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From "Gopal V (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (TEZ-3974) Tez: Correctness regression of TEZ-955 in TEZ-2937
Date Sat, 04 Aug 2018 01:33:00 GMT

    [ https://issues.apache.org/jira/browse/TEZ-3974?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16568979#comment-16568979
] 

Gopal V commented on TEZ-3974:
------------------------------

LGTM - +1

[~jaume]: can you give brief note about testing, because this is an issue which doesn't break
when running in a mini-cluster?

> Tez: Correctness regression of TEZ-955 in TEZ-2937
> --------------------------------------------------
>
>                 Key: TEZ-3974
>                 URL: https://issues.apache.org/jira/browse/TEZ-3974
>             Project: Apache Tez
>          Issue Type: Bug
>            Reporter: Gopal V
>            Priority: Critical
>         Attachments: TEZ-3974.1.patch
>
>
> TEZ-2937 might have introduced a race condition for Tez output events, along with TEZ-2237
> {code}
>       // Close the Outputs.
>       for (OutputSpec outputSpec : outputSpecs) {
>         String destVertexName = outputSpec.getDestinationVertexName();
>         initializedOutputs.remove(destVertexName);
>         List<Event> closeOutputEvents = ((LogicalOutputFrameworkInterface)outputsMap.get(destVertexName)).close();
>         sendTaskGeneratedEvents(closeOutputEvents,
>             EventProducerConsumerType.OUTPUT, taskSpec.getVertexName(),
>             destVertexName, taskSpec.getTaskAttemptID());
>       }
>       // Close the Processor.
>       processorClosed = true;
>       processor.close();
> {code}
> As part of TEZ-2237, the outputs send empty events when the output is closed without
being started (which happens in task init failures).
> These events are obsoleted when a task fails and this happens in the AM, but not before
the dispatcher looks at them.
> Depending on the timing, the empty events can escape obsoletion & be sent to a downstream
task.
> This gets marked as a SKIPPED event in the downstream task, which means that further
obsoletion events sent to the downstream task is ignored (because a zero byte fetch is not
repeated on node failure).
> So the downstream task can exit without actually waiting for the retry of the failed
task and cause silent dataloss in case where the retry succeeds in another attempt.
> So if processor.close() throws an exception, this introduce a race condition and if the
AM is too fast, we end up with correctness issues.
> This was originally reported in TEZ-955



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