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


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reading WAL log file from inside coprocessor
Jan Van Besien 2012-10-29, 09:42
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?

thanks,
Jan