[jira] [Commented] (HBASE-6496) Example ZK based scan policy

["Andrew Purtell (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-6496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13427594#comment-13427594 ] 

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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