How to control flow of a transactional stream with Spring Data R2DBC? - r2dbc

Question

Support for transactional streams seems to have been recently implemented but due to its newness, there are not many code examples.
Could someone show an example of a transactional stream that does a series of database inserts and then returns some value on success, but with a midstream checkpoint in between inserts that tests some condition and might roll back the transaction and return different values depending on the checkpoint result?

Answer 1

Reactive transactions follow the same pattern as imperative ones:
A transaction is started before running any user-space commands
Run user-space commands
Commit (or rollback)
A few aspects to note here: A connection is always associated with a materialization of a reactive sequence. What we know from a Thread-bound connection that is bound to an execution in imperative programming translates to an materialization in reactive programming.
So each (concurrent) execution gets a connection assigned.
Spring Data R2DBC has no support for savepoints. Take a look at the following code example that illustrates a decision to either commit or rollback:
DatabaseClient databaseClient = DatabaseClient.create(connectionFactory);
TransactionalOperator transactionalOperator = TransactionalOperator
.create(new R2dbcTransactionManager(connectionFactory));
transactionalOperator.execute(tx -> {
Mono<Void> insert = databaseClient.execute("INSERT INTO legoset VALUES(…)")
.then();
Mono<Long> select = databaseClient.execute("SELECT COUNT(*) FROM legoset")
.as(Long.class)
.fetch()
.first();
return insert.then(select.handle((count, sink) -> {
if(count > 10) {
tx.setRollbackOnly();
}
}));
}).as(StepVerifier::create).verifyComplete();
Notable aspects here are:
We're using TransactionalOperator instead of #Transactional.
The code in .handle() calls setRollbackOnly() to roll back the transaction.
Using #Transactional, you would typically use exceptions to signal a rollback condition.