DATAMONGO-1920 - Add support for MongoDB 4.0 transactions (snyc)

[christophstrobl]

ℹ️ Requires: MongoDB Server 3.7.9 ℹ️

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MongoDB Transactions

As of version 4, MongoDB will support Transactions. Transactions are built on top of Sessions and therefore require an active ClientSession.

⚠️ Unless you specify a MongoTransactionManager within your application context, transaction support is DISABLED. You may use setSessionSynchronization(ANY) to participate in ongoing non-native MongoDB transactions.

To get full programmatic control over transactions, you may want to use the session callback on MongoOperations.

An example of programmatic transaction control within a SessionCallback is shown below:

Programmatic transactions

ClientSession session = client.startSession(options); template.withSession(session) .execute(action -> { session.startTransaction(); try { Step step = // ...; action.insert(step); process(step); action.update(Step.class).apply(Update.set("state", // ... session.commitTransaction(); } catch (RuntimeException e) { session.abortTransaction(); } }, ClientSession::close) .subscribe();

The above example allows you to have full control over transactional behavior while using the session scoped MongoOperations instance within the callback to ensure the session is passed on to every server call.
To avoid some of the overhead that comes with this approach usage of a TransactionTemplate can take away some of the noise of manual transaction flow.

Transactions with TransactionTemplate

template.setSessionSynchronization(ANY); // ... TransactionTemplate txTemplate = new TransactionTemplate(anyTxManager); txTemplate.execute(new TransactionCallbackWithoutResult() { @Override protected void doInTransactionWithoutResult(TransactionStatus status) { Step step = // ...; template.insert(step); process(step); template.update(Step.class).apply(Update.set("state", // ...	}; });

Transactions with MongoTransactionManager

MongoTransactionManager is the gateway to the well known Spring transaction support. It allows applications to use managed transaction features of Spring.
The MongoTransactionManager binds a ClientSession to the thread. MongoTemplate detects those and operates on these resources which are associated with the transaction accordingly. MongoTemplate can also participate in other, ongoing transactions.

@Configuration static class Config extends AbstractMongoConfiguration { @Bean MongoTransactionManager transactionManager(MongoDbFactory dbFactory) { return new MongoTransactionManager(dbFactory);	} // ... } @Component public class StateService { @Transactional void someBusinessFunction(Step step) { template.insert(step); process(step); template.update(Step.class).apply(Update.set("state", // ...	}; });

⚠️ @Transactional(readOnly = true) advises MongoTransactionManager also to start a transaction adding theClientSession to outgoing requests.

[mp911de]

I wonder whether we should also provide a TransactionAwareMongoDbFactoryProxy. This could allow in-flight MongoTemplate creation that can participate in transactions if one is active.

[mp911de]

RefCount is incremented on each access. I think we should increment this one only on first access. Otherwise, multiple calls will reference the underlying resource multiple times without reducing refcount.

[mp911de]

NATIVE and ANY are hard to guess names. How about ALWAYS, ON_ACTUAL_TRANSACTION and NEVER (to omit transactional participation even if a Tx is active)?

[juergenzimmermann]

@christophstrobl Which artifact (and repo) is necessary to try you transactional implementation?

[christophstrobl]

@juergenzimmermann

<dependency> <groupId>org.springframework.data</groupId> <artifactId>spring-data-mongodb</artifactId> <version>2.1.0.DATAMONGO-1920-SNAPSHOT</version> </dependency>

via

<repositories> <repository> <id>spring-libs-snapshot</id> <url>https://repo.spring.io/libs-snapshot</url> </repository> </repositories>

[mp911de]

That's merged and polished now!