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https://issues.apache.org/jira/browse/HBASE-6496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13427594#comment-13427594
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Andrew Purtell commented on HBASE-6496:
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In an earlier iteration of CPs the coprocessor host provided an environment
variable like facility, so for all CPs installed on a region could share state
among themselves. This was a map provided by the CP host, so all CPs installed
on a region could share state. We could bring that back as a map like you
suggest but shared across all Observers in a RS, perhaps one map for every
class that asks for it. (So, shared across the RS but private to each CP
implementation.)
It could also be useful to have a controlled facility for ZK watchers anyway:
* Add to RegionServerServices a facility for getting ZK watchers on demand
* Add to the RegionCoprocessorEnvironment an API for getting ZK watchers from
the RS, handing them back to be reaped, and dealing with freeing up dangling
resources from any CP termination / unload.
* Extend above facility for shared watchers, one watcher per RS for a given CP,
perhaps again keyed on class name.
> 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.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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