
 		return collectionPublisher.flatMapMany(callback::doInCollection).onErrorMap(translateException());
 	}
@@ -466,7 +524,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		Assert.notNull(callback, "ReactiveCollectionCallback must not be null!");
 
 		Mono<MongoCollection<Document>> collectionPublisher = Mono
-				.fromCallable(() -> getAndPrepareCollection(getMongoDatabase(), collectionName));
+				.fromCallable(() -> getAndPrepareCollection(getDbInternal(), collectionName));
 
 		return collectionPublisher.flatMap(collection -> Mono.from(callback.doInCollection(collection)))
 				.onErrorMap(translateException());
@@ -548,7 +606,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 */
 	public Mono<Void> dropCollection(final String collectionName) {
 
-		return createMono(db -> db.getCollection(collectionName).drop()).doOnSuccess(success -> {
+		return createMono(collectionName, collection -> collection.drop()).doOnSuccess(success -> {
 			if (LOGGER.isDebugEnabled()) {
 				LOGGER.debug("Dropped collection [" + collectionName + "]");
 			}
@@ -564,6 +622,10 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	}
 
 	public MongoDatabase getMongoDatabase() {
+		return getDbInternal();
+	}
+
+	protected MongoDatabase getDbInternal() {
 		return mongoDatabaseFactory.getMongoDatabase();
 	}
 
@@ -619,7 +681,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		return createFlux(collectionName, collection -> {
 
 			Document mappedQuery = queryMapper.getMappedObject(query.getQueryObject(), getPersistentEntity(entityClass));
-			FindPublisher<Document> findPublisher = collection.find(mappedQuery).projection(new Document("_id", 1));
+			FindPublisher<Document> findPublisher = collection.find(mappedQuery, Document.class)
+					.projection(new Document("_id", 1));
 
 			findPublisher = query.getCollation().map(Collation::toMongoCollation).map(findPublisher::collation)
 					.orElse(findPublisher);
@@ -821,8 +884,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	private <O> Flux<O> aggregateAndMap(MongoCollection<Document> collection, List<Document> pipeline,
 			AggregationOptions options, ReadDocumentCallback<O> readCallback) {
 
-		AggregatePublisher<Document> cursor = collection.aggregate(pipeline).allowDiskUse(options.isAllowDiskUse())
+		AggregatePublisher<Document> cursor = collection.aggregate(pipeline, Document.class)
-				.useCursor(true);
+				.allowDiskUse(options.isAllowDiskUse()).useCursor(true);
 
 		if (options.getCollation().isPresent()) {
 			cursor = cursor.collation(options.getCollation().map(Collation::toMongoCollation).get());
@@ -988,7 +1051,12 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 					: queryMapper.getMappedObject(query.getQueryObject(),
 							entityClass == null ? null : mappingContext.getPersistentEntity(entityClass));
 
-			return collection.count(Document);
+			CountOptions options = new CountOptions();
+			if (query != null) {
+				query.getCollation().map(Collation::toMongoCollation).ifPresent(options::collation);
+			}
+
+			return collection.count(Document, options);
 		});
 	}
 
@@ -2197,7 +2265,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	private MongoCollection<Document> getAndPrepareCollection(MongoDatabase db, String collectionName) {
 
 		try {
-			MongoCollection<Document> collection = db.getCollection(collectionName);
+			MongoCollection<Document> collection = db.getCollection(collectionName, Document.class);
 			return prepareCollection(collection);
 		} catch (RuntimeException e) {
 			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
@@ -2235,6 +2303,15 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		return collection;
 	}
 
+	/**
+	 * @param database
+	 * @return
+	 * @since 2.1
+	 */
+	protected MongoDatabase prepareDatabase(MongoDatabase database) {
+		return database;
+	}
+
 	/**
 	 * Prepare the WriteConcern before any processing is done using it. This allows a convenient way to apply custom
 	 * settings in sub-classes. <br />
@@ -2323,7 +2400,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		Assert.notNull(action, "MongoDatabaseCallback must not be null!");
 
 		try {
-			MongoDatabase db = this.getMongoDatabase();
+			MongoDatabase db = this.getDbInternal();
 			return action.doInDatabase(db);
 		} catch (RuntimeException e) {
 			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
@@ -2484,7 +2561,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		public Publisher<Document> doInCollection(MongoCollection<Document> collection)
 				throws MongoException, DataAccessException {
 
-			FindPublisher<Document> publisher = collection.find(query);
+			FindPublisher<Document> publisher = collection.find(query, Document.class);
 
 			if (LOGGER.isDebugEnabled()) {
 
@@ -2526,13 +2603,13 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		public FindPublisher<Document> doInCollection(MongoCollection<Document> collection) {
 
 			FindPublisher<Document> findPublisher;
-			if (query == null || query.isEmpty()) {
+			if (ObjectUtils.isEmpty(query)) {
-				findPublisher = collection.find();
+				findPublisher = collection.find(Document.class);
 			} else {
-				findPublisher = collection.find(query);
+				findPublisher = collection.find(query, Document.class);
 			}
 
-			if (fields == null || fields.isEmpty()) {
+			if (ObjectUtils.isEmpty(fields)) {
 				return findPublisher;
 			} else {
 				return findPublisher.projection(fields);
@@ -2869,6 +2946,10 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 */
 	static class NoOpDbRefResolver implements DbRefResolver {
 
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.convert.DbRefResolver#resolveDbRef(org.springframework.data.mongodb.core.mapping.MongoPersistentProperty, org.springframework.data.mongodb.core.convert.DbRefResolverCallback)
+		 */
 		@Override
 		@Nullable
 		public Object resolveDbRef(@Nonnull MongoPersistentProperty property, @Nonnull DBRef dbref,
@@ -2876,6 +2957,10 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			return null;
 		}
 
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.convert.DbRefResolver#created(org.springframework.data.mongodb.core.mapping.MongoPersistentProperty, org.springframework.data.mongodb.core.mapping.MongoPersistentEntity, java.lang.Object)
+		 */
 		@Override
 		@Nullable
 		public DBRef createDbRef(org.springframework.data.mongodb.core.mapping.DBRef annotation,
@@ -2883,14 +2968,71 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			return null;
 		}
 
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.convert.DbRefResolver#fetch(com.mongodb.DBRef)
+		 */
 		@Override
 		public Document fetch(DBRef dbRef) {
 			return null;
 		}
 
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.convert.DbRefResolver#bulkFetch(java.util.List)
+		 */
 		@Override
 		public List<Document> bulkFetch(List<DBRef> dbRefs) {
 			return Collections.emptyList();
 		}
 	}
+
+	/**
+	 * {@link MongoTemplate} extension bound to a specific {@link ClientSession} that is applied when interacting with the
+	 * server through the driver API.
+	 * <p />
+	 * The prepare steps for {@link MongoDatabase} and {@link MongoCollection} proxy the target and invoke the desired
+	 * target method matching the actual arguments plus a {@link ClientSession}.
+	 *
+	 * @author Christoph Strobl
+	 * @since 2.1
+	 */
+	static class ReactiveSessionBoundMongoTemplate extends ReactiveMongoTemplate {
+
+		private final ReactiveMongoTemplate delegate;
+
+		/**
+		 * @param session must not be {@literal null}.
+		 * @param mongoDbFactory must not be {@literal null}.
+		 * @param mongoConverter must not be {@literal null}.
+		 */
+		ReactiveSessionBoundMongoTemplate(ClientSession session, ReactiveMongoTemplate that) {
+
+			super(that.mongoDatabaseFactory.withSession(session), that);
+
+			this.delegate = that;
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.ReactiveMongoTemplate#getCollection(java.lang.String)
+		 */
+		@Override
+		public MongoCollection<Document> getCollection(String collectionName) {
+
+			// native MongoDB objects that offer methods with ClientSession must not be proxied.
+			return delegate.getCollection(collectionName);
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.ReactiveMongoTemplate#getMongoDatabase()
+		 */
+		@Override
+		public MongoDatabase getMongoDatabase() {
+
+			// native MongoDB objects that offer methods with ClientSession must not be proxied.
+			return delegate.getMongoDatabase();
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveSessionCallback.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import org.reactivestreams.Publisher;
+import org.springframework.data.mongodb.core.query.Query;
+
+/**
+ * Callback interface for executing operations within a {@link com.mongodb.session.ClientSession} reactively.
+ *
+ * @author Christoph Strobl
+ * @since 2.1
+ */
+@FunctionalInterface
+public interface ReactiveSessionCallback<T> {
+
+	/**
+	 * Execute operations against a MongoDB instance via session bound {@link ReactiveMongoOperations}. The session is
+	 * inferred directly into the operation so that no further interaction is necessary.
+	 * <p />
+	 * Please note that only Spring Data specific abstractions like {@link ReactiveMongoOperations#find(Query, Class)} and
+	 * others are enhanced with the {@link com.mongodb.session.ClientSession}. When obtaining plain MongoDB gateway
+	 * objects like {@link com.mongodb.reactivestreams.client.MongoCollection} or
+	 * {@link om.mongodb.reactivestreams.client.MongoDatabase} via eg.
+	 * {@link ReactiveMongoOperations#getCollection(String)} we leave responsibility for
+	 * {@link com.mongodb.session.ClientSession} again up to the caller.
+	 *
+	 * @param operations will never be {@literal null}.
+	 * @return can be {@literal null}.
+	 */
+	Publisher<T> doInSession(ReactiveMongoOperations operations);
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveSessionScoped.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import reactor.core.publisher.Flux;
+
+import java.util.function.Consumer;
+
+import com.mongodb.session.ClientSession;
+
+/**
+ * Gateway interface to execute {@link ClientSession} bound operations against MongoDB via a
+ * {@link ReactiveSessionCallback}.
+ *
+ * @author Christoph Strobl
+ * @since 2.1
+ */
+public interface ReactiveSessionScoped {
+
+	/**
+	 * Executes the given {@link ReactiveSessionCallback} within the {@link com.mongodb.session.ClientSession}
+	 * <p/>
+	 * It is up to the caller to make sure the {@link com.mongodb.session.ClientSession} is {@link ClientSession#close()
+	 * closed} when done.
+	 *
+	 * @param action callback object that specifies the MongoDB action the callback action. Must not be {@literal null}.
+	 * @param <T> return type.
+	 * @return a result object returned by the action. Can be {@literal null}.
+	 */
+	default <T> Flux<T> execute(ReactiveSessionCallback<T> action) {
+		return execute(action, (session) -> {});
+	}
+
+	/**
+	 * Executes the given {@link ReactiveSessionCallback} within the {@link com.mongodb.session.ClientSession}
+	 * <p/>
+	 * It is up to the caller to make sure the {@link com.mongodb.session.ClientSession} is {@link ClientSession#close()
+	 * closed} when done.
+	 *
+	 * @param action callback object that specifies the MongoDB action the callback action. Must not be {@literal null}.
+	 * @param doFinally
+	 * @param <T> return type.
+	 * @return a result object returned by the action. Can be {@literal null}.
+	 */
+	<T> Flux<T> execute(ReactiveSessionCallback<T> action, Consumer<ClientSession> doFinally);
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/SessionCallback.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.lang.Nullable;
+
+/**
+ * Callback interface for executing operations within a {@link com.mongodb.session.ClientSession}.
+ *
+ * @author Christoph Strobl
+ * @since 2.1
+ */
+public interface SessionCallback<T> {
+
+	/**
+	 * Execute operations against a MongoDB instance via session bound {@link MongoOperations}. The session is inferred
+	 * directly into the operation so that no further interaction is necessary.
+	 * <p />
+	 * Please note that only Spring Data specific abstractions like {@link MongoOperations#find(Query, Class)} and others
+	 * are enhanced with the {@link com.mongodb.session.ClientSession}. When obtaining plain MongoDB gateway objects like
+	 * {@link com.mongodb.client.MongoCollection} or {@link com.mongodb.client.MongoDatabase} via eg.
+	 * {@link MongoOperations#getCollection(String)} we leave responsibility for {@link com.mongodb.session.ClientSession}
+	 * again up to the caller.
+	 *
+	 * @param operations will never be {@literal null}.
+	 * @return can be {@literal null}.
+	 */
+	@Nullable
+	T doInSession(MongoOperations operations);
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/SessionScoped.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import java.util.function.Consumer;
+
+import org.springframework.lang.Nullable;
+
+import com.mongodb.session.ClientSession;
+
+/**
+ * Gateway interface to execute {@link ClientSession} bound operations against MongoDB via a {@link SessionCallback}.
+ * <p />
+ * The very same bound {@link ClientSession} is used for all invocations of {@code execute} on the instance.
+ *
+ * @author Christoph Strobl
+ * @since 2.1
+ */
+public interface SessionScoped {
+
+	/**
+	 * Executes the given {@link SessionCallback} within the {@link com.mongodb.session.ClientSession}
+	 * <p/>
+	 * It is up to the caller to make sure the {@link com.mongodb.session.ClientSession} is {@link ClientSession#close()
+	 * closed} when done.
+	 *
+	 * @param action callback object that specifies the MongoDB action the callback action. Must not be {@literal null}.
+	 * @param <T> return type.
+	 * @return a result object returned by the action. Can be {@literal null}.
+	 */
+	@Nullable
+	default <T> T execute(SessionCallback<T> action) {
+		return execute(action, session -> {});
+	}
+
+	/**
+	 * Executes the given {@link SessionCallback} within the {@link com.mongodb.session.ClientSession}
+	 * <p/>
+	 * It is up to the caller to make sure the {@link com.mongodb.session.ClientSession} is {@link ClientSession#close()
+	 * closed} when done.
+	 *
+	 * @param action callback object that specifies the MongoDB action the callback action. Must not be {@literal null}.
+	 * @param doFinally
+	 * @param <T> return type.
+	 * @return a result object returned by the action. Can be {@literal null}.
+	 */
+	@Nullable
+	<T> T execute(SessionCallback<T> action, Consumer<ClientSession> doFinally);
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/SimpleMongoDbFactory.java

