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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-2542) Add mechanism to cancel scans
Date Mon, 24 Mar 2014 21:08:45 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-2542?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13945697#comment-13945697
] 

Josh Elser commented on ACCUMULO-2542:
--------------------------------------

Marc has done some work on this in the past (https://github.com/phrocker/accumulo-stopscan).
I remember him noting how this was difficult because infrastructure for running that Scan
will notice that it "stopped" and then try to restart it again (usually one of the nice qualities
of fetching data). I'm not sure if he ever came up with a workaround for this or chalked it
up to needing to rework the solution.

> Add mechanism to cancel scans
> -----------------------------
>
>                 Key: ACCUMULO-2542
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-2542
>             Project: Accumulo
>          Issue Type: Improvement
>          Components: tserver
>            Reporter: Michael Berman
>
> ScannerBase.close() closes all connections to a given scan, but if there's an iterator
spinning on the scan, it will keep running at least until a record makes it all the way out
of the iterator stack.  If you have a bug in an iterator, or a filter that doesn't match anything,
or a combiner that matches everything, then that could mean scanning an entire table.  It
would be nice if there were some mechanism to interrupt that process.
> We probably couldn't do anything about a really buggy externally provided iterator that
just spins without calling next() downstream (short of calling Thread.close(), which is probably
a bad idea), but accumulo iterators could periodically check for cancellation and bail gracefully
(maybe each time we're about to load a new block?).  It would be nice to provide a mechanism
for external iterators to check their cancellation status so they can have an opportunity
to be well behaved if they're doing anything that might take a while, but that seems less
important.



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