[RATIS-1216] State machine preAppend callback can throw exception without having leader to step down.

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.0.0
• Fix Version/s: 2.0.0
• Component/s: StateMachine
• Assignee: Ethan Rose
• Reporter: Aravindan Vijayan
• Created: 07/Dec/20 17:46
• Updated: 11/Dec/20 01:16
• Resolved: 11/Dec/20 01:16

Description

      try {
        state.appendLog(context);
      } catch (StateMachineException e) {
        // the StateMachineException is thrown by the SM in the preAppend stage.
        // Return the exception in a RaftClientReply.
        RaftClientReply exceptionReply = newExceptionReply(request, e);
        cacheEntry.failWithReply(exceptionReply);
        // leader will step down here
        if (getInfo().isLeader()) {
          leaderState.submitStepDownEvent(LeaderState.StepDownReason.STATE_MACHINE_EXCEPTION);
        }
        return CompletableFuture.completedFuture(exceptionReply);
      }

The state machine can throw a StateMachine exception, which has a flag to disable leader step down action. The default for this flag would be true.

Issue Links

• is required by: HDDS-4569 Add pre append gate and marker file to OM prepare state
• is required by: HDDS-3698 Ozone Non-Rolling upgrades
• links to: GitHub Pull Request #344