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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-2485) Persist Master in-memory state so on restart or failover, new instance can pick up where the old left off
Date Sat, 24 Apr 2010 04:03:49 GMT
Persist Master in-memory state so on restart or failover, new instance can pick up where the
old left off

                 Key: HBASE-2485
                 URL: https://issues.apache.org/jira/browse/HBASE-2485
             Project: Hadoop HBase
          Issue Type: Bug
            Reporter: stack
             Fix For: 0.20.5

Today there was some good stuff up on IRC on how transitions won't always make it across Master
failovers in multi-master deploy because transitions are kept in in-memory structure up in
the Master and so on master crash, the new master will be missing state  on startup (Todd
was main promulgator of this observation and of the opinion that while  master rewrite is
scheduled for 0.21, some part needs to be done for 0.20.5).  A few suggestions were made:
transitions should be file-backed somehow, etc.  Let this issue be about the subset we want
to do for 0.20.5.

Of the in-memory state queues, there is at least the master tasks queue -- process region
opens, closes, regionserver crashes, etc. -- where tasks must be done in order and IIRC, tasks
are fairly idempotent (at least in the server crash case, its multi-step and we'll put the
crash event back on the queue if we cannot do all steps in the one go).  Perhaps this queue
could be done using the new queue facility in zk 3.3.0 (I haven't looked to check if possible,
just suggesting).  Another suggestion was a file to which we'd append queue items, requeueing,
and marking the file with task complete, etc.  On Master restart or fail-over, we'd replay
the queue log.

There is also the Map of regions-in-transition.  Yesterday we learned that there is a bug
where server shutdown processing does not iterate the Map of regions-in-transition.  This
Map may hold regions that are in "opening" or "opened" state but haven't yet had the fact
added to .META. by master.  Meantime the hosting server can crash.  Regions that were opening
will stay in the regions-in-transition and those in opened-but-not-yet-added-to-meta will
go ahead and add a crashed server to .META. (Currently regions-in-transition does not record
server the region opening/open is happening on so it doesn't have enough info to be processed
as part of server shutdown).

Regions-in-transition also needs to be persistant.  On startup, regions-in-transition can
get kinda hectic on a big cluster.  Ordering is not so important here I believe.  A directory
in zk might work (For 1M regions in a big cluster, that'd be about 2M creates and 2M deletes
during startup -- thats too much?).  Or we could write a WAL-like log again of region  transitions
(We'd have to develop a little vocabulary) that got reread by a new master.

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