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https://issues.apache.org/jira/browse/HBASE-9291?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13860654#comment-13860654
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Andrew Purtell commented on HBASE-9291:
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Coped from
https://issues.apache.org/jira/browse/HBASE-6104?focusedCommentId=13860623&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13860623:
It's about conserving network bandwidth - we don't want to take the hit of
transferring the same data between client and server multiple times. For
example, with secondary indexing, we'd be tacking on data for every Put - if
you have a batch of 10,000, that's a lot of extra data. We could try to figure
out which Put is the "first one" for each region, but what if a split occurs
after we figure this out – this seems too brittle.
In the case of a Hash Join, we'd be sending over the compressed results of a
scan that ran over the smaller table (which gets joined against in a
coprocessor when the scan over the other table is ran). This can become very
large - imagine you're joining against a table with 10M rows. We would not want
to send this data for every region of the region server (or even multiple times
per region depending on how the scan gets parallelized on the client).
> Enable client to setAttribute that is sent once to each region server
> ---------------------------------------------------------------------
>
> Key: HBASE-9291
> URL: https://issues.apache.org/jira/browse/HBASE-9291
> Project: HBase
> Issue Type: New Feature
> Components: IPC/RPC
> Reporter: James Taylor
>
> Currently a Scan and Mutation allow the client to set its own attributes that
> get passed through the RPC layer and are accessible from a coprocessor. This
> is very handy, but breaks down if the amount of information is large, since
> this information ends up being sent again and again to every region. Clients
> can work around this with an endpoint "pre" and "post" coprocessor invocation
> that:
> 1) sends the information and caches it on the region server in the "pre"
> invocation
> 2) invokes the Scan or sends the batch of Mutations, and then
> 3) removes it in the "post" invocation.
> In this case, the client is forced to identify all region servers (ideally,
> all region servers that will be involved in the Scan/Mutation), make extra
> RPC calls, manage the caching of the information on the region server,
> age-out the information (in case the client dies before step (3) that clears
> the cached information), and must deal with the possibility of a split
> occurring while this operation is in-progress.
> Instead, it'd be much better if an attribute could be identified as a "region
> server" attribute in OperationWithAttributes and the HBase RPC layer would
> take care of doing the above.
> The use case where the above are necessary in Phoenix include:
> 1) Hash joins, where the results of the smaller side of a join scan are
> packaged up and sent to each region server, and
> 2) Secondary indexing, where the metadata of knowing a) which column
> family/column qualifier pairs and b) which part of the row key contributes to
> which indexes are sent to each region server that will process a batched put.
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