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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6496) Example ZK based scan policy
Date Wed, 08 Aug 2012 05:25:10 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13430877#comment-13430877
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Lars Hofhansl commented on HBASE-6496:
--------------------------------------

@Ted: I see...
There is no reliable way to have a singleton, though, since the instances could potentially
have been loaded by separate classloaders.

Hence the containsKey() check to avoid creating a ZKWatcher unless necessary. The putIfAbsent
just guards against the short race that another CP created the shared ZK between the contains
check and the put.
Could also synchronize on this CP's sharedData; that would be the equivalent of a class lock.
Taking the rare chance of unnecessarily creating a ZKWatcher seemed to be better approach.

                
> Example ZK based scan policy
> ----------------------------
>
>                 Key: HBASE-6496
>                 URL: https://issues.apache.org/jira/browse/HBASE-6496
>             Project: HBase
>          Issue Type: Sub-task
>            Reporter: Lars Hofhansl
>            Assignee: Lars Hofhansl
>             Fix For: 0.96.0, 0.94.2
>
>         Attachments: 6496-v2.txt, 6496.txt
>
>
> Provide an example of a RegionServer that listens to a ZK node to learn about what set
of KVs can safely be deleted during a compaction.

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