Switch to `Flux.flatMapSequential(…)` to prevent backpressure shaping.

We now use Flux.flatMapSequential(…) instead of concatMap as concatMap reduces the request size to 1. The change in backpressure/request size reduces parallelism and impacts the batch size by fetching 2 documents instead of considering the actual backpressure.

flatMapSequential doesn't tamper the requested amount while retaining the sequence order.

Closes: #4543
Original Pull Request: #4550

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

@@ -216,7 +216,7 @@ class DefaultReactiveBulkOperations extends BulkOperationsSupport implements Rea
 			collection = collection.withWriteConcern(defaultWriteConcern);
 		}
 
-		Flux<SourceAwareWriteModelHolder> concat = Flux.concat(models).flatMap(it -> {
+		Flux<SourceAwareWriteModelHolder> concat = Flux.concat(models).flatMapSequential(it -> {
 
 			if (it.model()instanceof InsertOneModel<Document> iom) {
 

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

@@ -1041,7 +1041,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			return (isOutOrMerge ? Flux.from(cursor.toCollection()) : Flux.from(cursor.first())).thenMany(Mono.empty());
 		}
 
-		return Flux.from(cursor).concatMap(readCallback::doWith);
+		return Flux.from(cursor).flatMapSequential(readCallback::doWith);
 	}
 
 	@Override
@@ -1088,7 +1088,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 				.withOptions(optionsBuilder.build());
 
 		return aggregate($geoNear, collection, Document.class) //
-				.concatMap(callback::doWith);
+				.flatMapSequential(callback::doWith);
 	}
 
 	@Override
@@ -1314,7 +1314,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		Assert.notNull(batchToSave, "Batch to insert must not be null");
 
-		return Flux.from(batchToSave).flatMap(collection -> insert(collection, collectionName));
+		return Flux.from(batchToSave).flatMapSequential(collection -> insert(collection, collectionName));
 	}
 
 	@Override
@@ -1382,7 +1382,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	@Override
 	public <T> Flux<T> insertAll(Mono<? extends Collection<? extends T>> objectsToSave) {
-		return Flux.from(objectsToSave).flatMap(this::insertAll);
+		return Flux.from(objectsToSave).flatMapSequential(this::insertAll);
 	}
 
 	protected <T> Flux<T> doInsertAll(Collection<? extends T> listToSave, MongoWriter<Object> writer) {
@@ -1433,7 +1433,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			return insertDocumentList(collectionName, documents).thenMany(Flux.fromIterable(tuples));
 		});
 
-		return insertDocuments.flatMap(tuple -> {
+		return insertDocuments.flatMapSequential(tuple -> {
 
 			Document document = tuple.getT2();
 			Object id = MappedDocument.of(document).getId();
@@ -1590,7 +1590,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 			return collectionToUse.insertMany(documents);
 
-		}).flatMap(s -> {
+		}).flatMapSequential(s -> {
 
 			return Flux.fromStream(documents.stream() //
 					.map(MappedDocument::of) //
@@ -2147,7 +2147,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			publisher = collation.map(Collation::toMongoCollation).map(publisher::collation).orElse(publisher);
 
 			return Flux.from(publisher)
-					.concatMap(new ReadDocumentCallback<>(mongoConverter, resultType, inputCollectionName)::doWith);
+					.flatMapSequential(new ReadDocumentCallback<>(mongoConverter, resultType, inputCollectionName)::doWith);
 		});
 	}
 
@@ -2215,7 +2215,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		return Flux.from(flux).collectList().filter(it -> !it.isEmpty())
 				.flatMapMany(list -> Flux.from(remove(operations.getByIdInQuery(list), entityClass, collectionName))
-						.flatMap(deleteResult -> Flux.fromIterable(list)));
+						.flatMapSequential(deleteResult -> Flux.fromIterable(list)));
 	}
 
 	/**
@@ -2674,7 +2674,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		return createFlux(collectionName, collection -> {
 			return Flux.from(preparer.initiateFind(collection, collectionCallback::doInCollection))
-					.concatMap(objectCallback::doWith);
+					.flatMapSequential(objectCallback::doWith);
 		});
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SimpleReactiveMongoRepository.java

