Aeron Cluster fails to start when appVersion >= 1.0.0

[zyulyaev]

See test case:
34586a2

Currently AppVersionValidator always receives appVersionUnderTest=0 as part of onNewLeadershipTerm. This happens due to ConsensusPublisher never encoding appVersion to the new leadership term message.

Relevant logs:

1 observations from 2024-10-19 13:05:04.419+0100 to 2024-10-19 13:05:04.419+0100 for:
 io.aeron.cluster.client.ClusterException: ERROR - incompatible version: 1.0.0 log=0.0.0
	at io.aeron.cluster.ConsensusModuleAgent.onNewLeadershipTerm(ConsensusModuleAgent.java:960)
	at io.aeron.cluster.ConsensusAdapter.onFragment(ConsensusAdapter.java:143)
	at io.aeron.FragmentAssembler.onFragment(FragmentAssembler.java:118)
	at io.aeron.logbuffer.TermReader.read(TermReader.java:76)
	at io.aeron.Image.poll(Image.java:324)
	at io.aeron.Subscription.poll(Subscription.java:195)
	at io.aeron.cluster.ConsensusAdapter.poll(ConsensusAdapter.java:69)
	at io.aeron.cluster.ConsensusModuleAgent.doWork(ConsensusModuleAgent.java:359)
	at org.agrona.concurrent.AgentRunner.doWork(AgentRunner.java:304)
	at org.agrona.concurrent.AgentRunner.workLoop(AgentRunner.java:296)
	at org.agrona.concurrent.AgentRunner.run(AgentRunner.java:162)
	at java.base/java.lang.Thread.run(Thread.java:1575)

As a side note: it would be very helpful to be able to distinguish whether we are validating a message in the log versus validating a snapshot. In our case we maintain backward compatibility of snapshotting logic, but maintaining backward compatibility of the log processing logic would be too much of a burden. Please let me know if I should create a separate feature request for that.

[vyazelenko]

@zyulyaev What is the use case for the appVersion validation? What are you trying to achieve?

[zyulyaev]

@vyazelenko We are trying to make sure that:

1. Logs are never processed by an app with version other than the one generating the logs. This ensures that the messages are processed in exactly the same way the LEADER did originally. We do not want to invest into maintaining log processing logic compatibility between versions.
2. Snapshot is never read by an app with version < the one creating the snapshot. We do maintain backward compatibility of snapshotting logic.