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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CURATOR-498) Protected Mode creation can mistake closing session's node causing problems for many recipes such as LeaderLatch
Date Mon, 28 Jan 2019 21:09:01 GMT

    [ https://issues.apache.org/jira/browse/CURATOR-498?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16754357#comment-16754357
] 

ASF GitHub Bot commented on CURATOR-498:
----------------------------------------

Github user shayshim commented on a diff in the pull request:

    https://github.com/apache/curator/pull/303#discussion_r251593572
  
    --- Diff: curator-client/src/main/java/org/apache/curator/utils/InjectSessionExpiration.java
---
    @@ -94,7 +89,7 @@ public static void injectSessionExpiration(ZooKeeper zooKeeper)
                 Object eventThread = eventThreadField.get(clientCnxn);
                 queueEventMethod.invoke(eventThread, event);
                 queueEventOfDeathMethod.invoke(eventThread);
    --- End diff --
    
    Why it is important to queue event of death immediately? It will be done for us soon by
this flow:
    ConnectionState.process -> handleExpiredSession -> ... -> ClientCnxn.disconnect
->...
    - because we just queued this WatchedEvent.
    Same thing for wakeupCnxnMethod.invoke.


> Protected Mode creation can mistake closing session's node causing problems for many
recipes such as LeaderLatch
> ----------------------------------------------------------------------------------------------------------------
>
>                 Key: CURATOR-498
>                 URL: https://issues.apache.org/jira/browse/CURATOR-498
>             Project: Apache Curator
>          Issue Type: Bug
>          Components: Framework
>    Affects Versions: 4.0.1, 4.1.0
>         Environment: ZooKeeper 3.4.13, Curator 4.1.0 (selecting explicitly 3.4.13), Linux
>            Reporter: Shay Shimony
>            Assignee: Jordan Zimmerman
>            Priority: Blocker
>             Fix For: 4.1.1
>
>         Attachments: CURATOR-498.png, HaWatcher.log, LeaderLatch0.java, ha.tar.gz, logs.tar.gz,
reproduction.tar.gz, reproduction2.tar.gz
>
>
> The Curator app I am working on uses the LeaderLatch to select a leader out of 6 clients.
> While testing my app, I noticed that when I make ZK lose its quorum for a while and then
restore it, then after Curator in my app restores it's connection to ZK - sometimes not all
the 6 clients are found in the latch path (using zkCli.sh). That is, I have 5 instead of 6.
> After investigating a little, I have a suspicion that LeaderLatch deleted the leader
in method setNode.
> To investigate it I copied the LeaderLatch code and added some log messages, and from
them it seems like very old create() background callback was surprisingly scheduled and corrupted
the current leader with its stale path name. Meaning, this old one called setNode with its
stale name, and set itself instead of the leader and deleted the leader. This leaves client
running, thinking it is the leader, while another leader is selected.
> If my analysis is correct then it seems like we need to make this obsolete create callback
cancelled (I think its session was suspended on 22:38:54 and then lost on 22:39:04 - so on
SUSPENDED cancel ongoing callbacks).
> Please see attached log file and modified LeaderLatch0.
>  
> In the log, note that on 22:39:26 it shows that 0000000485 is replaced by 0000000480
and then probably deleted.
> Note also that at 22:38:52, 34 seconds before, we can see that it was in the reset()
method ("RESET OUR PATH") and possibly triggered the creation of 0000000480 then.



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