Update count vs. estimatedCount documentation.

Closes #3055
Original pull request: #3541.

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

@@ -125,6 +125,11 @@ public interface ExecutableFindOperation {
 
 		/**
 		 * Get the number of matching elements.
+		 * <p />
+		 * This method uses an {@link com.mongodb.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions) aggregation
+		 * execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees shard,
+		 * session and transaction compliance. In case an inaccurate count satisfies the applications needs use
+		 * {@link MongoOperations#estimatedCount(String)} for empty queries instead.
 		 *
 		 * @return total number of matching elements.
 		 */

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

@@ -1160,6 +1160,12 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
+	 * <p />
+	 * This method uses an
+	 * {@link com.mongodb.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
+	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
+	 * shard, session and transaction compliance. In case an inaccurate count satisfies the applications needs use
+	 * {@link #estimatedCount(Class)} for empty queries instead.
 	 *
 	 * @param query the {@link Query} class that specifies the criteria used to find documents. Must not be
 	 *          {@literal null}.
@@ -1176,6 +1182,12 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
+	 * <p />
+	 * This method uses an
+	 * {@link com.mongodb.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
+	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
+	 * shard, session and transaction compliance. In case an inaccurate count satisfies the applications needs use
+	 * {@link #estimatedCount(String)} for empty queries instead.
 	 *
 	 * @param query the {@link Query} class that specifies the criteria used to find documents.
 	 * @param collectionName must not be {@literal null} or empty.
@@ -1187,6 +1199,9 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Estimate the number of documents, in the collection {@link #getCollectionName(Class) identified by the given type},
 	 * based on collection statistics.
+	 * <p />
+	 * Please make sure to read the MongoDB reference documentation about limitations on eg. sharded cluster or inside
+	 * transactions.
 	 *
 	 * @param entityClass must not be {@literal null}.
 	 * @return the estimated number of documents.
@@ -1200,6 +1215,9 @@ public interface MongoOperations extends FluentMongoOperations {
 
 	/**
 	 * Estimate the number of documents in the given collection based on collection statistics.
+	 * <p />
+	 * Please make sure to read the MongoDB reference documentation about limitations on eg. sharded cluster or inside
+	 * transactions.
 	 *
 	 * @param collectionName must not be {@literal null}.
 	 * @return the estimated number of documents.
@@ -1214,6 +1232,12 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
+	 * <p />
+	 * This method uses an
+	 * {@link com.mongodb.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
+	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
+	 * shard, session and transaction compliance. In case an inaccurate count satisfies the applications needs use
+	 * {@link #estimatedCount(String)} for empty queries instead.
 	 *
 	 * @param query the {@link Query} class that specifies the criteria used to find documents. Must not be
 	 *          {@literal null}.

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

@@ -106,6 +106,12 @@ public interface ReactiveFindOperation {
 
 		/**
 		 * Get the number of matching elements.
+		 * <p />
+		 * This method uses an
+		 * {@link com.mongodb.reactivestreams.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
+		 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but
+		 * guarantees shard, session and transaction compliance. In case an inaccurate count satisfies the applications
+		 * needs use {@link ReactiveMongoOperations#estimatedCount(String)} for empty queries instead.
 		 *
 		 * @return {@link Mono} emitting total number of matching elements. Never {@literal null}.
 		 */

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

@@ -940,6 +940,12 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
+	 * <p />
+	 * This method uses an
+	 * {@link com.mongodb.reactivestreams.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
+	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
+	 * shard, session and transaction compliance. In case an inaccurate count satisfies the applications needs use
+	 * {@link #estimatedCount(Class)} for empty queries instead.
 	 *
 	 * @param query the {@link Query} class that specifies the criteria used to find documents. Must not be
 	 *          {@literal null}.
@@ -956,6 +962,12 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
+	 * <p />
+	 * This method uses an
+	 * {@link com.mongodb.reactivestreams.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
+	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
+	 * shard, session and transaction compliance. In case an inaccurate count satisfies the applications needs use
+	 * {@link #estimatedCount(String)} for empty queries instead.
 	 *
 	 * @param query the {@link Query} class that specifies the criteria used to find documents.
 	 * @param collectionName must not be {@literal null} or empty.
@@ -971,6 +983,12 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * influence on the resulting number of documents found as those values are passed on to the server and potentially
 	 * limit the range and order within which the server performs the count operation. Use an {@literal unpaged} query to
 	 * count all matches.
+	 * <p />
+	 * This method uses an
+	 * {@link com.mongodb.reactivestreams.client.MongoCollection#countDocuments(org.bson.conversions.Bson, com.mongodb.client.model.CountOptions)
+	 * aggregation execution} even for empty {@link Query queries} which may have an impact on performance, but guarantees
+	 * shard, session and transaction compliance. In case an inaccurate count satisfies the applications needs use
+	 * {@link #estimatedCount(String)} for empty queries instead.
 	 *
 	 * @param query the {@link Query} class that specifies the criteria used to find documents. Must not be
 	 *          {@literal null}.
@@ -983,6 +1001,9 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Estimate the number of documents, in the collection {@link #getCollectionName(Class) identified by the given type},
 	 * based on collection statistics.
+	 * <p />
+	 * Please make sure to read the MongoDB reference documentation about limitations on eg. sharded cluster or inside
+	 * transactions.
 	 *
 	 * @param entityClass must not be {@literal null}.
 	 * @return a {@link Mono} emitting the estimated number of documents.
@@ -996,6 +1017,9 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 
 	/**
 	 * Estimate the number of documents in the given collection based on collection statistics.
+	 * <p />
+	 * Please make sure to read the MongoDB reference documentation about limitations on eg. sharded cluster or inside
+	 * transactions.
 	 *
 	 * @param collectionName must not be {@literal null}.
 	 * @return a {@link Mono} emitting the estimated number of documents.

src/main/asciidoc/reference/mongodb.adoc

@@ -2221,6 +2221,7 @@ With the introduction of <<mongo.transactions>> this was no longer possible beca
 So in version 2.x `MongoOperations.count()` would use the collection statistics if no transaction was in progress, and the aggregation variant if so.
 
 As of Spring Data MongoDB 3.x any `count` operation uses regardless the existence of filter criteria the aggregation-based count approach via MongoDBs `countDocuments`.
+If the application is fine with the limitations of working upon collection statistics `MongoOperations.estimatedCount()` offers an alternative.
 
 [NOTE]
 ====