@@ -17,18 +17,23 @@ package org.springframework.data.mongodb.core;
 
 import java.net.UnknownHostException;
 
+import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.dao.DataAccessException;
 import org.springframework.dao.support.PersistenceExceptionTranslator;
 import org.springframework.data.mongodb.MongoDbFactory;
+import org.springframework.data.mongodb.SessionAwareMethodInterceptor;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
+import com.mongodb.ClientSessionOptions;
 import com.mongodb.DB;
 import com.mongodb.MongoClient;
 import com.mongodb.MongoClientURI;
 import com.mongodb.WriteConcern;
+import com.mongodb.client.MongoCollection;
 import com.mongodb.client.MongoDatabase;
+import com.mongodb.session.ClientSession;
 
 /**
  * Factory to create {@link DB} instances from a {@link MongoClient} instance.
@@ -147,4 +152,120 @@ public class SimpleMongoDbFactory implements DisposableBean, MongoDbFactory {
 	public DB getLegacyDb() {
 		return mongoClient.getDB(databaseName);
 	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.MongoDbFactory#getSession(com.mongodb.ClientSessionOptions)
+	 */
+	@Override
+	public ClientSession getSession(ClientSessionOptions options) {
+		return mongoClient.startSession(options);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.MongoDbFactory#withSession(com.mongodb.session.ClientSession)
+	 */
+	@Override
+	public MongoDbFactory withSession(ClientSession session) {
+		return new ClientSessionBoundMongoDbFactory(session, this);
+	}
+
+	/**
+	 * {@link ClientSession} bound {@link MongoDbFactory} decorating the database with a
+	 * {@link SessionAwareMethodInterceptor}.
+	 * 
+	 * @author Christoph Strobl
+	 * @since 2.1
+	 */
+	static class ClientSessionBoundMongoDbFactory implements MongoDbFactory {
+
+		private final ClientSession session;
+		private final MongoDbFactory delegate;
+
+		ClientSessionBoundMongoDbFactory(ClientSession session, MongoDbFactory delegate) {
+
+			this.session = session;
+			this.delegate = delegate;
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.MongoDbFactory#getDb()
+		 */
+		@Override
+		public MongoDatabase getDb() throws DataAccessException {
+			return proxyMongoDatabase(delegate.getDb());
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.MongoDbFactory#getDb(java.lang.String)
+		 */
+		@Override
+		public MongoDatabase getDb(String dbName) throws DataAccessException {
+			return proxyMongoDatabase(delegate.getDb(dbName));
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.MongoDbFactory#getExceptionTranslator()
+		 */
+		@Override
+		public PersistenceExceptionTranslator getExceptionTranslator() {
+			return delegate.getExceptionTranslator();
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.MongoDbFactory#getLegacyDb()
+		 */
+		@Override
+		public DB getLegacyDb() {
+			return delegate.getLegacyDb();
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.MongoDbFactory#getSession(com.mongodb.ClientSessionOptions)
+		 */
+		@Override
+		public ClientSession getSession(ClientSessionOptions options) {
+			return delegate.getSession(options);
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.MongoDbFactory#withSession(com.mongodb.session.ClientSession)
+		 */
+		@Override
+		public MongoDbFactory withSession(ClientSession session) {
+			return delegate.withSession(session);
+		}
+
+		private MongoDatabase proxyMongoDatabase(MongoDatabase database) {
+			return createProxyInstance(session, database, MongoDatabase.class);
+		}
+
+		private MongoDatabase proxyDatabase(ClientSession session, MongoDatabase database) {
+			return createProxyInstance(session, database, MongoDatabase.class);
+		}
+
+		private MongoCollection proxyCollection(ClientSession session, MongoCollection collection) {
+			return createProxyInstance(session, collection, MongoCollection.class);
+		}
+
+		private <T> T createProxyInstance(ClientSession session, T target, Class<T> targetType) {
+
+			ProxyFactory factory = new ProxyFactory();
+			factory.setTarget(target);
+			factory.setInterfaces(targetType);
+			factory.setOpaque(true);
+
+			factory.addAdvice(new SessionAwareMethodInterceptor<>(session, target, MongoDatabase.class, this::proxyDatabase,
+					MongoCollection.class, this::proxyCollection));
+
+			return (T) factory.getProxy();
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/SimpleReactiveMongoDatabaseFactory.java

@@ -15,20 +15,27 @@
  */
 package org.springframework.data.mongodb.core;
 
+import reactor.core.publisher.Mono;
+
 import java.net.UnknownHostException;
 
+import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.dao.DataAccessException;
 import org.springframework.dao.support.PersistenceExceptionTranslator;
 import org.springframework.data.mongodb.ReactiveMongoDatabaseFactory;
+import org.springframework.data.mongodb.SessionAwareMethodInterceptor;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
+import com.mongodb.ClientSessionOptions;
 import com.mongodb.ConnectionString;
 import com.mongodb.WriteConcern;
 import com.mongodb.reactivestreams.client.MongoClient;
 import com.mongodb.reactivestreams.client.MongoClients;
+import com.mongodb.reactivestreams.client.MongoCollection;
 import com.mongodb.reactivestreams.client.MongoDatabase;
+import com.mongodb.session.ClientSession;
 
 /**
  * Factory to create {@link MongoDatabase} instances from a {@link MongoClient} instance.
@@ -129,4 +136,111 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 	public PersistenceExceptionTranslator getExceptionTranslator() {
 		return this.exceptionTranslator;
 	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.ReactiveMongoDbFactory#getSession(com.mongodb.ClientSessionOptions)
+	 */
+	@Override
+	public Mono<ClientSession> getSession(ClientSessionOptions options) {
+		return Mono.from(mongo.startSession(options));
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.ReactiveMongoDbFactory#withSession(com.mongodb.session.ClientSession)
+	 */
+	@Override
+	public ReactiveMongoDatabaseFactory withSession(ClientSession session) {
+		return new ClientSessionBoundMongoDbFactory(session, this);
+	}
+
+	/**
+	 * {@link ClientSession} bound {@link ReactiveMongoDatabaseFactory} decorating the database with a
+	 * {@link SessionAwareMethodInterceptor}.
+	 *
+	 * @author Christoph Strobl
+	 * @since 2.1
+	 */
+	static class ClientSessionBoundMongoDbFactory implements ReactiveMongoDatabaseFactory {
+
+		private final ClientSession session;
+		private final ReactiveMongoDatabaseFactory delegate;
+
+		ClientSessionBoundMongoDbFactory(ClientSession session, ReactiveMongoDatabaseFactory delegate) {
+
+			this.session = session;
+			this.delegate = delegate;
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getMongoDatabase()
+		 */
+		@Override
+		public MongoDatabase getMongoDatabase() throws DataAccessException {
+			return decorateDatabase(delegate.getMongoDatabase());
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getMongoDatabase(java.lang.String)
+		 */
+		@Override
+		public MongoDatabase getMongoDatabase(String dbName) throws DataAccessException {
+			return decorateDatabase(delegate.getMongoDatabase(dbName));
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getExceptionTranslator()
+		 */
+		@Override
+		public PersistenceExceptionTranslator getExceptionTranslator() {
+			return delegate.getExceptionTranslator();
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getSession(com.mongodb.ClientSessionOptions)
+		 */
+		@Override
+		public Mono<ClientSession> getSession(ClientSessionOptions options) {
+			return delegate.getSession(options);
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#withSession(com.mongodb.session.ClientSession)
+		 */
+		@Override
+		public ReactiveMongoDatabaseFactory withSession(ClientSession session) {
+			return delegate.withSession(session);
+		}
+
+		private MongoDatabase decorateDatabase(MongoDatabase database) {
+			return createProxyInstance(session, database, MongoDatabase.class);
+		}
+
+		private MongoDatabase proxyDatabase(ClientSession session, MongoDatabase database) {
+			return createProxyInstance(session, database, MongoDatabase.class);
+		}
+
+		private MongoCollection proxyCollection(ClientSession session, MongoCollection collection) {
+			return createProxyInstance(session, collection, MongoCollection.class);
+		}
+
+		private <T> T createProxyInstance(ClientSession session, T target, Class<T> targetType) {
+
+			ProxyFactory factory = new ProxyFactory();
+			factory.setTarget(target);
+			factory.setInterfaces(targetType);
+			factory.setOpaque(true);
+
+			factory.addAdvice(new SessionAwareMethodInterceptor<>(session, target, MongoDatabase.class, this::proxyDatabase,
+					MongoCollection.class, this::proxyCollection));
+
+			return (T) factory.getProxy();
+		}
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DefaultDbRefResolver.java

@@ -40,6 +40,7 @@ import org.springframework.cglib.proxy.MethodProxy;
 import org.springframework.dao.DataAccessException;
 import org.springframework.dao.InvalidDataAccessApiUsageException;
 import org.springframework.dao.support.PersistenceExceptionTranslator;
+import org.springframework.data.mongodb.ClientSessionException;
 import org.springframework.data.mongodb.LazyLoadingException;
 import org.springframework.data.mongodb.MongoDbFactory;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
@@ -51,7 +52,7 @@ import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
 
 import com.mongodb.DBRef;
-import com.mongodb.client.MongoDatabase;
+import com.mongodb.client.MongoCollection;
 import com.mongodb.client.model.Filters;
 
 /**
@@ -84,6 +85,18 @@ public class DefaultDbRefResolver implements DbRefResolver {
 		this.objenesis = new ObjenesisStd(true);
 	}
 
