Himanshu Vashishtha created HBASE-10378:
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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 as:
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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