@@ -109,7 +109,7 @@ public class SimpleReactiveMongoRepository<T, ID extends Serializable> implement
 
 		Assert.notNull(entityStream, "The given Publisher of entities must not be null");
 
-		return Flux.from(entityStream).flatMap(entity -> entityInformation.isNew(entity) ? //
+		return Flux.from(entityStream).flatMapSequential(entity -> entityInformation.isNew(entity) ? //
 				mongoOperations.insert(entity, entityInformation.getCollectionName()) : //
 				mongoOperations.save(entity, entityInformation.getCollectionName()));
 	}
@@ -167,7 +167,7 @@ public class SimpleReactiveMongoRepository<T, ID extends Serializable> implement
 
 		Assert.notNull(ids, "The given Publisher of Id's must not be null");
 
-		return Flux.from(ids).buffer().flatMap(this::findAllById);
+		return Flux.from(ids).buffer().flatMapSequential(this::findAllById);
 	}
 
 	@Override
@@ -297,7 +297,8 @@ public class SimpleReactiveMongoRepository<T, ID extends Serializable> implement
 
 		Assert.notNull(entities, "The given Publisher of entities must not be null");
 
-		return Flux.from(entities).flatMap(entity -> mongoOperations.insert(entity, entityInformation.getCollectionName()));
+		return Flux.from(entities)
+				.flatMapSequential(entity -> mongoOperations.insert(entity, entityInformation.getCollectionName()));
 	}
 
 	// -------------------------------------------------------------------------

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

@@ -36,6 +36,7 @@ import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicLong;
 import java.util.stream.Collectors;
 
 import org.assertj.core.api.Assertions;
@@ -698,6 +699,28 @@ public class ReactiveMongoTemplateUnitTests {
 		verify(collection).withReadPreference(ReadPreference.primaryPreferred());
 	}
 
+	@Test // GH-4543
+	void aggregateDoesNotLimitBackpressure() {
+
+		reset(collection);
+
+		AtomicLong request = new AtomicLong();
+		Publisher<Document> realPublisher = Flux.just(new Document()).doOnRequest(request::addAndGet);
+
+		doAnswer(invocation -> {
+			Subscriber<Document> subscriber = invocation.getArgument(0);
+			realPublisher.subscribe(subscriber);
+			return null;
+		}).when(aggregatePublisher).subscribe(any());
+
+		when(collection.aggregate(anyList())).thenReturn(aggregatePublisher);
+		when(collection.aggregate(anyList(), any(Class.class))).thenReturn(aggregatePublisher);
+
+		template.aggregate(newAggregation(Sith.class, project("id")), AutogenerateableId.class, Document.class).subscribe();
+
+		assertThat(request).hasValueGreaterThan(128);
+	}
+
 	@Test // DATAMONGO-1854
 	void aggreateShouldUseCollationFromOptionsEvenIfDefaultCollationIsPresent() {
 
@@ -1262,6 +1285,17 @@ public class ReactiveMongoTemplateUnitTests {
 		assertThat(results.get(0).id).isEqualTo("after-convert");
 	}
 
+	@Test // GH-4543
+	void findShouldNotLimitBackpressure() {
+
+		AtomicLong request = new AtomicLong();
+		stubFindSubscribe(new Document(), request);
+
+		template.find(new Query(), Person.class).subscribe();
+
+		assertThat(request).hasValueGreaterThan(128);
+	}
+
 	@Test // DATAMONGO-2479
 	void findByIdShouldInvokeAfterConvertCallbacks() {
 
@@ -1626,8 +1660,12 @@ public class ReactiveMongoTemplateUnitTests {
 	}
 
 	private void stubFindSubscribe(Document document) {
+		stubFindSubscribe(document, new AtomicLong());
+	}
+
+	private void stubFindSubscribe(Document document, AtomicLong request) {
 
-		Publisher<Document> realPublisher = Flux.just(document);
+		Publisher<Document> realPublisher = Flux.just(document).doOnRequest(request::addAndGet);
 
 		doAnswer(invocation -> {
 			Subscriber<Document> subscriber = invocation.getArgument(0);