+	/**
+	 * Creates a new {@link DefaultDbRefResolver} with the given {@link MongoDbFactory}.
+	 *
+	 * @param mongoDbFactory must not be {@literal null}.
+	 */
+	private DefaultDbRefResolver(DefaultDbRefResolver delegate) {
+
+		this.mongoDbFactory = delegate.mongoDbFactory;
+		this.exceptionTranslator = delegate.exceptionTranslator;
+		this.objenesis = delegate.objenesis;
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.convert.DbRefResolver#resolveDbRef(org.springframework.data.mongodb.core.mapping.MongoPersistentProperty, org.springframework.data.mongodb.core.convert.DbRefResolverCallback)
@@ -126,9 +139,7 @@ public class DefaultDbRefResolver implements DbRefResolver {
 	public Document fetch(DBRef dbRef) {
 
 		StringUtils.hasText(dbRef.getDatabaseName());
-		return (StringUtils.hasText(dbRef.getDatabaseName()) ? mongoDbFactory.getDb(dbRef.getDatabaseName())
+		return getCollection(dbRef).find(Filters.eq("_id", dbRef.getId())).first();
-				: mongoDbFactory.getDb()).getCollection(dbRef.getCollectionName(), Document.class)
-						.find(Filters.eq("_id", dbRef.getId())).first();
 	}
 
 	/*
@@ -158,9 +169,7 @@ public class DefaultDbRefResolver implements DbRefResolver {
 			ids.add(ref.getId());
 		}
 
-		MongoDatabase db = mongoDbFactory.getDb();
+		List<Document> result = getCollection(refs.iterator().next()) //
-
-		List<Document> result = db.getCollection(collection) //
 				.find(new Document("_id", new Document("$in", ids))) //
 				.into(new ArrayList<>());
 
@@ -466,6 +475,11 @@ public class DefaultDbRefResolver implements DbRefResolver {
 				} catch (RuntimeException ex) {
 
 					DataAccessException translatedException = this.exceptionTranslator.translateExceptionIfPossible(ex);
+
+					if (translatedException instanceof ClientSessionException) {
+						throw new LazyLoadingException("Unable to lazily resolve DBRef! Invalid session state.", ex);
+					}
+
 					throw new LazyLoadingException("Unable to lazily resolve DBRef!",
 							translatedException != null ? translatedException : ex);
 				}
@@ -474,4 +488,17 @@ public class DefaultDbRefResolver implements DbRefResolver {
 			return result;
 		}
 	}
+
+	/**
+	 * Customization hook for obtaining the {@link MongoCollection} for a given {@link DBRef}.
+	 *
+	 * @param dbref must not be {@literal null}.
+	 * @return the {@link MongoCollection} the given {@link DBRef} points to.
+	 * @since 2.1
+	 */
+	protected MongoCollection<Document> getCollection(DBRef dbref) {
+
+		return (StringUtils.hasText(dbref.getDatabaseName()) ? mongoDbFactory.getDb(dbref.getDatabaseName())
+				: mongoDbFactory.getDb()).getCollection(dbref.getCollectionName(), Document.class);
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java

@@ -147,8 +147,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 	 */
 	public void setTypeMapper(@Nullable MongoTypeMapper typeMapper) {
 		this.typeMapper = typeMapper == null
-				? new DefaultMongoTypeMapper(DefaultMongoTypeMapper.DEFAULT_TYPE_KEY, mappingContext)
+				? new DefaultMongoTypeMapper(DefaultMongoTypeMapper.DEFAULT_TYPE_KEY, mappingContext) : typeMapper;
-				: typeMapper;
 	}
 
 	/*
@@ -557,8 +556,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		}
 
 		MongoPersistentEntity<?> entity = isSubtype(prop.getType(), obj.getClass())
-				? mappingContext.getRequiredPersistentEntity(obj.getClass())
+				? mappingContext.getRequiredPersistentEntity(obj.getClass()) : mappingContext.getRequiredPersistentEntity(type);
-				: mappingContext.getRequiredPersistentEntity(type);
 
 		Object existingValue = accessor.get(prop);
 		Document document = existingValue instanceof Document ? (Document) existingValue : new Document();
@@ -777,8 +775,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		}
 
 		return conversions.hasCustomWriteTarget(key.getClass(), String.class)
-				? (String) getPotentiallyConvertedSimpleWrite(key)
+				? (String) getPotentiallyConvertedSimpleWrite(key) : key.toString();
-				: key.toString();
 	}
 
 	/**
@@ -1438,8 +1435,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		}
 
 		List<Document> referencedRawDocuments = dbrefs.size() == 1
-				? Collections.singletonList(readRef(dbrefs.iterator().next()))
+				? Collections.singletonList(readRef(dbrefs.iterator().next())) : bulkReadRefs(dbrefs);
-				: bulkReadRefs(dbrefs);
 		String collectionName = dbrefs.iterator().next().getCollectionName();
 
 		List<T> targeList = new ArrayList<>(dbrefs.size());

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MongoConverter.java

@@ -69,7 +69,6 @@ public interface MongoConverter
 		Assert.notNull(targetType, "TargetType must not be null!");
 		Assert.notNull(dbRefResolver, "DbRefResolver must not be null!");
 
-
 		if (targetType != Object.class && ClassUtils.isAssignable(targetType, source.getClass())) {
 			return (T) source;
 		}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/SessionAwareMethodInterceptorUnitTests.java

@@ -0,0 +1,182 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb;
+
+import static org.assertj.core.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.*;
+import static org.mockito.Mockito.any;
+
+import java.lang.reflect.Method;
+import java.lang.reflect.Proxy;
+
+import org.bson.Document;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+import org.springframework.aop.framework.ProxyFactory;
+import org.springframework.data.mongodb.SessionAwareMethodInterceptor.MethodCache;
+import org.springframework.test.util.ReflectionTestUtils;
+import org.springframework.util.ClassUtils;
+
+import com.mongodb.MongoClient;
+import com.mongodb.client.MongoCollection;
+import com.mongodb.client.MongoDatabase;
+import com.mongodb.session.ClientSession;
+
+/**
+ * @author Christoph Strobl
+ */
+@RunWith(MockitoJUnitRunner.class)
+public class SessionAwareMethodInterceptorUnitTests {
+
+	@Mock ClientSession session;
+	@Mock MongoCollection<Document> targetCollection;
+	@Mock MongoDatabase targetDatabase;
+
+	MongoCollection collection;
+	MongoDatabase database;
+
+	@Before
+	public void setUp() {
+
+		collection = createProxyInstance(session, targetCollection, MongoCollection.class);
+		database = createProxyInstance(session, targetDatabase, MongoDatabase.class);
+	}
+
+	@Test // DATAMONGO-1880
+	public void proxyFactoryOnCollectionDelegatesToMethodWithSession() {
+
+		collection.find();
+
+		verify(targetCollection).find(eq(session));
+	}
+
+	@Test // DATAMONGO-1880
+	public void proxyFactoryOnCollectionWithSessionInArgumentListProceedsWithExecution() {
+
+		ClientSession yetAnotherSession = mock(ClientSession.class);
+		collection.find(yetAnotherSession);
+
+		verify(targetCollection).find(eq(yetAnotherSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void proxyFactoryOnDatabaseDelegatesToMethodWithSession() {
+
+		database.drop();
+
+		verify(targetDatabase).drop(eq(session));
+	}
+
+	@Test // DATAMONGO-1880
+	public void proxyFactoryOnDatabaseWithSessionInArgumentListProceedsWithExecution() {
+
+		ClientSession yetAnotherSession = mock(ClientSession.class);
+		database.drop(yetAnotherSession);
+
+		verify(targetDatabase).drop(eq(yetAnotherSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void justMoveOnIfNoOverloadWithSessionAvailable() {
+
+		collection.getReadPreference();
+
+		verify(targetCollection).getReadPreference();
+	}
+
+	@Test // DATAMONGO-1880
+	public void usesCacheForMethodLookup() {
+
+		MethodCache cache = (MethodCache) ReflectionTestUtils.getField(SessionAwareMethodInterceptor.class, "METHOD_CACHE");
+		Method countMethod = ClassUtils.getMethod(MongoCollection.class, "count");
+
+		assertThat(cache.contains(countMethod, MongoCollection.class)).isFalse();
+
+		collection.count();
+
+		assertThat(cache.contains(countMethod, MongoCollection.class)).isTrue();
+	}
+
+	@Test // DATAMONGO-1880
+	public void cachesNullForMethodsThatDoNotHaveASessionOverload() {
+
+		MethodCache cache = (MethodCache) ReflectionTestUtils.getField(SessionAwareMethodInterceptor.class, "METHOD_CACHE");
+		Method readConcernMethod = ClassUtils.getMethod(MongoCollection.class, "getReadConcern");
+
+		assertThat(cache.contains(readConcernMethod, MongoCollection.class)).isFalse();
+
+		collection.getReadConcern();
+
+		collection.getReadConcern();
+
+		assertThat(cache.contains(readConcernMethod, MongoCollection.class)).isTrue();
+		assertThat(cache.lookup(readConcernMethod, MongoCollection.class)).isEmpty();
+	}
+
+	@Test // DATAMONGO-1880
+	public void proxiesNewDbInstanceReturnedByMethdod() {
+
+		MongoDatabase otherDb = mock(MongoDatabase.class);
+		when(targetDatabase.withCodecRegistry(any())).thenReturn(otherDb);
+
+		MongoDatabase target = database.withCodecRegistry(MongoClient.getDefaultCodecRegistry());
+		assertThat(target).isInstanceOf(Proxy.class).isNotSameAs(database).isNotSameAs(targetDatabase);
+
+		target.drop();
+
+		verify(otherDb).drop(eq(session));
+	}
+
+	@Test // DATAMONGO-1880
+	public void proxiesNewCollectionInstanceReturnedByMethdod() {
+
+		MongoCollection otherCollection = mock(MongoCollection.class);
+		when(targetCollection.withCodecRegistry(any())).thenReturn(otherCollection);
+
+		MongoCollection target = collection.withCodecRegistry(MongoClient.getDefaultCodecRegistry());
+		assertThat(target).isInstanceOf(Proxy.class).isNotSameAs(collection).isNotSameAs(targetCollection);
+
+		target.drop();
+
+		verify(otherCollection).drop(eq(session));
+	}
+
+	private MongoDatabase proxyDatabase(ClientSession session, MongoDatabase database) {
+		return createProxyInstance(session, database, MongoDatabase.class);
+	}
+
+	private MongoCollection proxyCollection(ClientSession session, MongoCollection collection) {
+		return createProxyInstance(session, collection, MongoCollection.class);
+	}
+
+	private <T> T createProxyInstance(ClientSession session, T target, Class<T> targetType) {
+
+		ProxyFactory factory = new ProxyFactory();
+		factory.setTarget(target);
+		factory.setInterfaces(targetType);
+		factory.setOpaque(true);
+
+		factory.addAdvice(new SessionAwareMethodInterceptor<>(session, target, MongoDatabase.class, this::proxyDatabase,
+				MongoCollection.class, this::proxyCollection));
+
+		return (T) factory.getProxy();
+	}
+
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/ClientSessionTests.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.bson.Document;
+import org.junit.Before;
+import org.junit.ClassRule;
+import org.junit.Test;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.test.util.MongoVersionRule;
+import org.springframework.data.util.Version;
+
+import com.mongodb.ClientSessionOptions;
+import com.mongodb.MongoClient;
+import com.mongodb.session.ClientSession;
+
+/**
+ * @author Christoph Strobl
+ */
+public class ClientSessionTests {
+
+	public static @ClassRule MongoVersionRule REQUIRES_AT_LEAST_3_6_0 = MongoVersionRule.atLeast(Version.parse("3.6.0"));
+
+	MongoTemplate template;
+	MongoClient client;
+
+	@Before
+	public void setUp() {
+
+		client = new MongoClient();
+		template = new MongoTemplate(client, "reflective-client-session-tests");
+		template.getDb().getCollection("test").drop();
+
+		template.getDb().getCollection("test").insertOne(new Document("_id", "id-1").append("value", "spring"));
+	}
+
+	@Test // DATAMONGO-1880
+	public void shouldApplyClientSession() {
+
+		ClientSession session = client.startSession(ClientSessionOptions.builder().causallyConsistent(true).build());
+
+		assertThat(session.getOperationTime()).isNull();
+
+		Document doc = template.withSession(() -> session)
+				.execute(action -> action.findOne(new Query(), Document.class, "test"));
+
+		assertThat(doc).isNotNull();
+		assertThat(session.getOperationTime()).isNotNull();
+		assertThat(session.getServerSession().isClosed()).isFalse();
+
+		session.close();
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/DefaultBulkOperationsIntegrationTests.java

