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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:
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@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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