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From "kishore gopalakrishna (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HELIX-621) Missing listener notification of LiveInstances changes (and possibly other state change)
Date Thu, 14 Jan 2016 21:54:39 GMT

    [ https://issues.apache.org/jira/browse/HELIX-621?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15098956#comment-15098956
] 

kishore gopalakrishna commented on HELIX-621:
---------------------------------------------

Thanks for letting us know. I was also worried about it. We rely heavily on this piece of
code.

You can disable flapping altogether by setting a system env. You can probably find this in
the code.

Good suggestion, currently we don't give any callback (apart from logging) when we detect
flapping and disconnect from the cluster.  Feel free to provide a patch for this.

> Missing listener notification of LiveInstances changes (and possibly other state change)
> ----------------------------------------------------------------------------------------
>
>                 Key: HELIX-621
>                 URL: https://issues.apache.org/jira/browse/HELIX-621
>             Project: Apache Helix
>          Issue Type: Bug
>          Components: helix-core
>    Affects Versions: 0.6.5
>            Reporter: Marco P.
>
> I noticed sometimes my LiveInstanceChangeListener was not notified of an instance disconnecting.
> Digging a little bit I found out:
>  - A reliable way to consistently reproduce this problem
>  - The problem does not seem to be limited to LiveInstances, it can happen to other listeners
using the same strategy
> This is bad as an application relies on notifications, and its view of the system (LiveInstances
or else) can get very outdated.
> The problem at the core is this logic:
> 1) Set watch W on some path P
> 2) Event E1 modifies P triggering W
> 3) The callback for W re-sets W on P
> If however a second Event E2 modifies between 2 and 3, W will not trigger (until P is
modified again).
> An example of why this is bad:
>  - 2 live instances L1, L2 and a spectator S watching them.
> 1) L1 disconnects
> 2) S's watch on LIVEINSTANCES fires
> 3) S reads the children of LIVEINSTANCES: {L2}
> 3) L2 disconnects
> 4) S's notifies LiveInstanceChangeListeners and goes back to watching LIVEINSTANCES
> The application receives a notification that the live instances now consist of {L2}.

> And no further notification until another instance joins.
> The reality is that no instances are live.
> Again, this is not limited to LIVEINSTANCES, although that's the one I can reliably reproduce.
> Fixing this is not trivial, it requires firing the watch again when re-setting it IF
the version of the watched node change since the last time the watch fired.



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