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HBase, mail # user - reading WAL log file from inside coprocessor

Jan Van Besien 2012-10-29, 09:42
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Re: reading WAL log file from inside coprocessor
lars hofhansl 2012-10-29, 15:34
I filed HBASE-4844 a while ago but never actually provided a patch.
If you can comment on that jira with the exact use case that'd be helpful.

If you wanted to work on a patch that'd be cooler even :)

-- Lars

 From: Jan Van Besien <[EMAIL PROTECTED]>
Sent: Monday, October 29, 2012 2:42 AM
Subject: reading WAL log file from inside coprocessor
I am implementing some functionality as a HBase coprocessor which relies on reading WALEdits from the HBase WAL.

I have to requirements:

- get notified whenever a new WALEdit is written to the WAL
- have access to the actual WAL log file such that I can read from it whenever I want

In a way, this is very similar to how HBase replication works.

The first requirement is easy enough, using the WALObserver coprocessor interface. It is for the second requirement however, that I am asking this question.

HBase replication uses the WALActionsListener interface to get notified when the log files are rolled. There doesn't seem to be a way to do that in a WALObserver (coprocessor interface) though.

I thought of adding a WALActionsListener myself from inside the coprocessor code, but I would need to make sure that it is registered before the preLogRolled method is invoked for the first time.

I also looked at HBASE-4132, but I believe this talks about the WALActionsListener and not the WALObserver.
So.. any hints on how to do WAL reading (like replication) from inside a coprocessor?

Jan Van Besien 2012-10-29, 15:50