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POCelery Task Chaining/Worker Release
 

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  1. POCelery Task Chaining/Worker Release
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    <p>If I write a celery task that calls other celery tasks, can I release the parent task/worker without waiting for the downstream tasks to finish?</p> <p><strong>The situation:</strong> I am working with an API that returns some data and the arguments for the next API call. I want to put all the data behind the API into a database. My current method is to query the API for the batch to work on, start some downstream processors, then recursively re-call the API+processing chain. I fear this will lock up workers waiting for all the recursive API calls to finish, when the workers do not care about the results of their children.</p> <p><strong>Pseudocode:</strong></p> <pre><code>@task def apiPing(start=None): """ Returns a dict of 5 elements, starting at the *start* element, or the beginning of the list if start is not specified. Also present in the dict is 'remaining', indicating how many elements are left in the API's list""" return json.loads(api(start)) @task def processList(data) """ Takes a result from API ping, starts a task to store each element and a chain to recall the API and process that.""" for element in data: store(element).delay() if data['remaining']!=0: chain = chain(apiPing.s(data['last']), processList.s()) chain.delay() </code></pre> <p>I understand from <a href="https://stackoverflow.com/questions/17219474/django-is-this-chaining-tasks%20here11">here</a> that the above is very close to bad; I do not want workers handling processList() to be locked up until all of the data in the API is handled. Is there a way to start the downstream tasks and release the parent worker, or refactor the above to not lock up workers?</p> <p>Testing reveals that workers are in fact locked this way:</p> <pre><code>from celery import task from time import sleep @task def parent(): print "In parent" child.apply_async() print "Out of parent" @task def child(): print "In child" sleep(10) print "Out of child" [2013-08-05 18:37:29,264: WARNING/PoolWorker-4] In parent [2013-08-05 18:37:31,278: WARNING/PoolWorker-2] In child [2013-08-05 18:37:41,285: WARNING/PoolWorker-2] Out of child [2013-08-05 18:37:41,298: WARNING/PoolWorker-4] Out of parent </code></pre>
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    1. PTQuestion
    1. USCommunity
    1. USbwarren2
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    1. COCatching the result of my call to parent() above was what was keeping things open. Invoking parent.s().delay() and not catching the result caused the parent task to close in 3/100 of a second rather than waiting for child to finish.
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      1. POCelery Task Chaining/Worker Release
      1. USbwarren2
    2. COMaybe you should follow http://docs.celeryproject.org/en/latest/userguide/canvas.html#chains best practice?
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      1. POCelery Task Chaining/Worker Release
      1. USadam
 

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