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Ken Huang commented on CURATOR-644: ----------------------------------- [~tison] If you read through the original thread: https://issues.apache.org/jira/browse/CURATOR-504. You will see that this race condition would cause a delete, create (new latch node) chain and results in a huge number of new latch nodes in zk in a very short time (2.6 million of znodes (for about 2 hours) as reported in the original thread), which is the same we encountered here. What we expected is to avoid such a delete, create chain when race condition happens. > CLONE - Race conditions in LeaderLatch after reconnecting to ensemble > --------------------------------------------------------------------- > > Key: CURATOR-644 > URL: https://issues.apache.org/jira/browse/CURATOR-644 > Project: Apache Curator > Issue Type: Bug > Affects Versions: 4.1.0 > Reporter: Ken Huang > Assignee: Jordan Zimmerman > Priority: Minor > > Clone from CURATOR-504. > We use LeaderLatch in a lot of places in our system and when ZooKeeper > ensemble is unstable and clients are reconnecting to logs are full of > messages like the following: > {{{}[2017-08-31 > 19:18:34,562][ERROR][org.apache.curator.framework.recipes.leader.LeaderLatch] > Can't find our node. Resetting. Index: -1 {{}}}} > According to the > [implementation|https://github.com/apache/curator/blob/4251fe328908e5fca37af034fabc190aa452c73f/curator-recipes/src/main/java/org/apache/curator/framework/recipes/leader/LeaderLatch.java#L529-L536], > this can happen in two cases: > * When internal state `ourPath` is null > * When the list of latches does not have the expected one. > I believe we hit the first condition because of races that occur after client > reconnects to ZooKeeper. > * Client reconnects to ZooKeeper and LeaderLatch gets the event and calls > reset method which set the internal state (`ourPath`) to null, removes old > latch and creates a new one. This happens in thread > "Curator-ConnectionStateManager-0". > * Almost simultaneously, LeaderLatch gets another even NodeDeleted > ([here|https://github.com/apache/curator/blob/4251fe328908e5fca37af034fabc190aa452c73f/curator-recipes/src/main/java/org/apache/curator/framework/recipes/leader/LeaderLatch.java#L543-L554]) > and tries to re-read the list of latches and check leadership. This happens > in the thread "main-EventThread". > Therefore, sometimes there is a situation when method `checkLeadership` is > called when `ourPath` is null. -- This message was sent by Atlassian Jira (v8.20.7#820007)