[jira] [Updated] (ACCUMULO-2268) Use conditinal mutations to update metadata table

["Keith Turner (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ACCUMULO-2268?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Keith Turner updated ACCUMULO-2268:
-----------------------------------

    Description: 
For correctness Accumulo requires that only one tablet server at a time serve a tablet.   Inorder to enforce this Accumulo uses zookeeper locks.  It assumed that a 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 caused by bugs in Accumulo, sometimes it 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.

  was:
For correctness Accumulo requires that only one tablet server at a time serve a tablet.   Inorder to enforce this Accumulo uses zookeeper locks.  It assumed that a when a tablet server dies, it will lose its lock and 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 caused by bugs in Accumulo, sometimes it 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 1.6 Accumulo added contditional mutations.  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.


> Use conditinal 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.  It assumed that a 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 caused by bugs in Accumulo, sometimes it 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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