Hi, I'm curious what the best practices are for handling zxid rollover in a ZK ensemble. We have a few five-node ZK ensembles (some 3.4.8 and some 3.3.6) and they periodically rollover their zxid. We see the following in the system logs on the leader node:
2017-05-22 12:54:14,117 [myid:15] - ERROR [ProcessThread(sid:15 cport:-1)::ZooKeeperCriticalThread@49] - Severe unrecoverable error, from thread : ProcessThread(sid:15 cport:-1): org.apache.zookeeper.server.RequestProcessor$RequestProcessorException: zxid lower 32 bits have rolled over, forcing re-election, and therefore new epoch start >From my best understanding of the code, this exception will end up causing the leader to enter shutdown(): https://github.com/apache/zookeeper/blob/09cd5db55446a4b390f82e3548b929f19e33430d/src/java/main/org/apache/zookeeper/server/ZooKeeperServer.java#L464-L464 This shuts down the zookeeper instance from servicing requests, but the JVM is still actually running. What we experience is that while this ZK instance is still running, the remaining follower nodes can't re-elect a leader (at least within 15 mins) and quorum is offline. Our remediation so far has been to restart the original leader node, at which point the cluster recovers. The two questions I have are: 1. Should the remaining 4 nodes be able to re-elect a leader after zxid rollover without intervention (restarting)? 2. If the leader enters shutdown() state after a zxid rollover, is there any scenario where it will return to started? If not, how are others handling this scenario -- maybe a healthcheck that kills/restarts an instance that is in shutdown state? Cheers, Mike -- Mike Heffner <m...@librato.com> Librato, Inc.
