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Date: Thu, 15 Jan 2015 05:32:34 +0000 (UTC)
From: "Andrew Purtell (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Message-ID: <JIRA.12767655.1421295716000.89374.1421299954487@Atlassian.JIRA>
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Subject: [jira] [Commented] (HBASE-12865) WALs may be deleted before they
 are replicated to peers
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    [ https://issues.apache.org/jira/browse/HBASE-12865?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14278281#comment-14278281 ] 

Andrew Purtell commented on HBASE-12865:
----------------------------------------

It seems we should introduce a timed lock (a ZK lock recipe) on the replication znode to lock out changes when the ReplicationLogCleaner is enumerating over its children.

> WALs may be deleted before they are replicated to peers
> -------------------------------------------------------
>
>                 Key: HBASE-12865
>                 URL: https://issues.apache.org/jira/browse/HBASE-12865
>             Project: HBase
>          Issue Type: Bug
>          Components: Replication
>            Reporter: Liu Shaohui
>
> By design, ReplicationLogCleaner guarantee that the WALs  being in replication queue can't been deleted by the HMaster. The ReplicationLogCleaner gets the WAL set from zookeeper by scanning the replication zk node. But it may get uncompleted WAL set during replication failover for the scan operation is not atomic.
> For example: There are three region servers: rs1, rs2, rs3, and peer id 10.  The layout of replication zookeeper nodes is:
> {code}
> /hbase/replication/rs/rs1/10/wals
>                      /rs2/10/wals
>                      /rs3/10/wals
> {code}
> - t1: the ReplicationLogCleaner finished scanning the replication queue of rs1, and start to scan the queue of rs2.
> - t2: region server rs3 is down, and rs1 take over rs3's replication queue. The new layout is
> {code}
> /hbase/replication/rs/rs1/10/wals
>                      /rs1/10-rs3/wals
>                      /rs2/10/wals
>                      /rs3
> {code}
> - t3, the ReplicationLogCleaner finished scanning the queue of rs2, and start to scan the node of rs3. But the the queue has been moved to  "replication/rs1/10-rs3/WALS"
> So the  ReplicationLogCleaner will miss the WALs of rs3 in peer 10 and the hmaster may delete these WALs before they are replicated to peer clusters.
> We encountered this problem in our cluster and I think it's a serious bug for replication.
> Suggestions are welcomed to fix this bug. thx~


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