Spring @Transactional - isolation, propagation

Can someone explain the isolation & propagation parameters in the @Transactional annotation via a real-world example?

Basically when and why should I choose to change their default values?

Good question, although not a trivial one to answer.
Propagation

Defines how transactions relate to each other. Common options:

• REQUIRED: Code will always run in a transaction. Creates a new transaction or reuses one if available.

• REQUIRES_NEW: Code will always run in a new transaction. Suspends the current transaction if one exists.

The default value for @Transactional is REQUIRED, and this is often what you want.
Isolation

Defines the data contract between transactions.

• ISOLATION_READ_UNCOMMITTED: Allows dirty reads.

• ISOLATION_READ_COMMITTED: Does not allow dirty reads.

• ISOLATION_REPEATABLE_READ: If a row is read twice in the same transaction, the result will always be the same.

• ISOLATION_SERIALIZABLE: Performs all transactions in a sequence.

The different levels have different performance characteristics in a multi-threaded application. I think if you understand the dirty reads concept you will be able to select a good option.

Defaults may vary between difference databases. As an example, for MariaDB it is REPEATABLE READ.

Example of when a dirty read can occur:

thread 1   thread 2      
      |         |
    write(x)    |
      |         |
      |        read(x)
      |         |
    rollback    |
      v         v 
           value (x) is now dirty (incorrect)

So a sane default (if such can be claimed) could be ISOLATION_READ_COMMITTED, which only lets you read values which have already been committed by other running transactions, in combination with a propagation level of REQUIRED. Then you can work from there if your application has other needs.

A practical example of where a new transaction will always be created when entering the provideService routine and completed when leaving:

public class FooService {
    private Repository repo1;
    private Repository repo2;

    @Transactional(propagation=Propagation.REQUIRES_NEW)
    public void provideService() {
        repo1.retrieveFoo();
        repo2.retrieveFoo();
    }
}

Had we instead used REQUIRED, the transaction would remain open if the transaction was already open when entering the routine.
Note also that the result of a rollback could be different as several executions could take part in the same transaction.

We can easily verify the behaviour with a test and see how results differ with propagation levels:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations="classpath:/fooService.xml")
public class FooServiceTests {

    private @Autowired TransactionManager transactionManager;
    private @Autowired FooService fooService;

    @Test
    public void testProvideService() {
        TransactionStatus status = transactionManager.getTransaction(new DefaultTransactionDefinition());
        fooService.provideService();
        transactionManager.rollback(status);
        // assert repository values are unchanged ... 
}

With a propagation level of

-
REQUIRES_NEW: we would expect fooService.provideService() was NOT rolled back since it created its own sub-transaction.

-
REQUIRED: we would expect everything was rolled back and the backing store was unchanged.

Stack Overflow Q#8490852, score: 554