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From "Marco P. (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HELIX-621) Missing listener notification of LiveInstances changes (and possibly other state change)
Date Fri, 08 Jan 2016 19:50:40 GMT
Marco P. created HELIX-621:

             Summary: 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or else) can get very outdated.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

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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