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From Vladimir Rodionov <vladrodio...@gmail.com>
Subject HBase reads, isolation levels and RegionScanner internal locking
Date Thu, 11 Sep 2014 22:58:18 GMT
Hi, all

We have two isolation levels in (used to be in Scan) in Query now. See:
https://issues.apache.org/jira/browse/HBASE-11936

I moved isolation levels API from Scan upward to Query class. The reason:
this API was not available for Get operations. The rationals? Improve
performance of get and multi-gets over the same region.

As many of you aware, RegionScannerImpl is heavily synchronized on internal
region's lock.  Now some questions:

1. Is it safe to bypass this locking (in next() call) in READ_UNCOMMITTED
mode?
We will do all necessary checks, of course, before calling nextRaw().
2. What was the reason of this locking in a first place for reads in
READ_COMMITTED mode? Except obvious - no-dirty-reads allowed? Can someone
tell me what else bad can happen?

-Vladimir

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