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https://issues.apache.org/jira/browse/HBASE-19166?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16238332#comment-16238332
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Josh Elser commented on HBASE-19166:
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bq. What if an hbase2 server goes down with some content in WAL ? The WAL may 
be opened by hbase 1.x server.

Ahh, rolling upgrade scenario. Didn't think about that one. Good catch. As-is, 
we'd cause some cascading failures until recovery was picked up by a 2.0 RS 
(which would be scary for a big cluster).

> Add translation for handling hbase.regionserver.wal.WALEdit
> -----------------------------------------------------------
>
>                 Key: HBASE-19166
>                 URL: https://issues.apache.org/jira/browse/HBASE-19166
>             Project: HBase
>          Issue Type: Bug
>          Components: wal
>            Reporter: Ted Yu
>            Assignee: stack
>            Priority: Blocker
>             Fix For: 2.0.0-beta-1
>
>
> For hlog generated by 1.x, using WALPlayer from hbase2 would result in:
> {code}
> 2017-11-02 21:22:40,907 INFO  [main] mapreduce.Job: Task Id : 
> attempt_1509641483571_0003_m_000000_0, Status : FAILED
> Error: java.lang.ClassCastException: 
> org.apache.hadoop.hbase.regionserver.wal.WALEdit cannot be cast to 
> org.apache.hadoop.hbase.wal.WALEdit
>         at 
> org.apache.hadoop.hbase.mapreduce.WALPlayer$WALCellMapper.map(WALPlayer.java:143)
>         at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:146)
>         at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:793)
> {code}
> HBASE-16479 relocated WALEdit.
> Chatting with Enis, he mentioned adding translation for handling 
> hbase.regionserver.wal.WALEdit
> This way, WAL from 1.x can be recognized by hbase-2



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