@@ -23,11 +23,14 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.Optional;
 
+import com.mongodb.BulkWriteException;
 import org.bson.Document;
+import org.hamcrest.core.IsInstanceOf;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.dao.DuplicateKeyException;
 import org.springframework.data.mongodb.core.BulkOperations.BulkMode;
 import org.springframework.data.mongodb.core.DefaultBulkOperations.BulkOperationContext;
 import org.springframework.data.mongodb.core.convert.QueryMapper;
@@ -102,10 +105,12 @@ public class DefaultBulkOperationsIntegrationTests {
 		try {
 			createBulkOps(BulkMode.ORDERED).insert(documents).execute();
 			fail();
-		} catch (MongoBulkWriteException e) {
+		} catch (DuplicateKeyException e) {
-			assertThat(e.getWriteResult().getInsertedCount(), is(1)); // fails after first error
+
-			assertThat(e.getWriteErrors(), notNullValue());
+			assertThat(e.getCause(), IsInstanceOf.instanceOf(MongoBulkWriteException.class));
-			assertThat(e.getWriteErrors().size(), is(1));
+			assertThat(((MongoBulkWriteException)e.getCause()).getWriteResult().getInsertedCount(), is(1)); // fails after first error
+			assertThat(((MongoBulkWriteException)e.getCause()).getWriteErrors(), notNullValue());
+			assertThat(((MongoBulkWriteException)e.getCause()).getWriteErrors().size(), is(1));
 		}
 	}
 
@@ -125,10 +130,12 @@ public class DefaultBulkOperationsIntegrationTests {
 		try {
 			createBulkOps(BulkMode.UNORDERED).insert(documents).execute();
 			fail();
-		} catch (MongoBulkWriteException e) {
+		} catch (DuplicateKeyException e) {
-			assertThat(e.getWriteResult().getInsertedCount(), is(2)); // two docs were inserted
+
-			assertThat(e.getWriteErrors(), notNullValue());
+			assertThat(e.getCause(), IsInstanceOf.instanceOf(MongoBulkWriteException.class));
-			assertThat(e.getWriteErrors().size(), is(1));
+			assertThat(((MongoBulkWriteException)e.getCause()).getWriteResult().getInsertedCount(), is(2)); // two docs were inserted
+			assertThat(((MongoBulkWriteException)e.getCause()).getWriteErrors(), notNullValue());
+			assertThat(((MongoBulkWriteException)e.getCause()).getWriteErrors().size(), is(1));
 		}
 	}
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/DefaultBulkOperationsUnitTests.java

@@ -19,6 +19,7 @@ import static org.assertj.core.api.Assertions.*;
 import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.Mockito.*;
 import static org.mockito.Mockito.any;
+import static org.mockito.Mockito.eq;
 import static org.springframework.data.mongodb.core.query.Criteria.*;
 import static org.springframework.data.mongodb.core.query.Query.*;
 
@@ -34,6 +35,7 @@ import org.mockito.Captor;
 import org.mockito.Mock;
 import org.mockito.junit.MockitoJUnitRunner;
 import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.MongoDbFactory;
 import org.springframework.data.mongodb.core.BulkOperations.BulkMode;
 import org.springframework.data.mongodb.core.DefaultBulkOperations.BulkOperationContext;
 import org.springframework.data.mongodb.core.convert.DbRefResolver;
@@ -48,6 +50,7 @@ import org.springframework.data.mongodb.core.query.Collation;
 import org.springframework.data.mongodb.core.query.Update;
 
 import com.mongodb.client.MongoCollection;
+import com.mongodb.client.MongoDatabase;
 import com.mongodb.client.model.DeleteManyModel;
 import com.mongodb.client.model.UpdateManyModel;
 import com.mongodb.client.model.UpdateOneModel;
@@ -62,8 +65,10 @@ import com.mongodb.client.model.WriteModel;
 @RunWith(MockitoJUnitRunner.class)
 public class DefaultBulkOperationsUnitTests {
 
-	@Mock MongoTemplate template;
+	MongoTemplate template;
+	@Mock MongoDatabase database;
 	@Mock MongoCollection<Document> collection;
+	@Mock MongoDbFactory factory;
 	@Mock DbRefResolver dbRefResolver;
 	@Captor ArgumentCaptor<List<WriteModel<Document>>> captor;
 	MongoConverter converter;
@@ -79,7 +84,10 @@ public class DefaultBulkOperationsUnitTests {
 
 		converter = new MappingMongoConverter(dbRefResolver, mappingContext);
 
-		when(template.getCollection(anyString())).thenReturn(collection);
+		template = new MongoTemplate(factory, converter);
+
+		when(factory.getDb()).thenReturn(database);
+		when(database.getCollection(anyString(), eq(Document.class))).thenReturn(collection);
 
 		ops = new DefaultBulkOperations(template, "collection-1",
 				new BulkOperationContext(BulkMode.ORDERED,

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/MongoExceptionTranslatorUnitTests.java

@@ -137,14 +137,13 @@ public class MongoExceptionTranslatorUnitTests {
 
 	private void checkTranslatedMongoException(Class<? extends Exception> clazz, int code) {
 
-		try {
+		DataAccessException translated = translator.translateExceptionIfPossible(new MongoException(code, ""));
-			translator.translateExceptionIfPossible(new MongoException(code, ""));
+
-			fail("Expected exception of type " + clazz.getName() + "!");
+		assertThat("Expected exception of type " + clazz.getName() + "!", translated, is(not(nullValue())));
-		} catch (NestedRuntimeException e) {
+
-			Throwable cause = e.getRootCause();
+		Throwable cause = translated.getRootCause();
-			assertThat(cause, is(instanceOf(MongoException.class)));
+		assertThat(cause, is(instanceOf(MongoException.class)));
-			assertThat(((MongoException) cause).getCode(), is(code));
+		assertThat(((MongoException) cause).getCode(), is(code));
-		}
 	}
 
 	private static void expectExceptionWithCauseMessage(NestedRuntimeException e,

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/MongoTemplateUnitTests.java

@@ -129,8 +129,8 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 		when(factory.getExceptionTranslator()).thenReturn(exceptionTranslator);
 		when(db.getCollection(Mockito.any(String.class), eq(Document.class))).thenReturn(collection);
 		when(db.runCommand(Mockito.any(), Mockito.any(Class.class))).thenReturn(commandResultDocument);
-		when(collection.find(Mockito.any(org.bson.Document.class))).thenReturn(findIterable);
+		when(collection.find(Mockito.any(org.bson.Document.class), any(Class.class))).thenReturn(findIterable);
-		when(collection.mapReduce(Mockito.any(), Mockito.any())).thenReturn(mapReduceIterable);
+		when(collection.mapReduce(Mockito.any(), Mockito.any(), eq(Document.class))).thenReturn(mapReduceIterable);
 		when(collection.count(any(Bson.class), any(CountOptions.class))).thenReturn(1L);
 		when(collection.aggregate(any(List.class), any())).thenReturn(aggregateIterable);
 		when(collection.withReadPreference(any())).thenReturn(collection);
@@ -324,7 +324,7 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 
 		BasicQuery query = new BasicQuery("{'foo':'bar'}");
 		template.findAllAndRemove(query, VersionedEntity.class);
-		verify(collection, times(1)).find(Matchers.eq(query.getQueryObject()));
+		verify(collection, times(1)).find(Mockito.eq(query.getQueryObject()), Mockito.any(Class.class));
 	}
 
 	@Test // DATAMONGO-566
@@ -445,7 +445,7 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 		when(output.iterator()).thenReturn(cursor);
 		when(cursor.hasNext()).thenReturn(false);
 
-		when(collection.mapReduce(anyString(), anyString())).thenReturn(output);
+		when(collection.mapReduce(anyString(), anyString(), eq(Document.class))).thenReturn(output);
 
 		Query query = new BasicQuery("{'foo':'bar'}");
 		query.limit(100);
@@ -466,7 +466,7 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 		when(output.iterator()).thenReturn(cursor);
 		when(cursor.hasNext()).thenReturn(false);
 
-		when(collection.mapReduce(anyString(), anyString())).thenReturn(output);
+		when(collection.mapReduce(anyString(), anyString(), eq(Document.class))).thenReturn(output);
 
 		Query query = new BasicQuery("{'foo':'bar'}");
 
@@ -487,7 +487,7 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 		when(output.iterator()).thenReturn(cursor);
 		when(cursor.hasNext()).thenReturn(false);
 
-		when(collection.mapReduce(anyString(), anyString())).thenReturn(output);
+		when(collection.mapReduce(anyString(), anyString(), eq(Document.class))).thenReturn(output);
 
 		template.mapReduce("collection", "function(){}", "function(key,values){}", new MapReduceOptions().limit(1000),
 				Wrapper.class);
@@ -506,7 +506,7 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 		when(output.iterator()).thenReturn(cursor);
 		when(cursor.hasNext()).thenReturn(false);
 
-		when(collection.mapReduce(anyString(), anyString())).thenReturn(output);
+		when(collection.mapReduce(anyString(), anyString(), eq(Document.class))).thenReturn(output);
 
 		Query query = new BasicQuery("{'foo':'bar'}");
 		query.limit(100);
@@ -810,6 +810,18 @@ public class MongoTemplateUnitTests extends MongoOperationsUnitTests {
 				equalTo(new Document("locale", "fr")));
 	}
 
+	@Test // DATAMONGO-1880
+	public void countShouldUseCollationWhenPresent() {
+
+		template.count(new BasicQuery("{}").collation(Collation.of("fr")), AutogenerateableId.class);
+
+		ArgumentCaptor<CountOptions> options = ArgumentCaptor.forClass(CountOptions.class);
+		verify(collection).count(any(), options.capture());
+
+		assertThat(options.getValue().getCollation(),
+				is(equalTo(com.mongodb.client.model.Collation.builder().locale("fr").build())));
+	}
+
 	@Test // DATAMONGO-1733
 	public void appliesFieldsWhenInterfaceProjectionIsClosedAndQueryDoesNotDefineFields() {
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/ReactiveClientSessionTests.java

