[jira] [Commented] (ACCUMULO-2268) Use conditional mutations to update metadata table

["Christopher Tubbs (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ACCUMULO-2268?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13884894#comment-13884894 ] 

Christopher Tubbs commented on ACCUMULO-2268:
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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 operations.

> Use conditional mutations to update metadata table
> --------------------------------------------------
>
>                 Key: ACCUMULO-2268
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-2268
>             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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