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From "Sergey Shelukhin (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8701) distributedLogReplay need to apply wal edits in the receiving order of those edits
Date Fri, 07 Jun 2013 22:58:20 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8701?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13678546#comment-13678546
] 

Sergey Shelukhin commented on HBASE-8701:
-----------------------------------------

[~lhofhansl] same TS can also result from system clock issues (and these will happen somewhere
on any large cluster; however these can also cause TS-s to reverse, so we can probably ignore
this case); then, one no-TS put + one explicit TS put where explicit TS happens to be a realistic
time value that is the same as the implicit one. It might theoretically also happen if HBase
can do two puts within one TS value from the same client sequentially... but yeah the defined-ness
of the behavior prevents many optimizations. It is indeed spelled out in the book now though.
                
> distributedLogReplay need to apply wal edits in the receiving order of those edits
> ----------------------------------------------------------------------------------
>
>                 Key: HBASE-8701
>                 URL: https://issues.apache.org/jira/browse/HBASE-8701
>             Project: HBase
>          Issue Type: Bug
>          Components: MTTR
>            Reporter: Jeffrey Zhong
>            Assignee: Jeffrey Zhong
>             Fix For: 0.98.0, 0.95.2
>
>
> This issue happens in distributedLogReplay mode when recovering multiple puts of the
same key + version(timestamp). After replay, the value is nondeterministic of the key
> h5. The original concern situation raised from [~eclark]:
> For all edits the rowkey is the same.
> There's a log with: [ A (ts = 0), B (ts = 0) ]
> Replay the first half of the log.
> A user puts in C (ts = 0)
> Memstore has to flush
> A new Hfile will be created with [ C, A ] and MaxSequenceId = C's seqid.
> Replay the rest of the Log.
> Flush
> The issue will happen in similar situation like Put(key, t=T) in WAL1 and Put(key,t=T)
in WAL2
> h5. Below is the option I'd like to use:
> a) During replay, we pass wal file name hash in each replay batch and original wal sequence
id of each edit to the receiving RS
> b) Once a wal is recovered, playing RS send a signal to the receiving RS so the receiving
RS can flush
> c) In receiving RS, different WAL file of a region sends edits to different memstores.(We
can visualize this in high level as sending changes to a new region object with name(origin
region name + wal name hash) and use the original sequence Ids.) 
> d) writes from normal traffic(allow writes during recovery) are put in normal memstores
as of today and flush normally with new sequenceIds.
> h5. The other alternative options are listed below for references:
> Option one
> a) disallow writes during recovery
> b) during replay, we pass original wal sequence ids
> c) hold flush till all wals of a recovering region are replayed. Memstore should hold
because we only recover unflushed wal edits. For edits with same key + version, whichever
with larger sequence Id wins.
> Option two
> a) During replay, we pass original wal sequence ids
> b) for each wal edit, we store each edit's original sequence id along with its key. 
> c) during scanning, we use the original sequence id if it's present otherwise its store
file sequence Id
> d) compaction can just leave put with max sequence id
> Please let me know if you have better ideas.

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