@@ -0,0 +1,134 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import static org.assertj.core.api.Assertions.*;
+
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Supplier;
+
+import org.bson.Document;
+import org.junit.Before;
+import org.junit.ClassRule;
+import org.junit.Test;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.test.util.MongoVersionRule;
+import org.springframework.data.util.Version;
+
+import com.mongodb.ClientSessionOptions;
+import com.mongodb.reactivestreams.client.MongoClient;
+import com.mongodb.reactivestreams.client.MongoClients;
+import com.mongodb.session.ClientSession;
+
+/**
+ * @author Christoph Strobl
+ */
+public class ReactiveClientSessionTests {
+
+	public static @ClassRule MongoVersionRule REQUIRES_AT_LEAST_3_6_0 = MongoVersionRule.atLeast(Version.parse("3.6.0"));
+
+	MongoClient client;
+	ReactiveMongoTemplate template;
+
+	@Before
+	public void setUp() {
+
+		client = MongoClients.create();
+
+		template = new ReactiveMongoTemplate(client, "reflective-client-session-tests");
+
+		StepVerifier.create(template.dropCollection("test")).verifyComplete();
+
+		StepVerifier.create(template.insert(new Document("_id", "id-1").append("value", "spring"), "test"))
+				.expectNextCount(1).verifyComplete();
+	}
+
+	@Test // DATAMONGO-1880
+	public void shouldApplyClientSession() {
+
+		ClientSession session = Mono
+				.from(client.startSession(ClientSessionOptions.builder().causallyConsistent(true).build())).block();
+
+		assertThat(session.getOperationTime()).isNull();
+
+		StepVerifier.create(template.withSession(() -> session).execute(action -> action.findAll(Document.class, "test")))
+				.expectNextCount(1).verifyComplete();
+
+		assertThat(session.getOperationTime()).isNotNull();
+		assertThat(session.getServerSession().isClosed()).isFalse();
+
+		session.close();
+	}
+
+	@Test // DATAMONGO-1880
+	public void useMonoInCallback() {
+
+		ClientSession session = Mono
+				.from(client.startSession(ClientSessionOptions.builder().causallyConsistent(true).build())).block();
+
+		assertThat(session.getOperationTime()).isNull();
+
+		StepVerifier
+				.create(
+						template.withSession(() -> session).execute(action -> action.findOne(new Query(), Document.class, "test")))
+				.expectNextCount(1).verifyComplete();
+
+		assertThat(session.getOperationTime()).isNotNull();
+		assertThat(session.getServerSession().isClosed()).isFalse();
+
+		session.close();
+	}
+
+	@Test // DATAMONGO-1880
+	public void reusesClientSessionInSessionScopedCallback() {
+
+		ClientSession session = Mono
+				.from(client.startSession(ClientSessionOptions.builder().causallyConsistent(true).build())).block();
+		CountingSessionSupplier sessionSupplier = new CountingSessionSupplier(session);
+
+		ReactiveSessionScoped sessionScoped = template.withSession(sessionSupplier);
+
+		sessionScoped.execute(action -> action.findOne(new Query(), Document.class, "test")).blockFirst();
+		assertThat(sessionSupplier.getInvocationCount()).isEqualTo(1);
+
+		sessionScoped.execute(action -> action.findOne(new Query(), Document.class, "test")).blockFirst();
+		assertThat(sessionSupplier.getInvocationCount()).isEqualTo(1);
+	}
+
+	static class CountingSessionSupplier implements Supplier<ClientSession> {
+
+		AtomicInteger invocationCount = new AtomicInteger(0);
+		final ClientSession session;
+
+		public CountingSessionSupplier(ClientSession session) {
+			this.session = session;
+		}
+
+		@Override
+		public ClientSession get() {
+
+			invocationCount.incrementAndGet();
+			return session;
+		}
+
+		int getInvocationCount() {
+			return invocationCount.get();
+		}
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/ReactiveMongoTemplateUnitTests.java

@@ -88,8 +88,8 @@ public class ReactiveMongoTemplateUnitTests {
 		when(db.getCollection(any())).thenReturn(collection);
 		when(db.getCollection(any(), any())).thenReturn(collection);
 		when(db.runCommand(any(), any(Class.class))).thenReturn(runCommandPublisher);
-		when(collection.find()).thenReturn(findPublisher);
+		when(collection.find(any(Class.class))).thenReturn(findPublisher);
-		when(collection.find(Mockito.any(Document.class))).thenReturn(findPublisher);
+		when(collection.find(any(Document.class), any(Class.class))).thenReturn(findPublisher);
 		when(findPublisher.projection(any())).thenReturn(findPublisher);
 		when(findPublisher.limit(anyInt())).thenReturn(findPublisher);
 		when(findPublisher.collation(any())).thenReturn(findPublisher);

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/ReactiveSessionBoundMongoTemplateUnitTests.java

@@ -0,0 +1,319 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import static org.assertj.core.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.*;
+import static org.mockito.Mockito.any;
+import static org.mockito.Mockito.anyBoolean;
+import static org.mockito.Mockito.anyInt;
+import static org.mockito.Mockito.anyList;
+import static org.mockito.Mockito.anyString;
+
+import java.lang.reflect.Proxy;
+
+import com.mongodb.reactivestreams.client.MongoClient;
+import org.bson.Document;
+import org.bson.codecs.BsonValueCodec;
+import org.bson.codecs.configuration.CodecRegistry;
+import org.junit.Before;
+import org.junit.Ignore;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+import org.reactivestreams.Publisher;
+import org.springframework.data.geo.Metrics;
+import org.springframework.data.geo.Point;
+import org.springframework.data.mongodb.ReactiveMongoDatabaseFactory;
+import org.springframework.data.mongodb.core.ReactiveMongoTemplate.NoOpDbRefResolver;
+import org.springframework.data.mongodb.core.ReactiveMongoTemplate.ReactiveSessionBoundMongoTemplate;
+import org.springframework.data.mongodb.core.SimpleReactiveMongoDatabaseFactory.ClientSessionBoundMongoDbFactory;
+import org.springframework.data.mongodb.core.aggregation.Aggregation;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
+import org.springframework.data.mongodb.core.query.NearQuery;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
+
+import com.mongodb.client.model.CountOptions;
+import com.mongodb.client.model.DeleteOptions;
+import com.mongodb.client.model.FindOneAndUpdateOptions;
+import com.mongodb.client.model.UpdateOptions;
+import com.mongodb.reactivestreams.client.AggregatePublisher;
+import com.mongodb.reactivestreams.client.DistinctPublisher;
+import com.mongodb.reactivestreams.client.FindPublisher;
+import com.mongodb.reactivestreams.client.MongoCollection;
+import com.mongodb.reactivestreams.client.MongoDatabase;
+import com.mongodb.session.ClientSession;
+
+/**
+ * @author Christoph Strobl
+ */
+@RunWith(MockitoJUnitRunner.Silent.class)
+public class ReactiveSessionBoundMongoTemplateUnitTests {
+
+	private static final String COLLECTION_NAME = "collection-1";
+
+	ReactiveSessionBoundMongoTemplate template;
+	MongoMappingContext mappingContext;
+	MappingMongoConverter converter;
+
+	ReactiveMongoDatabaseFactory factory;
+
+	@Mock MongoCollection collection;
+	@Mock MongoDatabase database;
+	@Mock ClientSession clientSession;
+	@Mock FindPublisher findPublisher;
+	@Mock AggregatePublisher aggregatePublisher;
+	@Mock DistinctPublisher distinctPublisher;
+	@Mock Publisher resultPublisher;
+	@Mock MongoClient client;
+	@Mock CodecRegistry codecRegistry;
+
+	@Before
+	public void setUp() {
+
+		when(client.getDatabase(anyString())).thenReturn(database);
+		when(codecRegistry.get(any(Class.class))).thenReturn(new BsonValueCodec());
+		when(database.getCodecRegistry()).thenReturn(codecRegistry);
+		when(database.getCollection(anyString())).thenReturn(collection);
+		when(database.getCollection(anyString(), any())).thenReturn(collection);
+		when(database.listCollectionNames(any(ClientSession.class))).thenReturn(findPublisher);
+		when(database.createCollection(any(ClientSession.class), any(), any())).thenReturn(resultPublisher);
+		when(database.runCommand(any(ClientSession.class), any(), any(Class.class))).thenReturn(resultPublisher);
+		when(collection.find(any(ClientSession.class))).thenReturn(findPublisher);
+		when(collection.find(any(ClientSession.class), any(Document.class))).thenReturn(findPublisher);
+		when(collection.find(any(ClientSession.class), any(Class.class))).thenReturn(findPublisher);
+		when(collection.find(any(ClientSession.class), any(), any())).thenReturn(findPublisher);
+		when(collection.deleteMany(any(ClientSession.class), any(), any())).thenReturn(resultPublisher);
+		when(collection.insertOne(any(ClientSession.class), any(Document.class))).thenReturn(resultPublisher);
+		when(collection.aggregate(any(ClientSession.class), anyList(), any(Class.class))).thenReturn(aggregatePublisher);
+		when(collection.count(any(ClientSession.class), any(), any(CountOptions.class))).thenReturn(resultPublisher);
+		when(collection.drop(any(ClientSession.class))).thenReturn(resultPublisher);
+		when(collection.findOneAndUpdate(any(ClientSession.class), any(), any(), any())).thenReturn(resultPublisher);
+		when(collection.distinct(any(ClientSession.class), any(), any(), any())).thenReturn(distinctPublisher);
+		when(collection.updateOne(any(ClientSession.class), any(), any(), any(UpdateOptions.class)))
+				.thenReturn(resultPublisher);
+		when(collection.updateMany(any(ClientSession.class), any(), any(), any(UpdateOptions.class)))
+				.thenReturn(resultPublisher);
+		when(collection.dropIndex(any(ClientSession.class), anyString())).thenReturn(resultPublisher);
+		when(findPublisher.projection(any())).thenReturn(findPublisher);
+		when(findPublisher.limit(anyInt())).thenReturn(findPublisher);
+		when(findPublisher.collation(any())).thenReturn(findPublisher);
+		when(findPublisher.first()).thenReturn(resultPublisher);
+		when(aggregatePublisher.allowDiskUse(anyBoolean())).thenReturn(aggregatePublisher);
+		when(aggregatePublisher.useCursor(anyBoolean())).thenReturn(aggregatePublisher);
+
+		factory = new SimpleReactiveMongoDatabaseFactory(client, "foo");
+
+		this.mappingContext = new MongoMappingContext();
+		this.converter = new MappingMongoConverter(new NoOpDbRefResolver(), mappingContext);
+		this.template = new ReactiveSessionBoundMongoTemplate(clientSession, new ReactiveMongoTemplate(factory, converter));
+	}
+
+	@Test // DATAMONGO-1880
+	public void executeUsesProxiedCollectionInCallback() {
+
+		template.execute("collection", MongoCollection::find).subscribe();
+
+		verify(collection, never()).find();
+		verify(collection).find(eq(clientSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void executeUsesProxiedDatabaseInCallback() {
+
+		template.execute(MongoDatabase::listCollectionNames).subscribe();
+
+		verify(database, never()).listCollectionNames();
+		verify(database).listCollectionNames(eq(clientSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void findOneUsesProxiedCollection() {
+
+		template.findOne(new Query(), Person.class).subscribe();
+
+		verify(collection).find(eq(clientSession), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void findShouldUseProxiedCollection() {
+
+		template.find(new Query(), Person.class).subscribe();
+
+		verify(collection).find(eq(clientSession), any(Class.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void findAllShouldUseProxiedCollection() {
+
+		template.findAll(Person.class).subscribe();
+
+		verify(collection).find(eq(clientSession), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void executeCommandShouldUseProxiedDatabase() {
+
+		template.executeCommand("{}").subscribe();
+
+		verify(database).runCommand(eq(clientSession), any(), any(Class.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void removeShouldUseProxiedCollection() {
+
+		template.remove(new Query(), Person.class).subscribe();
+
+		verify(collection).deleteMany(eq(clientSession), any(), any(DeleteOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void insertShouldUseProxiedCollection() {
+
+		template.insert(new Person()).subscribe();
+
+		verify(collection).insertOne(eq(clientSession), any(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void aggregateShouldUseProxiedCollection() {
+
+		template.aggregate(Aggregation.newAggregation(Aggregation.project("foo")), COLLECTION_NAME, Person.class)
+				.subscribe();
+
+		verify(collection).aggregate(eq(clientSession), anyList(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void collectionExistsShouldUseProxiedDatabase() {
+
+		template.collectionExists(Person.class).subscribe();
+
+		verify(database).listCollectionNames(eq(clientSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void countShouldUseProxiedCollection() {
+
+		template.count(new Query(), Person.class).subscribe();
+
+		verify(collection).count(eq(clientSession), any(), any(CountOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void createCollectionShouldUseProxiedDatabase() {
+
+		template.createCollection(Person.class).subscribe();
+
+		verify(database).createCollection(eq(clientSession), anyString(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void dropShouldUseProxiedCollection() {
+
+		template.dropCollection(Person.class).subscribe();
+
+		verify(collection).drop(eq(clientSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void findAndModifyShouldUseProxiedCollection() {
+
+		template.findAndModify(new Query(), new Update().set("foo", "bar"), Person.class).subscribe();
+
+		verify(collection).findOneAndUpdate(eq(clientSession), any(), any(), any(FindOneAndUpdateOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void findDistinctShouldUseProxiedCollection() {
+
+		template.findDistinct(new Query(), "firstName", Person.class, String.class).subscribe();
+
+		verify(collection).distinct(eq(clientSession), anyString(), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void geoNearShouldUseProxiedDatabase() {
+
+		template.geoNear(NearQuery.near(new Point(0, 0), Metrics.NEUTRAL), Person.class).subscribe();
+
+		verify(database).runCommand(eq(clientSession), any(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880, DATAMONGO-1889
+	@Ignore("No group by yet - DATAMONGO-1889")
+	public void groupShouldUseProxiedDatabase() {
+
+		// template.group(COLLECTION_NAME, GroupBy.key("firstName"), Person.class).subscribe();
+
+		verify(database).runCommand(eq(clientSession), any(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880, DATAMONGO-1890
+	@Ignore("No map reduce yet on template - DATAMONGO-1890")
+	public void mapReduceShouldUseProxiedCollection() {
+
+		// template.mapReduce(COLLECTION_NAME, "foo", "bar", Person.class);
+
+		verify(collection).mapReduce(eq(clientSession), anyString(), anyString(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void updateFirstShouldUseProxiedCollection() {
+
+		template.updateFirst(new Query(), Update.update("foo", "bar"), Person.class).subscribe();
+
+		verify(collection).updateOne(eq(clientSession), any(), any(), any(UpdateOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void updateMultiShouldUseProxiedCollection() {
+
+		template.updateMulti(new Query(), Update.update("foo", "bar"), Person.class).subscribe();
+
+		verify(collection).updateMany(eq(clientSession), any(), any(), any(UpdateOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void upsertShouldUseProxiedCollection() {
+
+		template.upsert(new Query(), Update.update("foo", "bar"), Person.class).subscribe();
+
+		verify(collection).updateOne(eq(clientSession), any(), any(), any(UpdateOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void getCollectionShouldShouldJustReturnTheCollection/*No ClientSession binding*/() {
+		assertThat(template.getCollection(COLLECTION_NAME)).isNotInstanceOf(Proxy.class);
+	}
+
+	@Test // DATAMONGO-1880
+	public void getDbShouldJustReturnTheDatabase/*No ClientSession binding*/() {
+		assertThat(template.getMongoDatabase()).isNotInstanceOf(Proxy.class);
+	}
+
+	@Test // DATAMONGO-1880
+	public void indexOpsShouldUseProxiedCollection() {
+
+		template.indexOps(COLLECTION_NAME).dropIndex("index-name").subscribe();
+
+		verify(collection).dropIndex(eq(clientSession), eq("index-name"));
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/SessionBoundMongoTemplateTests.java

