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From "ramkrishna.s.vasudevan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10378) Divide HLog interface into User and Implementor specific interfaces
Date Tue, 21 Jan 2014 04:55:21 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10378?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13877189#comment-13877189

ramkrishna.s.vasudevan commented on HBASE-10378:

We could discuss offline too, not an issue. and then consolidate our thoughts here. 
>bq.Or, have the grouping logic in WAL impl and rewrite the getWAL to use this grouping
knowledge? i.e., getWAL passes a Hregioninfo to the underlying Group-WAL-impl, and the underlying
Group-wal impl returns a WALService instance based on its grouping.
I would prefer this.  Let HRS not deal with these things.  The HRS at any point of time should
only say instantiate WAL and based on what grouping or mechanism we select those many HLogs
should be instantiated and the supporting feature like syncer,writers and replication for
every HLog should be done inside the Group-WAL impl.  
Another API which I was not sure how to go about was the rollWriter() called from admin. Currently
it returns set of regions that were flushed on calling rollWriter.  
Now it should be an array of regions or a list of regions.  We can discuss offline, fine with

> 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
>         Attachments: 10378-1.patch
> 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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