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From "Himanshu Vashishtha (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-10378) Divide HLog interface into User and Implementor specific interfaces
Date Sun, 19 Jan 2014 00:43:19 GMT
Himanshu Vashishtha created HBASE-10378:

             Summary: Divide HLog interface into User and Implementor specific interfaces
                 Key: HBASE-10378
                 URL: https://issues.apache.org/jira/browse/HBASE-10378
             Project: HBase
          Issue Type: Sub-task
          Components: wal
            Reporter: Himanshu Vashishtha

HBASE-5937 introduces the HLog interface as a first step to support multiple WAL implementations.
This interface is a good start, but has some limitations/drawbacks in its current state, such
1) There is no clear distinction b/w User and Implementor APIs, and it provides APIs both
for WAL users (append, sync, etc) and also WAL implementors (Reader/Writer interfaces, etc).
There are APIs which are very much implementation specific (getFileNum, etc) and a user such
as a RegionServer shouldn't know about it.
2) There are about 14 methods in FSHLog which are not present in HLog interface but are used
at several places in the unit test code. These tests typecast HLog to FSHLog, which makes
it very difficult to test multiple WAL implementations without doing some ugly checks.

I'd like to propose some changes in HLog interface that would ease the multi WAL story:

1) Have two interfaces WAL and WALService. WAL provides APIs for implementors. WALService
provides APIs for users (such as RegionServer).
2) A skeleton implementation of the above two interface as the base class for other WAL implementations
(AbstractWAL). It provides required fields for all subclasses (fs, conf, log dir, etc). Make
a minimal set of test only methods and add this set in AbstractWAL.
3) HLogFactory returns a WALService reference when creating a WAL instance; if a user need
to access impl specific APIs (there are unit tests which get WAL from a HRegionServer and
then call impl specific APIs), use AbstractWAL type casting,
4) Make TestHLog abstract and let all implementors provide their respective test class which
extends TestHLog (TestFSHLog, for example).

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