@@ -0,0 +1,298 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import static org.assertj.core.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.*;
+
+import lombok.Data;
+
+import java.lang.reflect.InvocationHandler;
+import java.lang.reflect.Proxy;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+import org.aopalliance.aop.Advice;
+import org.bson.Document;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.ClassRule;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.mockito.Mockito;
+import org.springframework.aop.Advisor;
+import org.springframework.aop.framework.Advised;
+import org.springframework.dao.DataAccessException;
+import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.ClientSessionException;
+import org.springframework.data.mongodb.LazyLoadingException;
+import org.springframework.data.mongodb.MongoDbFactory;
+import org.springframework.data.mongodb.SessionAwareMethodInterceptor;
+import org.springframework.data.mongodb.core.MongoTemplate.SessionBoundMongoTemplate;
+import org.springframework.data.mongodb.core.aggregation.Aggregation;
+import org.springframework.data.mongodb.core.convert.DbRefResolver;
+import org.springframework.data.mongodb.core.convert.DefaultDbRefResolver;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
+import org.springframework.data.mongodb.core.convert.MongoConverter;
+import org.springframework.data.mongodb.core.convert.MongoCustomConversions;
+import org.springframework.data.mongodb.core.mapping.DBRef;
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.test.util.MongoVersionRule;
+import org.springframework.data.util.Version;
+import org.springframework.test.util.ReflectionTestUtils;
+
+import com.mongodb.ClientSessionOptions;
+import com.mongodb.MongoClient;
+import com.mongodb.client.MongoCollection;
+import com.mongodb.client.MongoDatabase;
+import com.mongodb.session.ClientSession;
+
+/**
+ * Integration tests for {@link SessionBoundMongoTemplate} operating up an active {@link ClientSession}.
+ *
+ * @author Christoph Strobl
+ */
+public class SessionBoundMongoTemplateTests {
+
+	public static @ClassRule MongoVersionRule REQUIRES_AT_LEAST_3_6_0 = MongoVersionRule.atLeast(Version.parse("3.6.0"));
+
+	public @Rule ExpectedException exception = ExpectedException.none();
+
+	MongoTemplate template;
+	SessionBoundMongoTemplate sessionBoundTemplate;
+	ClientSession session;
+	volatile List<MongoCollection<Document>> spiedCollections = new ArrayList<>();
+	volatile List<MongoDatabase> spiedDatabases = new ArrayList<>();
+
+	@Before
+	public void setUp() {
+
+		MongoClient client = new MongoClient();
+
+		MongoDbFactory factory = new SimpleMongoDbFactory(client, "session-bound-mongo-template-tests") {
+
+			@Override
+			public MongoDatabase getDb() throws DataAccessException {
+
+				MongoDatabase spiedDatabse = Mockito.spy(super.getDb());
+				spiedDatabases.add(spiedDatabse);
+				return spiedDatabse;
+			}
+		};
+
+		session = client.startSession(ClientSessionOptions.builder().build());
+
+		this.template = new MongoTemplate(factory);
+
+		this.sessionBoundTemplate = new SessionBoundMongoTemplate(session,
+				new MongoTemplate(factory, getDefaultMongoConverter(factory))) {
+
+			@Override
+			protected MongoCollection<Document> prepareCollection(MongoCollection<Document> collection) {
+
+				InvocationHandler handler = Proxy.getInvocationHandler(collection);
+
+				Advised advised = (Advised) ReflectionTestUtils.getField(handler, "advised");
+
+				for (Advisor advisor : advised.getAdvisors()) {
+					Advice advice = advisor.getAdvice();
+					if (advice instanceof SessionAwareMethodInterceptor) {
+
+						MongoCollection<Document> spiedCollection = Mockito
+								.spy((MongoCollection<Document>) ReflectionTestUtils.getField(advice, "target"));
+						spiedCollections.add(spiedCollection);
+
+						ReflectionTestUtils.setField(advice, "target", spiedCollection);
+					}
+				}
+
+				return super.prepareCollection(collection);
+			}
+		};
+	}
+
+	@After
+	public void tearDown() {
+		session.close();
+	}
+
+	@Test // DATAMONGO-1880
+	public void findDelegatesToMethodWithSession() {
+
+		sessionBoundTemplate.find(new Query(), Person.class);
+
+		verify(operation(0)).find(eq(session), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void fluentFindDelegatesToMethodWithSession() {
+
+		sessionBoundTemplate.query(Person.class).all();
+
+		verify(operation(0)).find(eq(session), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void aggregateDelegatesToMethoddWithSession() {
+
+		sessionBoundTemplate.aggregate(Aggregation.newAggregation(Aggregation.project("firstName")), Person.class,
+				Person.class);
+
+		verify(operation(0)).aggregate(eq(session), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void collectionExistsDelegatesToMethodWithSession() {
+
+		sessionBoundTemplate.collectionExists(Person.class);
+
+		verify(command(0)).listCollectionNames(eq(session));
+	}
+
+	@Test // DATAMONGO-1880
+	public void shouldLoadDbRefWhenSessionIsActive() {
+
+		Person person = new Person("Kylar Stern");
+
+		template.save(person);
+
+		WithDbRef wdr = new WithDbRef();
+		wdr.id = "id-1";
+		wdr.personRef = person;
+
+		template.save(wdr);
+
+		WithDbRef result = sessionBoundTemplate.findById(wdr.id, WithDbRef.class);
+
+		assertThat(result.personRef).isEqualTo(person);
+	}
+
+	@Test // DATAMONGO-1880
+	public void shouldErrorOnLoadDbRefWhenSessionIsClosed() {
+
+		exception.expect(ClientSessionException.class);
+
+		Person person = new Person("Kylar Stern");
+
+		template.save(person);
+
+		WithDbRef wdr = new WithDbRef();
+		wdr.id = "id-1";
+		wdr.personRef = person;
+
+		template.save(wdr);
+
+		session.close();
+
+		sessionBoundTemplate.findById(wdr.id, WithDbRef.class);
+	}
+
+	@Test // DATAMONGO-1880
+	public void shouldLoadLazyDbRefWhenSessionIsActive() {
+
+		Person person = new Person("Kylar Stern");
+
+		template.save(person);
+
+		WithLazyDbRef wdr = new WithLazyDbRef();
+		wdr.id = "id-1";
+		wdr.personRef = person;
+
+		template.save(wdr);
+
+		WithLazyDbRef result = sessionBoundTemplate.findById(wdr.id, WithLazyDbRef.class);
+
+		assertThat(result.getPersonRef()).isEqualTo(person);
+	}
+
+	@Test // DATAMONGO-1880
+	public void shouldErrorOnLoadLazyDbRefWhenSessionIsClosed() {
+
+		exception.expect(LazyLoadingException.class);
+		exception.expectMessage("Invalid session state");
+
+		Person person = new Person("Kylar Stern");
+
+		template.save(person);
+
+		WithLazyDbRef wdr = new WithLazyDbRef();
+		wdr.id = "id-1";
+		wdr.personRef = person;
+
+		template.save(wdr);
+
+		WithLazyDbRef result = null;
+		try {
+			result = sessionBoundTemplate.findById(wdr.id, WithLazyDbRef.class);
+		} catch (Exception e) {
+			fail("Someting went wrong, seems the session was already closed when reading.", e);
+		}
+
+		session.close(); // now close the session
+
+		assertThat(result.getPersonRef()).isEqualTo(person); // resolve the lazy loading proxy
+	}
+
+	@Data
+	static class WithDbRef {
+
+		@Id String id;
+		@DBRef Person personRef;
+	}
+
+	@Data
+	static class WithLazyDbRef {
+
+		@Id String id;
+		@DBRef(lazy = true) Person personRef;
+
+		public Person getPersonRef() {
+			return personRef;
+		}
+	}
+
+	// --> Just some helpers for testing
+
+	MongoCollection<Document> operation(int index) {
+		return spiedCollections.get(index);
+	}
+
+	MongoDatabase command(int index) {
+		return spiedDatabases.get(index);
+	}
+
+	private MongoConverter getDefaultMongoConverter(MongoDbFactory factory) {
+
+		DbRefResolver dbRefResolver = new DefaultDbRefResolver(factory);
+		MongoCustomConversions conversions = new MongoCustomConversions(Collections.emptyList());
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		mappingContext.setSimpleTypeHolder(conversions.getSimpleTypeHolder());
+		mappingContext.afterPropertiesSet();
+
+		MappingMongoConverter converter = new MappingMongoConverter(dbRefResolver, mappingContext);
+		converter.setCustomConversions(conversions);
+		converter.afterPropertiesSet();
+
+		return converter;
+	}
+
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/SessionBoundMongoTemplateUnitTests.java

