[jira] [Comment Edited] (CURATOR-504) Race conditions in LeaderLatch after reconnecting to ensemble

["Jordan Zimmerman (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CURATOR-504?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16757750#comment-16757750 ] 

Jordan Zimmerman edited comment on CURATOR-504 at 1/31/19 9:56 PM:
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Hey [~yuri.tceretian] - nice to see you over "here" :D

This sounds like it might be related to something we're about to fix: https://issues.apache.org/jira/browse/CURATOR-498 - please look that over and see if it corresponds.

Update: Looking at the graphic posted it looks a lot like what we're seeing in CURATOR-498.


was (Author: randgalt):
Hey [~yuri.tceretian] - nice to see you over "here" :D

This sounds like it might be related to something we're about to fix: https://issues.apache.org/jira/browse/CURATOR-498 - please look that over and see if it corresponds.

> Race conditions in LeaderLatch after reconnecting to ensemble
> -------------------------------------------------------------
>
>                 Key: CURATOR-504
>                 URL: https://issues.apache.org/jira/browse/CURATOR-504
>             Project: Apache Curator
>          Issue Type: Bug
>    Affects Versions: 4.1.0
>            Reporter: Yuri Tceretian
>            Assignee: Jordan Zimmerman
>            Priority: Minor
>         Attachments: 51868597-65791000-231c-11e9-9bfa-1def62bc3ea1.png
>
>
> 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.
> Below is an approximate diagram of what happens:
> !51868597-65791000-231c-11e9-9bfa-1def62bc3ea1.png|width=1261,height=150!



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