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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-3901) QJM: send 'heartbeat' messages to JNs even when they are out-of-sync
Date Fri, 07 Sep 2012 03:28:07 GMT
Todd Lipcon created HDFS-3901:

             Summary: 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

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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