@@ -0,0 +1,358 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core;
+
+import static org.mockito.Mockito.*;
+import static org.springframework.data.mongodb.test.util.Assertions.*;
+
+import java.lang.reflect.Proxy;
+import java.util.Collections;
+
+import org.bson.Document;
+import org.bson.codecs.BsonValueCodec;
+import org.bson.codecs.configuration.CodecRegistry;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+import org.springframework.data.geo.Metrics;
+import org.springframework.data.geo.Point;
+import org.springframework.data.mongodb.MongoDbFactory;
+import org.springframework.data.mongodb.core.BulkOperations.BulkMode;
+import org.springframework.data.mongodb.core.MongoTemplate.SessionBoundMongoTemplate;
+import org.springframework.data.mongodb.core.aggregation.Aggregation;
+import org.springframework.data.mongodb.core.convert.DefaultDbRefResolver;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
+import org.springframework.data.mongodb.core.mapreduce.GroupBy;
+import org.springframework.data.mongodb.core.query.NearQuery;
+import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
+
+import com.mongodb.MongoClient;
+import com.mongodb.client.AggregateIterable;
+import com.mongodb.client.DistinctIterable;
+import com.mongodb.client.FindIterable;
+import com.mongodb.client.MapReduceIterable;
+import com.mongodb.client.MongoCollection;
+import com.mongodb.client.MongoCursor;
+import com.mongodb.client.MongoDatabase;
+import com.mongodb.client.MongoIterable;
+import com.mongodb.client.model.CountOptions;
+import com.mongodb.client.model.DeleteOptions;
+import com.mongodb.client.model.FindOneAndUpdateOptions;
+import com.mongodb.client.model.UpdateOptions;
+import com.mongodb.session.ClientSession;
+
+/**
+ * Unit test for {@link SessionBoundMongoTemplate} making sure a proxied {@link MongoCollection} and
+ * {@link MongoDatabase} is used for executing high level commands like {@link MongoOperations#find(Query, Class)}
+ * provided by Spring Data. Those commands simply handing over MongoDB base types for interaction like when obtaining a
+ * {@link MongoCollection} via {@link MongoOperations#getCollection(String)} shall not be proxied as the user can
+ * control the behavior by using the methods dedicated for {@link ClientSession} directly.
+ *
+ * @author Christoph Strobl
+ */
+@RunWith(MockitoJUnitRunner.Silent.class)
+public class SessionBoundMongoTemplateUnitTests {
+
+	private static final String COLLECTION_NAME = "collection-1";
+
+	SessionBoundMongoTemplate template;
+
+	MongoDbFactory factory;
+
+	@Mock MongoCollection collection;
+	@Mock MongoDatabase database;
+	@Mock MongoClient client;
+	@Mock ClientSession clientSession;
+	@Mock FindIterable findIterable;
+	@Mock MongoIterable mongoIterable;
+	@Mock DistinctIterable distinctIterable;
+	@Mock AggregateIterable aggregateIterable;
+	@Mock MapReduceIterable mapReduceIterable;
+	@Mock MongoCursor cursor;
+	@Mock CodecRegistry codecRegistry;
+
+	MappingMongoConverter converter;
+	MongoMappingContext mappingContext;
+
+	@Before
+	public void setUp() {
+
+		when(client.getDatabase(anyString())).thenReturn(database);
+		when(codecRegistry.get(any(Class.class))).thenReturn(new BsonValueCodec());
+		when(database.getCodecRegistry()).thenReturn(codecRegistry);
+		when(database.getCollection(anyString(), any())).thenReturn(collection);
+		when(database.listCollectionNames(any(ClientSession.class))).thenReturn(mongoIterable);
+		when(collection.find(any(ClientSession.class), any(), any())).thenReturn(findIterable);
+		when(collection.aggregate(any(ClientSession.class), anyList(), any())).thenReturn(aggregateIterable);
+		when(collection.distinct(any(ClientSession.class), any(), any(), any())).thenReturn(distinctIterable);
+		when(collection.mapReduce(any(ClientSession.class), any(), any(), any())).thenReturn(mapReduceIterable);
+		when(findIterable.iterator()).thenReturn(cursor);
+		when(aggregateIterable.collation(any())).thenReturn(aggregateIterable);
+		when(aggregateIterable.allowDiskUse(anyBoolean())).thenReturn(aggregateIterable);
+		when(aggregateIterable.batchSize(anyInt())).thenReturn(aggregateIterable);
+		when(aggregateIterable.map(any())).thenReturn(aggregateIterable);
+		when(aggregateIterable.useCursor(anyBoolean())).thenReturn(aggregateIterable);
+		when(aggregateIterable.into(any())).thenReturn(Collections.emptyList());
+		when(mongoIterable.iterator()).thenReturn(cursor);
+		when(distinctIterable.map(any())).thenReturn(distinctIterable);
+		when(distinctIterable.into(any())).thenReturn(Collections.emptyList());
+		when(mapReduceIterable.sort(any())).thenReturn(mapReduceIterable);
+		when(mapReduceIterable.filter(any())).thenReturn(mapReduceIterable);
+		when(mapReduceIterable.map(any())).thenReturn(mapReduceIterable);
+		when(mapReduceIterable.iterator()).thenReturn(cursor);
+		when(cursor.hasNext()).thenReturn(false);
+		when(findIterable.projection(any())).thenReturn(findIterable);
+
+		factory = new SimpleMongoDbFactory(client, "foo");
+
+		this.mappingContext = new MongoMappingContext();
+		this.converter = new MappingMongoConverter(new DefaultDbRefResolver(factory), mappingContext);
+		this.template = new SessionBoundMongoTemplate(clientSession, new MongoTemplate(factory, converter));
+	}
+
+	@Test // DATAMONGO-1880
+	public void executeUsesProxiedCollectionInCallback() {
+
+		template.execute("collection", MongoCollection::find);
+
+		verify(collection, never()).find();
+		verify(collection).find(eq(clientSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void executeUsesProxiedDatabaseInCallback() {
+
+		template.execute(MongoDatabase::listCollectionNames);
+
+		verify(database, never()).listCollectionNames();
+		verify(database).listCollectionNames(eq(clientSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void findOneUsesProxiedCollection() {
+
+		template.findOne(new Query(), Person.class);
+
+		verify(collection).find(eq(clientSession), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void findShouldUseProxiedCollection() {
+
+		template.find(new Query(), Person.class);
+
+		verify(collection).find(eq(clientSession), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void findAllShouldUseProxiedCollection() {
+
+		template.findAll(Person.class);
+
+		verify(collection).find(eq(clientSession), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void executeCommandShouldUseProxiedDatabase() {
+
+		template.executeCommand("{}");
+
+		verify(database).runCommand(eq(clientSession), any(), any(Class.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void removeShouldUseProxiedCollection() {
+
+		template.remove(new Query(), Person.class);
+
+		verify(collection).deleteMany(eq(clientSession), any(), any(DeleteOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void insertShouldUseProxiedCollection() {
+
+		template.insert(new Person());
+
+		verify(collection).insertOne(eq(clientSession), any(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void aggregateShouldUseProxiedCollection() {
+
+		template.aggregate(Aggregation.newAggregation(Aggregation.project("foo")), COLLECTION_NAME, Person.class);
+
+		verify(collection).aggregate(eq(clientSession), anyList(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void aggregateStreamShouldUseProxiedCollection() {
+
+		template.aggregateStream(Aggregation.newAggregation(Aggregation.project("foo")), COLLECTION_NAME, Person.class);
+
+		verify(collection).aggregate(eq(clientSession), anyList(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void collectionExistsShouldUseProxiedDatabase() {
+
+		template.collectionExists(Person.class);
+
+		verify(database).listCollectionNames(eq(clientSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void countShouldUseProxiedCollection() {
+
+		template.count(new Query(), Person.class);
+
+		verify(collection).count(eq(clientSession), any(), any(CountOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void createCollectionShouldUseProxiedDatabase() {
+
+		template.createCollection(Person.class);
+
+		verify(database).createCollection(eq(clientSession), anyString(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void dropShouldUseProxiedCollection() {
+
+		template.dropCollection(Person.class);
+
+		verify(collection).drop(eq(clientSession));
+	}
+
+	@Test // DATAMONGO-1880
+	public void findAndModifyShouldUseProxiedCollection() {
+
+		template.findAndModify(new Query(), new Update().set("foo", "bar"), Person.class);
+
+		verify(collection).findOneAndUpdate(eq(clientSession), any(), any(), any(FindOneAndUpdateOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void findDistinctShouldUseProxiedCollection() {
+
+		template.findDistinct(new Query(), "firstName", Person.class, String.class);
+
+		verify(collection).distinct(eq(clientSession), anyString(), any(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void geoNearShouldUseProxiedDatabase() {
+
+		when(database.runCommand(any(ClientSession.class), any(), eq(Document.class)))
+				.thenReturn(new Document("results", Collections.emptyList()));
+		template.geoNear(NearQuery.near(new Point(0, 0), Metrics.NEUTRAL), Person.class);
+
+		verify(database).runCommand(eq(clientSession), any(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void groupShouldUseProxiedDatabase() {
+
+		when(database.runCommand(any(ClientSession.class), any(), eq(Document.class)))
+				.thenReturn(new Document("retval", Collections.emptyList()));
+
+		template.group(COLLECTION_NAME, GroupBy.key("firstName"), Person.class);
+
+		verify(database).runCommand(eq(clientSession), any(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void mapReduceShouldUseProxiedCollection() {
+
+		template.mapReduce(COLLECTION_NAME, "foo", "bar", Person.class);
+
+		verify(collection).mapReduce(eq(clientSession), anyString(), anyString(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void streamShouldUseProxiedCollection() {
+
+		template.stream(new Query(), Person.class);
+
+		verify(collection).find(eq(clientSession), any(), eq(Document.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void updateFirstShouldUseProxiedCollection() {
+
+		template.updateFirst(new Query(), Update.update("foo", "bar"), Person.class);
+
+		verify(collection).updateOne(eq(clientSession), any(), any(), any(UpdateOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void updateMultiShouldUseProxiedCollection() {
+
+		template.updateMulti(new Query(), Update.update("foo", "bar"), Person.class);
+
+		verify(collection).updateMany(eq(clientSession), any(), any(), any(UpdateOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void upsertShouldUseProxiedCollection() {
+
+		template.upsert(new Query(), Update.update("foo", "bar"), Person.class);
+
+		verify(collection).updateOne(eq(clientSession), any(), any(), any(UpdateOptions.class));
+	}
+
+	@Test // DATAMONGO-1880
+	public void getCollectionShouldShouldJustReturnTheCollection/*No ClientSession binding*/() {
+		assertThat(template.getCollection(COLLECTION_NAME)).isNotInstanceOf(Proxy.class);
+	}
+
+	@Test // DATAMONGO-1880
+	public void getDbShouldJustReturnTheDatabase/*No ClientSession binding*/() {
+		assertThat(template.getDb()).isNotInstanceOf(Proxy.class);
+	}
+
+	@Test // DATAMONGO-1880
+	public void indexOpsShouldUseProxiedCollection() {
+
+		template.indexOps(COLLECTION_NAME).dropIndex("index-name");
+
+		verify(collection).dropIndex(eq(clientSession), eq("index-name"));
+	}
+
+	@Test // DATAMONGO-1880
+	public void bulkOpsShouldUseProxiedCollection() {
+
+		BulkOperations bulkOps = template.bulkOps(BulkMode.ORDERED, COLLECTION_NAME);
+		bulkOps.insert(new Document());
+
+		bulkOps.execute();
+
+		verify(collection).bulkWrite(eq(clientSession), anyList(), any());
+	}
+
+	@Test // DATAMONGO-1880
+	public void scriptOpsShouldUseProxiedDatabase() {
+
+		when(database.runCommand(eq(clientSession), any())).thenReturn(new Document("retval", new Object()));
+		template.scriptOps().call("W-O-P-R");
+
+		verify(database).runCommand(eq(clientSession), any());
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ReactiveAggregationUnitTests.java

