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From "James Taylor (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-6137) RegionServer-level context and start/stop life-cycle methods for observer coprocessor
Date Wed, 30 May 2012 19:29:24 GMT
James Taylor created HBASE-6137:

             Summary: RegionServer-level context and start/stop life-cycle methods for observer
                 Key: HBASE-6137
                 URL: https://issues.apache.org/jira/browse/HBASE-6137
             Project: HBase
          Issue Type: New Feature
          Components: coprocessors
    Affects Versions: 0.94.0
            Reporter: James Taylor

Coprocessors are a great way for an application to affect server-side processing. We're using
observer coprocessors via the postScannerOpen to enable a scan to do aggregation. There's
currently no way, however, to store/share state across coprocessor invocations on the regions
within a region server. Ideally, we'd like to be able to have a context object that allows
state to be shared across coprocessor invocation for the regions on the same region server.
This would save us the setup cost for "compiling" our aggregators again for each region. Also
useful, would be:
- a start/stop method invocation on this new region server context object before the first
region invocation and after the last region invocation on a given region server.
- a way to pass state to the start/stop method from the client. The scan.setAttribute works
well for passing state for the invocation on each region, but ideally something that would
allow state to be passed just once per region server. One use case would be to pass a cache
of the row data for a hash join implementation, where we wouldn't want to pass this information
for every region.

Our current work around is to either take the hit of the extra setup costs for the coprocessor
invocation on each region or use an Endpoint coprocessor to initialize state prior to the
client scan that will cause coprocessor invocations.

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