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https://issues.apache.org/jira/browse/HBASE-2001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12781036#action_12781036
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Andrew Purtell commented on HBASE-2001:
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One application of this is like in process map reduce on the HBase cluster.
Coprocessors accept list of writables in and produce a list of writables out,
like a mapper. Client library doing collation and aggregation of per region
invocations would be functionally like a reducer.
> Coprocessors: Colocate arbitrary code with regions
> --------------------------------------------------
>
> Key: HBASE-2001
> URL: https://issues.apache.org/jira/browse/HBASE-2001
> Project: Hadoop HBase
> Issue Type: Sub-task
> Reporter: Andrew Purtell
> Assignee: Andrew Purtell
>
> "Support arbitrary code that runs run next to each region in table. As
> regions split and move, coprocessor code should automatically move also."
> Use classloader which looks on HDFS.
> Associate a list of classes to load with each table. Put this in HRI so it
> inherits from table but can be changed on a per region basis (so then those
> region specific changes can inherited by daughters).
> Not completely arbitrary code, should require implementation of an interface
> with callbacks for:
> * Open
> * Close
> * Split
> * Compact
> * (Multi)get and scanner next()
> * (Multi)put
> * (Multi)delete
> Add method to HRegionInterface for invoking coprocessor methods and
> retrieving results.
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