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From "Varun Saxena (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (YARN-2962) ZKRMStateStore: Limit the number of znodes under a znode
Date Wed, 08 Jun 2016 20:29:21 GMT

    [ https://issues.apache.org/jira/browse/YARN-2962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15321386#comment-15321386

Varun Saxena commented on YARN-2962:

Have rebased the patch and fixed comments given by Daniel. Haven't tested it in my setup after
rebase but tests pass and should be good for review.

As per current patch, there can be a race if 2 RMs' become active at same time. Do we need
to handle it because I have never come across this scenario.
Basically when we store an app, we first create parent app node as per split(if it does not
exist) and then create the child znode which stores app data. These 2 operations are not carried
out within the same fencing though.
And when we remove an app, we first delete the app znode containing app data, then get children
for parent app node to check if there are no more child znodes, and if there are no more child
znodes, delete the parent app node. These 3 operations are not done in a single fencing either
which can lead to a race between creating and deleting parent app node.

We can however get rid of this potential race by doing these operations within a single fencing
by creating fencing lock nodes explicitly first and then carrying out these operations one
by one(cant be done in single transaction as we have to check how many children exist for
the parent app node). Thoughts ?

> ZKRMStateStore: Limit the number of znodes under a znode
> --------------------------------------------------------
>                 Key: YARN-2962
>                 URL: https://issues.apache.org/jira/browse/YARN-2962
>             Project: Hadoop YARN
>          Issue Type: Improvement
>          Components: resourcemanager
>    Affects Versions: 2.6.0
>            Reporter: Karthik Kambatla
>            Assignee: Varun Saxena
>            Priority: Critical
>         Attachments: YARN-2962.01.patch, YARN-2962.04.patch, YARN-2962.05.patch, YARN-2962.2.patch,
> We ran into this issue where we were hitting the default ZK server message size configs,
primarily because the message had too many znodes even though they individually they were
all small.

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