@@ -59,8 +59,8 @@ public class ReactiveAggregationUnitTests {
 		template = new ReactiveMongoTemplate(factory);
 
 		when(mongoClient.getDatabase("db")).thenReturn(db);
-		when(db.getCollection(INPUT_COLLECTION)).thenReturn(collection);
+		when(db.getCollection(eq(INPUT_COLLECTION), any(Class.class))).thenReturn(collection);
-		when(collection.aggregate(any())).thenReturn(publisher);
+		when(collection.aggregate(anyList(), any(Class.class))).thenReturn(publisher);
 		when(publisher.allowDiskUse(any())).thenReturn(publisher);
 		when(publisher.useCursor(any())).thenReturn(publisher);
 		when(publisher.collation(any())).thenReturn(publisher);

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/DefaultDbRefResolverUnitTests.java

@@ -16,7 +16,8 @@
 package org.springframework.data.mongodb.core.convert;
 
 import static org.assertj.core.api.Assertions.*;
-import static org.mockito.ArgumentMatchers.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.Mockito.*;
 
 import java.util.Arrays;
@@ -59,8 +60,8 @@ public class DefaultDbRefResolverUnitTests {
 	public void setUp() {
 
 		when(factoryMock.getDb()).thenReturn(dbMock);
-		when(dbMock.getCollection(anyString())).thenReturn(collectionMock);
+		when(dbMock.getCollection(anyString(), any(Class.class))).thenReturn(collectionMock);
-		when(collectionMock.find(Mockito.any(Document.class))).thenReturn(cursorMock);
+		when(collectionMock.find(any(Document.class))).thenReturn(cursorMock);
 
 		resolver = new DefaultDbRefResolver(factoryMock);
 	}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/index/MongoPersistentEntityIndexCreatorUnitTests.java

@@ -66,7 +66,6 @@ import com.mongodb.client.model.IndexOptions;
 public class MongoPersistentEntityIndexCreatorUnitTests {
 
 	private @Mock MongoDbFactory factory;
-	private @Mock ApplicationContext context;
 	private @Mock MongoDatabase db;
 	private @Mock MongoCollection<org.bson.Document> collection;
 	private MongoTemplate mongoTemplate;
@@ -84,7 +83,7 @@ public class MongoPersistentEntityIndexCreatorUnitTests {
 
 		when(factory.getDb()).thenReturn(db);
 		when(factory.getExceptionTranslator()).thenReturn(new MongoExceptionTranslator());
-		when(db.getCollection(collectionCaptor.capture())).thenReturn(collection);
+		when(db.getCollection(collectionCaptor.capture(), Mockito.eq(org.bson.Document.class))).thenReturn((MongoCollection) collection);
 
 		mongoTemplate = new MongoTemplate(factory);
 
@@ -192,7 +191,7 @@ public class MongoPersistentEntityIndexCreatorUnitTests {
 
 		ArgumentCaptor<String> collectionNameCapturer = ArgumentCaptor.forClass(String.class);
 
-		verify(db, times(1)).getCollection(collectionNameCapturer.capture());
+		verify(db, times(1)).getCollection(collectionNameCapturer.capture(), Mockito.any());
 		assertThat(collectionNameCapturer.getValue(), equalTo("wrapper"));
 	}
 
@@ -204,14 +203,13 @@ public class MongoPersistentEntityIndexCreatorUnitTests {
 
 		ArgumentCaptor<String> collectionNameCapturer = ArgumentCaptor.forClass(String.class);
 
-		verify(db, times(1)).getCollection(collectionNameCapturer.capture());
+		verify(db, times(1)).getCollection(collectionNameCapturer.capture(), Mockito.any());
 		assertThat(collectionNameCapturer.getValue(), equalTo("indexedDocumentWrapper"));
 	}
 
 	@Test(expected = DataAccessException.class) // DATAMONGO-1125
 	public void createIndexShouldUsePersistenceExceptionTranslatorForNonDataIntegrityConcerns() {
 
-		when(factory.getExceptionTranslator()).thenReturn(new MongoExceptionTranslator());
 		doThrow(new MongoException(6, "HostUnreachable")).when(collection).createIndex(Mockito.any(org.bson.Document.class),
 				Mockito.any(IndexOptions.class));
 
@@ -223,7 +221,6 @@ public class MongoPersistentEntityIndexCreatorUnitTests {
 	@Test(expected = ClassCastException.class) // DATAMONGO-1125
 	public void createIndexShouldNotConvertUnknownExceptionTypes() {
 
-		when(factory.getExceptionTranslator()).thenReturn(new MongoExceptionTranslator());
 		doThrow(new ClassCastException("o_O")).when(collection).createIndex(Mockito.any(org.bson.Document.class),
 				Mockito.any(IndexOptions.class));
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/performance/ReactivePerformanceTests.java

@@ -74,7 +74,7 @@ public class ReactivePerformanceTests {
 	private static final StopWatch watch = new StopWatch();
 	private static final Collection<String> IGNORED_WRITE_CONCERNS = Arrays.asList("MAJORITY", "REPLICAS_SAFE",
 			"FSYNC_SAFE", "FSYNCED", "JOURNAL_SAFE", "JOURNALED", "REPLICA_ACKNOWLEDGED");
-	private static final int COLLECTION_SIZE = 1024-2018 * 1024-2018 * 256; // 256 MB
+	private static final int COLLECTION_SIZE = 1024 - 2018 * 1024 - 2018 * 256; // 256 MB
 	private static final Collection<String> COLLECTION_NAMES = Arrays.asList("template", "driver", "person");
 
 	MongoClient mongo;
@@ -96,8 +96,8 @@ public class ReactivePerformanceTests {
 
 		converter = new MappingMongoConverter(new DbRefResolver() {
 			@Override
-			public Object resolveDbRef(MongoPersistentProperty property, DBRef dbref,
+			public Object resolveDbRef(MongoPersistentProperty property, DBRef dbref, DbRefResolverCallback callback,
-					DbRefResolverCallback callback, DbRefProxyHandler proxyHandler) {
+					DbRefProxyHandler proxyHandler) {
 				return null;
 			}
 

src/main/asciidoc/index.adoc

@@ -27,6 +27,7 @@ include::{spring-data-commons-docs}/repositories.adoc[]
 include::reference/introduction.adoc[]
 include::reference/mongodb.adoc[]
 include::reference/reactive-mongodb.adoc[]
+include::reference/client-session.adoc[]
 include::reference/mongo-repositories.adoc[]
 include::reference/reactive-mongo-repositories.adoc[]
 include::{spring-data-commons-docs}/auditing.adoc[]

src/main/asciidoc/new-features.adoc

@@ -9,6 +9,7 @@
 * <<mongo.jsonSchema,`$jsonSchema` support>> for queries and collection creation.
 * <<change-streams, Change Stream support>> for imperative and reactive drivers.
 * Tailable cursors for imperative driver.
+* <<mongo.sessions, MongoDB Session>> support for the imperative and reactive Template API.
 
 [[new-features.2-0-0]]
 == What's new in Spring Data MongoDB 2.0

src/main/asciidoc/reference/client-session.adoc

@@ -0,0 +1,79 @@
+[[mongo.sessions]]
+= MongoDB Sessions
+
+As of version 3.6 MongoDB supports a concept of Sessions. The use of sessions enables MongoDBs https://docs.mongodb.com/manual/core/read-isolation-consistency-recency/#causal-consistency[Causal Consistency] model guaranteeing to execute operations in an order that respect their causal relationships. Those are split into ``ServerSession``s and ``ClientSession``s. In the following when we speak of session we refer to `ClientSession`.
+
+WARNING: Operations within a client session are not isolated from operations outside the session.
+
+Both `MongoOperations` and `ReactiveMongoOperations` provide gateway methods for tying a `ClientSession` to the operations themselves. Within the callback all operations on `MongoCollection` and `MongoDatabase` are called with the provided session via a `Proxy` without the need to add it manually. This means that a potential call to `MongoCollection#find()` is delegated to `MongoCollection#find(ClientSession)`.
+
+NOTE: Methods like `(Reactive)MongoOperations#getCollection` returning native MongoDB java driver gateway objects, such as `MongoCollection`, that themselves offer dedicated methods for `ClientSession` will *NOT* be wrapped by the `Proxy`. So please make sure to provide the `ClientSession` where needed when interacting directly with a `MongoCollection` or `MongoDatabase` and not via one of the `#excute` callbacks on `MongoOperations`.
+
+.ClientSession with MongoOperations.
+====
+[source,java]
+----
+ClientSessionOptions sessionOptions = ClientSessionOptions.builder()
+    .causallyConsistent(true)
+    .build();
+
+ClientSession session = client.startSession(sessionOptions); <1>
+
+template.withSession(() -> session)
+    .execute(action -> {
+
+        Query query = query(where("name").is("Durzo Blint"));
+        Person durzo = action.findOne(query, Person.class);  <2>
+
+        Person azoth = new Person("Kylar Stern");
+        azoth.setMaster(durzo);
+
+        action.insert(azoth);                                <2>
+
+        return azoth;
+    });
+
+session.close()                                              <4>
+----
+<1> Obtain a new session from the server.
+<2> Use `MongoOperation` methods as before. The `ClientSession` gets applied automatically.
+<3> Important! Do not forget to close the session.
+====
+
+WARNING: When dealing with ``DBRef``s, especially lazily loaded ones, it is essential to **not** close the `ClientSession` before all data is loaded.
+
+The reactive counterpart uses the very same building blocks as the imperative one.
+
+.ClientSession with ReactiveMongoOperations.
+====
+[source,java]
+----
+ClientSessionOptions sessionOptions = ClientSessionOptions.builder()
+    .causallyConsistent(true)
+    .build();
+
+Publisher<ClientSession> session = client.startSession(sessionOptions); <1>
+
+template.withSession(session)
+    .execute(action -> {
+
+        Query query = query(where("name").is("Durzo Blint"));
+        return action.findOne(query, Person.class)
+            .flatMap(durzo -> {
+
+                Person azoth = new Person("Kylar Stern");
+                azoth.setMaster(durzo);
+
+                return action.insert(azoth);                            <2>
+            });
+    }, ClientSession::close)                                            <4>
+    .subscribe();
+----
+<1> Obtain a `Publisher` for new session retrieval.
+<2> Use `MongoOperation` methods as before. The `ClientSession` is obtained and applied automatically.
+<3> Important! Do not forget to close the session.
+====
+
+By using a `Publisher` providing the actual session you can defer session acquisition to the point of actual subscription.
+Still you need to close the session when done in order to not pollute the server with stale sessions. Use the `doFinally` hook on `execute` to call `ClientSession#close()` when you don't need the session any more.
+In case you prefer having more control over the session itself, you can always obtain the `ClientSession` via the driver and provide it via a `Supplier`.