Hoss Man created SOLR-13486: ------------------------------- Summary: race condition between leader's "replay on startup" and non-leader's "recover from leader" can leave replicas out of sync (TestCloudConsistency) Key: SOLR-13486 URL: https://issues.apache.org/jira/browse/SOLR-13486 Project: Solr Issue Type: Bug Security Level: Public (Default Security Level. Issues are Public) Reporter: Hoss Man
I've been investigating some jenkins failures from TestCloudConsistency, which at first glance suggest a problem w/replica(s) recovering after a network partition from the leader - but in digging into the logs the root cause acturally seems to be a thread race conditions when a replica (the leader) is first registered... * The {{ZkContainer.registerInZk(...)}} method (which is called by {{CoreContainer.registerCore(...)}} & {{CoreContainer.load()}}) is typically run in a background thread (via the {{ZkContainer.coreZkRegister}} ExecutorService) * {{ZkContainer.registerInZk(...)}} delegates to {{ZKController.register(...)}} which is ultimately responsible for checking if there are any "old" tlogs on disk, and if so handling the "Replaying tlog for <URL> during startup" logic * Because this happens in a background thread, other logic/requests can be handled by this core/replica in the meantime - before it starts (or while in the middle of) replaying the tlogs ** Notably: *leader's that have not yet replayed tlogs on startup will erroneously respond to RTG / Fingerprint / PeerSync requests from other replicas w/incomplete data* ...In general, it seems scary / fishy to me that a replica can (aparently) become *ACTIVE* before it's finished it's {{registerInZk}} + "Replaying tlog ... during startup" logic ... particularly since this can happen even for replicas that are/become leaders. It seems like this could potentially cause a whole host of problems, only one of which manifests in this particular test failure: * *BEFORE* replicaX's "coreZkRegister" thread reaches the "Replaying tlog ... during startup" check: ** replicaX can recognize (via zk terms) that it should be the leader(X) ** this leaderX can then instruct some other replicaY to recover from it ** replicaY can send RTG / PeerSync / FetchIndex requests to the leaderX (either on it's own volition, or because it was instructed to by leaderX) in an attempt to recover *** the responses to these recovery requests will not include updates in the tlog files that existed on leaderX prior to startup that hvae not yet been replayed * *AFTER* replicaY has finished it's recovery, leaderX's "Replaying tlog ... during startup" can finish ** replicaY now thinks it is in sync with leaderX, but leaderX has (replayed) updates the other replicas know nothing about -- This message was sent by Atlassian JIRA (v7.6.3#76005) --------------------------------------------------------------------- To unsubscribe, e-mail: dev-unsubscr...@lucene.apache.org For additional commands, e-mail: dev-h...@lucene.apache.org