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67 lines
2.1 KiB
Markdown
67 lines
2.1 KiB
Markdown
---
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title: "RQ: Simple job queues for Python"
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layout: contrib
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---
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This document describes how RQ works internally when enqueuing or dequeueing.
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## Enqueueing internals
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Whenever a function call gets enqueued, RQ does two things:
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* It creates a job instance representing the delayed function call and persists
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it in a Redis [hash][h]; and
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* It pushes the given job's ID onto the requested Redis queue.
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All jobs are stored in Redis under the `rq:job:` prefix, for example:
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rq:job:55528e58-9cac-4e05-b444-8eded32e76a1
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The keys of such a job [hash][h] are:
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created_at => '2012-02-13 14:35:16+0000'
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enqueued_at => '2012-02-13 14:35:16+0000'
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origin => 'default'
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data => <pickled representation of the function call>
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description => "count_words_at_url('http://nvie.com')"
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Depending on whether or not the job has run successfully or has failed, the
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following keys are available, too:
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ended_at => '2012-02-13 14:41:33+0000'
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result => <pickled return value>
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exc_info => <exception information>
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[h]: http://redis.io/topics/data-types#hashes
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## Dequeueing internals
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Whenever a dequeue is requested, an RQ worker does two things:
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* It pops a job ID from the queue, and fetches the job data belonging to that
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job ID;
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* It starts executing the function call.
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* If the job succeeds, its return value is written to the `result` hash key and
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the hash itself is expired after 500 seconds; or
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* If the job fails, the exception information is written to the `exc_info`
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hash key and the job ID is pushed onto the `failed` queue.
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## Cancelling jobs
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Any job ID that is encountered by a worker for which no job hash is found in
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Redis is simply ignored. This makes it easy to cancel jobs by simply removing
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the job hash. In Python:
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```python
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from rq import cancel_job
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cancel_job('2eafc1e6-48c2-464b-a0ff-88fd199d039c')
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```
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Note that it is irrelevant on which queue the job resides. When a worker
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eventually pops the job ID from the queue and notes that the Job hash does not
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exist (anymore), it simply discards the job ID and continues with the next.
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