James Taylor created HBASE-6137:
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Summary: RegionServer-level context and start/stop life-cycle
methods for observer coprocessor
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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