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From "Shaun Senecal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-1740) Zookeeper 3.3.4 loses ephemeral nodes under stress
Date Wed, 06 Nov 2013 08:43:18 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-1740?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13814708#comment-13814708

Shaun Senecal commented on ZOOKEEPER-1740:

My understanding is that this issue was closed as "Not a problem" because it wasn't reprodicible
under 3.4 at the time.  However, the behaviour seems to be the same and I am able to consistently
reproduce it under 3.4.5 using the app in ZOOKEEPER-1809.  Perhaps this ticket can remain
closed, as it pertains to 3.3, and any further investigation happens on ZOOKEEPER-1809?

> Zookeeper 3.3.4 loses ephemeral nodes under stress
> --------------------------------------------------
>                 Key: ZOOKEEPER-1740
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1740
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: server
>    Affects Versions: 3.3.4
>            Reporter: Neha Narkhede
>            Assignee: Neha Narkhede
>            Priority: Critical
> The current behavior of zookeeper for ephemeral nodes is that session expiration and
ephemeral node deletion is not an atomic operation. 
> The side-effect of the above zookeeper behavior in Kafka, for certain corner cases, is
that ephemeral nodes can be lost even if the session is not expired. The sequence of events
that can lead to lossy ephemeral nodes is as follows - 
> 1. The session expires on the client, it assumes the ephemeral nodes are deleted, so
it establishes a new session with zookeeper and tries to re-create the ephemeral nodes. 
> 2. However, when it tries to re-create the ephemeral node,zookeeper throws back a NodeExists
error code. Now this is legitimate during a session disconnect event (since zkclient automatically
retries the 
> operation and raises a NodeExists error). Also by design, Kafka server doesn't have multiple
zookeeper clients create the same ephemeral node, so Kafka server assumes the NodeExists is
> 3. However, after a few seconds zookeeper deletes that ephemeral node. So from the client's
perspective, even though the client has a new valid session, its ephemeral node is gone. 
> This behavior is triggered due to very long fsync operations on the zookeeper leader.
When the leader wakes up from such a long fsync operation, it has several sessions to expire.
And the time between the session expiration and the ephemeral node deletion is magnified.
Between these 2 operations, a zookeeper client can issue a ephemeral node creation operation,
that could've appeared to have succeeded, but the leader later deletes the ephemeral node
leading to permanent ephemeral node loss from the client's perspective. 
> Thread from zookeeper mailing list: http://zookeeper.markmail.org/search/?q=Zookeeper+3.3.4#query:Zookeeper%203.3.4%20date%3A201307%20+page:1+mid:zma242a2qgp6gxvx+state:results
> The way to reproduce this behavior is as follows -
> 1. Bring up a zookeeper 3.3.4 cluster and create several sessions with ephemeral ndoes
on it using zkclient. Make sure the session expiration callback is implemented and it re-registers
the ephemeral node.
> 2. Run the following script on the zookeeper leader -
> while true
>  do
>    kill -STOP $1
>    sleep 8
>    kill -CONT $1
>    sleep 60
>  done
> 3. Run another script to check for existence of ephemeral nodes.
> This script shows that zookeeper loses the ephemeral nodes and the clients still have
a valid session.

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