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From "Christopher Tubbs (JIRA)" <>
Subject [jira] [Commented] (ACCUMULO-2268) Use conditional mutations to update metadata table
Date Tue, 28 Jan 2014 23:19:12 GMT


Christopher Tubbs commented on ACCUMULO-2268:

For reference, the rudimentary framework for the refactoring that is mentioned is located
in o.a.a.core.metadata.MetadataServicer. It doesn't do much now, but the ideas was to move
stuff in to clean up client code (TabletLocationObtainer, for example) and other metadata

> Use conditional mutations to update metadata table
> --------------------------------------------------
>                 Key: ACCUMULO-2268
>                 URL:
>             Project: Accumulo
>          Issue Type: Improvement
>            Reporter: Keith Turner
>             Fix For: 1.7.0
> For correctness Accumulo requires that only one tablet server at a time serve a tablet.
  Inorder to enforce this Accumulo uses zookeeper locks.  Its assumed when a tablet server
lock disappears that the tablet server will kill itself.  Therefore a tablet thats 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 its the Java GC or swapping (and the tserver does die), sometimes its
problems w/ zookeeper (like the zk thread that reports lock lost dies).
> In Accumulo 1.6 contditional 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 required the tablet
location to be the current tserver that would prevent a zombie tserver from adding an extraneuous
file to the metadata table for a tablet.
> [~ctubbsii] has discussed refactoring all metadata code so that its more modular and
works w/ 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 w/ the right abstraction layer.

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