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From "Zhen Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HELIX-504) Controller should avoid resetting watches on removed paths
Date Tue, 23 Sep 2014 17:11:34 GMT

    [ https://issues.apache.org/jira/browse/HELIX-504?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14145049#comment-14145049
] 

Zhen Zhang commented on HELIX-504:
----------------------------------

On investigating zookeeper watch leakage problem, it turns out to be a zookeeper issue:
https://issues.apache.org/jira/browse/ZOOKEEPER-442

For zookeeper before 3.5.0, we can't  remove watches that are no longer of interests. The
only way to remove a watch is to trigger it; that is, if it is a DataWatch, we need to trigger
a data change on the watching path, or if it is a ChildWatch, we need to trigger a child change
on the watching path. Unfortunately, if we are watching a path that has been deleted, unless
we re-create the path, there is no way we can remove the watch.

Here are some of the most common scenarios where we will have dead zookeeper watches on zookeeper
server side even though we unregister all the listeners on the zookeeper client side:

- When we drop a resource group from a cluster, we may have dead watches on ideal-state, participant
current-state, and external-view
- When we remove an instance from a cluster, we may have dead watches on current-state, participant-config,
and participant messages
- When we use property store with caches enabled by zookeeper watches, we may have dead watches
on all removed paths

Will create separate jiras to track the issue.

> Controller should avoid resetting watches on removed paths
> ----------------------------------------------------------
>
>                 Key: HELIX-504
>                 URL: https://issues.apache.org/jira/browse/HELIX-504
>             Project: Apache Helix
>          Issue Type: Bug
>            Reporter: Kanak Biscuitwala
>            Assignee: Zhen Zhang
>
> Right now, if a participant has a session change, the old current state will be removed,
but the controller despite unregistering the listener, still reset the watch on the path deleted
event. We should avoid resetting this path on delete and let the controller explicitly set
watches/listeners on new sessions.



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