Spring #Transactional and Neo4j OGM session.getTransaction() - neo4j

Question

What is the correlation between Spring org.springframework.transaction.annotation.Transactional annotation and Neo4j OGM org.neo4j.ogm.session.Session.getTransaction() method.
I'm trying to access the current transaction via session.getTransaction() inside of the method annotated with Spring #Transactional but always getting null.
I have added a following code inside of my Spring MVC RestController method:
Transaction tx = session.beginTransaction();
try {
for (int i = 0; i < 10; i++) {
initializeNode(node);
}
}
tx.commit();
} catch (Throwable th) {
logger.error("Error while inserting mock data", th);
th.printStackTrace();
} finally {
tx.close();
}
in case of the following method:
private void initializeNode(TestNode node) {
System.out.println(session.getTransaction());
}
it prints current tx - so far everything is okay.
But in case of the following method:
private void initializeNode(TestNode node) {
System.out.println(session.getTransaction());
User admin = userDao.findByUsername("admin");
}
first time it prints current tx and then null... transaction disappear before commit for a some reason..
this is findByUsername method:
#Service
#Transactional
public class UserDaoImpl implements UserDao {
#Override
#Transactional(readOnly = true)
public User findByUsername(String username) {
return userRepository.findByUsername(username);
}
...
}
Right after that on commit I'm getting a following exception:
org.neo4j.ogm.exception.TransactionManagerException: Transaction is not current for this thread
at org.neo4j.ogm.session.transaction.DefaultTransactionManager.commit(DefaultTransactionManager.java:100)
at org.neo4j.ogm.transaction.AbstractTransaction.commit(AbstractTransaction.java:83)
at org.neo4j.ogm.drivers.embedded.transaction.EmbeddedTransaction.commit(EmbeddedTransaction.java:77)
What am I doing wrong ? Why transaction disappears ?

Answer 1

There are several issues and themes going on in this question. I will try and break them down and hopefully at the end it will all make sense.
As of the latest release of Spring Data Neo4j (4.1.x) there is no correlation between Spring's #Transactional and the Neo4j OGM's Session.getTransaction() or Session.beginTransaction() when called directly.
In your first two code blocks you are completely managing your OGM session lifecycle directly. Spring is not involved at all at this point and as you say it executes as expected.
In your updated third code block you are now expecting the session that you have manually opened to work with your Spring managed DAO. What will happen here is depends on the Neo4j Driver you are using with SDN but essentially because your DAO has the #Transactional annotation, Spring will intercept the call and start a brand new transaction all on its own on top of the one you are manually managing. At this point, we can't make any guarantees about the behaviour but the easiest explanation would be to say that it will be unexpected (again, depending on the driver used).
So how can you fix this?
I'm going to assume you want to use Spring Transactions and Spring Data Neo4j. If that's the case you will want to start by:
Changing your DAO to use Spring Data Repositories. This gives you a lot of free persistence functionality like finders, saves, deletes etc.
Putting the #Transactional annotation around the unit of work you want to accomplish. You might have a method that calls userRepository.findByUserName(), modifies that user and calls userRepository.save(user). In a web environment this is typically some sort of service method.
Removing any code that manually starts or ends an OGM session transaction.
You can find a very short code sample here and a longer code sample here.
A more comprehensive guide can also be found here.
In Spring Data Neo4j 4.2.x we hope to introduce some more powerful and friendlier #Transactional behaviour so keep posted for that update.