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From "Aaron T. Myers (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3901) QJM: send 'heartbeat' messages to JNs even when they are out-of-sync
Date Fri, 07 Sep 2012 22:00:08 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3901?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13451040#comment-13451040
] 

Aaron T. Myers commented on HDFS-3901:
--------------------------------------

Looks like this patch is missing the "Stopwatch" class, and so doesn't compile.
                
> QJM: send 'heartbeat' messages to JNs even when they are out-of-sync
> --------------------------------------------------------------------
>
>                 Key: HDFS-3901
>                 URL: https://issues.apache.org/jira/browse/HDFS-3901
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>    Affects Versions: QuorumJournalManager (HDFS-3077)
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hdfs-3901.txt
>
>
> Currently, if one of the JNs has fallen out of sync with the writer (eg because it went
down), it will be marked as such until the next log roll. This causes the writer to no longer
send any RPCs to it. This means that the JN's metrics will no longer reflect up-to-date information
on how far laggy they are.
> This patch will introduce a heartbeat() RPC that has no effect except to update the JN's
view of the latest committed txid. When the writer is talking to an out-of-sync logger, it
will send these heartbeat messages once a second.
> In a future patch we can extend the heartbeat functionality so that NNs periodically
check their connections to JNs if no edits arrive, such that a fenced NN won't accidentally
continue to serve reads indefinitely.

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