[jira] [Commented] (HBASE-10378) Divide HLog interface into User and Implementor specific interfaces

["Sean Busbey (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-10378?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14102596#comment-14102596 ] 

Sean Busbey commented on HBASE-10378:
-------------------------------------

This has turned into a bit of a time sink. Now that I have small tests passing, I'm posting some WIP to get suggestions early about parts that are worth breaking off into different tickets. Note that the RB patch is definitely not something that's ready to go in.

* [github branch|https://github.com/busbey/hbase/tree/HBASE-10378] - has both the original work from this ticket rebased onto master and then follow on changes.
* [reviewboard for the net changes on top of master|https://reviews.apache.org/r/24857/] (warning: it's 10 pages)

RB description has the major compatibility goals and current failings. The github WIP commit message has questions that I'm trying to work out in my head. I'd love to get feedback on the compatibility stuff and suggestions on the open questions.

> 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
>            Assignee: Sean Busbey
>         Attachments: 10378-1.patch, 10378-2.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 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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