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From "Noble Paul (JIRA)" <>
Subject [jira] [Commented] (SOLR-5681) Make the OverseerCollectionProcessor multi-threaded
Date Tue, 08 Apr 2014 08:45:17 GMT


Noble Paul commented on SOLR-5681:

Although we call the stuff a queue in ZK , ZK internally has no queue. You can just read or
delete an item from anywhere. Keep an in memory list of tasks getting processed . Read a few
items in batch (from the head of course) decide what can be run now w/o interfering with other
running tasks and pass it over to a thread. When the tasks are completed , remove them from
the in memory list and delete from the zk directly using delete() command instead of queue.remove()

> Make the OverseerCollectionProcessor multi-threaded
> ---------------------------------------------------
>                 Key: SOLR-5681
>                 URL:
>             Project: Solr
>          Issue Type: Improvement
>          Components: SolrCloud
>            Reporter: Anshum Gupta
>            Assignee: Anshum Gupta
> Right now, the OverseerCollectionProcessor is single threaded i.e submitting anything
long running would have it block processing of other mutually exclusive tasks.
> When OCP tasks become optionally async (SOLR-5477), it'd be good to have truly non-blocking
behavior by multi-threading the OCP itself.
> For example, a ShardSplit call on Collection1 would block the thread and thereby, not
processing a create collection task (which would stay queued in zk) though both the tasks
are mutually exclusive.
> Here are a few of the challenges:
> * Mutual exclusivity: Only let mutually exclusive tasks run in parallel. An easy way
to handle that is to only let 1 task per collection run at a time.
> * ZK Distributed Queue to feed tasks: The OCP consumes tasks from a queue. The task from
the workQueue is only removed on completion so that in case of a failure, the new Overseer
can re-consume the same task and retry. A queue is not the right data structure in the first
place to look ahead i.e. get the 2nd task from the queue when the 1st one is in process. Also,
deleting tasks which are not at the head of a queue is not really an 'intuitive' thing.
> Proposed solutions for task management:
> * Task funnel and peekAfter(): The parent thread is responsible for getting and passing
the request to a new thread (or one from the pool). The parent method uses a peekAfter(last
element) instead of a peek(). The peekAfter returns the task after the 'last element'. Maintain
this request information and use it for deleting/cleaning up the workQueue.
> * Another (almost duplicate) queue: While offering tasks to workQueue, also offer them
to a new queue (call it volatileWorkQueue?). The difference is, as soon as a task from this
is picked up for processing by the thread, it's removed from the queue. At the end, the cleanup
is done from the workQueue.

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