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From "Josh Elser (JIRA)" <>
Subject [jira] [Updated] (ACCUMULO-2268) Use conditional mutations to update metadata table
Date Sat, 04 Apr 2015 02:48:33 GMT


Josh Elser updated ACCUMULO-2268:
    Fix Version/s:     (was: 1.7.0)

> Use conditional mutations to update metadata table
> --------------------------------------------------
>                 Key: ACCUMULO-2268
>                 URL:
>             Project: Accumulo
>          Issue Type: Improvement
>            Reporter: Keith Turner
>             Fix For: 1.8.0
> For correctness Accumulo requires that only one tablet server at a time serve a tablet.
  In order to enforce this constraint, Accumulo uses zookeeper locks.  It's assumed when a
tablet server lock disappears that the tablet server will kill itself.  Therefore a tablet
that's assigned to a dead tablet server can be safely reassigned.  However sometimes tablet
servers continue to operate for a period of time after losing their locks.  Sometimes this
is caused by bugs in Accumulo, sometimes it's the Java GC or swapping (and the tserver does
die), sometimes it's problems with zookeeper (like the zk thread that reports lock lost dies).
> In Accumulo 1.6 conditional mutations were added.  Making all tablet metadata updates
use conditional mutations could make multiply-assigned tablets less able to do damage.   
> For example if after a minor compaction, the metadata update mutation could require the
tablet location to be the current tserver: it would prevent a zombie tserver from adding an
extraneous file to the metadata table for a tablet.
> [~ctubbsii] has discussed refactoring all metadata code so that its more modular and
works with zookeeper (for root tablet) and metadata table using same API.  This solution could
depend on that.  It may also be useful to make the root tablet operate more like a regular
tablet and store its list of files in zookeeper.  Then the root tablet could benefit from
these changes with the right abstraction layer.

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