Introduce SessionSynchronization.NEVER to disable transactional participation.

SessionSynchronization.NEVER bypasses all transactional integration in cases where applications do not want to make use of transactions so that transaction inspection overhead is avoided.

Original Pull Request: #3809

Author: mp911de

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoDatabaseUtils.java

@@ -104,7 +104,8 @@ private static MongoDatabase doGetMongoDatabase(@Nullable String dbName, MongoDa
 
 		Assert.notNull(factory, "Factory must not be null!");
 
-		if (!TransactionSynchronizationManager.isSynchronizationActive()) {
+		if (sessionSynchronization == SessionSynchronization.NEVER
+				|| !TransactionSynchronizationManager.isSynchronizationActive()) {
 			return StringUtils.hasText(dbName) ? factory.getMongoDatabase(dbName) : factory.getMongoDatabase();
 		}
 

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/ReactiveMongoDatabaseUtils.java

@@ -138,6 +138,10 @@ private static Mono<MongoDatabase> doGetMongoDatabase(@Nullable String dbName, R
 
 		Assert.notNull(factory, "DatabaseFactory must not be null!");
 
+		if (sessionSynchronization == SessionSynchronization.NEVER) {
+			return getMongoDatabaseOrDefault(dbName, factory);
+		}
+
 		return TransactionSynchronizationManager.forCurrentTransaction()
 				.filter(TransactionSynchronizationManager::isSynchronizationActive) //
 				.flatMap(synchronizationManager -> {

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/SessionSynchronization.java

@@ -15,13 +15,20 @@
  */
 package org.springframework.data.mongodb;
 
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.ReactiveMongoTemplate;
+
 /**
- * {@link SessionSynchronization} is used along with {@link org.springframework.data.mongodb.core.MongoTemplate} to
- * define in which type of transactions to participate if any.
+ * {@link SessionSynchronization} is used along with {@code MongoTemplate} to define in which type of transactions to
+ * participate if any.
  *
  * @author Christoph Strobl
  * @author Mark Paluch
  * @since 2.1
+ * @see MongoTemplate#setSessionSynchronization(SessionSynchronization)
+ * @see MongoDatabaseUtils#getDatabase(MongoDatabaseFactory, SessionSynchronization)
+ * @see ReactiveMongoTemplate#setSessionSynchronization(SessionSynchronization)
+ * @see ReactiveMongoDatabaseUtils#getDatabase(ReactiveMongoDatabaseFactory, SessionSynchronization)
  */
 public enum SessionSynchronization {
 
@@ -34,5 +41,12 @@ public enum SessionSynchronization {
 	/**
 	 * Synchronize with native MongoDB transactions initiated via {@link MongoTransactionManager}.
 	 */
-	ON_ACTUAL_TRANSACTION;
+	ON_ACTUAL_TRANSACTION,
+
+	/**
+	 * Do not participate in ongoing transactions.
+	 *
+	 * @since 3.2.5
+	 */
+	NEVER;
 }

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/MongoDatabaseUtilsUnitTests.java

@@ -109,6 +109,30 @@ void shouldNotStartSessionWhenNoTransactionOngoing() {
 		verify(dbFactory, never()).withSession(any(ClientSession.class));
 	}
 
+	@Test // GH-3760
+	void shouldJustReturnDatabaseIfSessionSynchronizationDisabled() throws Exception {
+
+		when(dbFactory.getMongoDatabase()).thenReturn(db);
+
+		JtaTransactionManager txManager = new JtaTransactionManager(userTransaction);
+		TransactionTemplate txTemplate = new TransactionTemplate(txManager);
+
+		txTemplate.execute(new TransactionCallbackWithoutResult() {
+
+			@Override
+			protected void doInTransactionWithoutResult(TransactionStatus transactionStatus) {
+
+				MongoDatabaseUtils.getDatabase(dbFactory, SessionSynchronization.NEVER);
+
+				assertThat(TransactionSynchronizationManager.hasResource(dbFactory)).isFalse();
+			}
+		});
+
+		verify(userTransaction).getStatus();
+		verifyNoMoreInteractions(userTransaction);
+		verifyNoInteractions(session);
+	}
+
 	@Test // DATAMONGO-1920
 	void shouldParticipateInOngoingJtaTransactionWithCommitWhenSessionSychronizationIsAny() throws Exception {
 

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/ReactiveMongoDatabaseUtilsUnitTests.java

@@ -88,6 +88,20 @@ void isTransactionActiveShouldLookupTxForActiveTransactionSynchronizationViaTxMa
 		}).as(StepVerifier::create).expectNext(true).verifyComplete();
 	}
 
+	@Test // GH-3760
+	void shouldJustReturnDatabaseIfSessionSynchronizationDisabled() {
+
+		when(databaseFactory.getMongoDatabase()).thenReturn(Mono.just(db));
+
+		ReactiveMongoDatabaseUtils.getDatabase(databaseFactory, SessionSynchronization.NEVER) //
+				.as(StepVerifier::create) //
+				.expectNextCount(1) //
+				.verifyComplete();
+
+		verify(databaseFactory, never()).getSession(any());
+		verify(databaseFactory, never()).withSession(any(ClientSession.class));
+	}
+
 	@Test // DATAMONGO-2265
 	void shouldNotStartSessionWhenNoTransactionOngoing() {