Lower the replication factor for transactions topic when using embedded Kafka broker for tests

[dlyash]

Expected Behavior

It would be nice if the embedded broker started by @EmbeddedKafka (or the similar JUnit Rule) defaulted the replication factor for the transaction state topic to the number of brokers, or a min(<number-of-brokers>, 3).

Current Behavior
The embedded broker starts with the default configuration of transaction.state.log.replication.factor = 3 which doesn't make sense for a single-broker cluster, which is quite common in tests:

org.apache.kafka.common.errors.InvalidReplicationFactorException: Replication factor: 3 larger than available brokers: 1.

Context
It's not a big deal, as it can be resolved by explicitly setting broker properties like follows. But a meaningful default would save some time for thousands of developers googling the problem again and again.

@EmbeddedKafka(brokerProperties={
    "transaction.state.log.replication.factor=1"
})

[sobychacko]

@dlyash The tests we have in the framework for transactions that use EmbeddedKafka do exactly that - i.e., setting that property transaction.state.log.replication.factor to 1. Should we document that and not make any code changes primarily because retrieving the number of brokers consistently might be challenging? What do you think?

[dlyash]

@sobychacko A few lines in the documentation surely won't hurt. Technically it's already documented in Kafka's own documentation:

Note that, by default, transactions require a cluster of at least three brokers 
which is the recommended setting for production; for development you can change this, 
by adjusting broker setting transaction.state.log.replication.factor.

But I bet 99% of developers will only look for them once running into the issue and then will end up on the StackOverflow anyway.

As a developer, I would appreciate the testing framework taking care of it for me. That's why I was suggesting the programmatic solution.

---

.. retrieving the number of brokers consistently might be challenging

Oh, is it? I didn't dive too deep into the code, but I was imagining something like this in the EmbeddedKafkaContextCustomizer, but I may be simplifying things of course:

properties.putIfAbsent("transaction.state.log.replication.factor", Math.min(embeddedKafka.count(), 3));

[sobychacko]

Ok, you can give it a try and see if that works and is stable. If so, feel free to submit a PR. Thanks!