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92 lines
3.1 KiB
92 lines
3.1 KiB
Celery Based Background Tasks |
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============================= |
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Celery is a task queue for Python with batteries included. It used to |
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have a Flask integration but it became unnecessary after some |
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restructuring of the internals of Celery with Version 3. This guide fills |
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in the blanks in how to properly use Celery with Flask but assumes that |
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you generally already read the `First Steps with Celery |
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<http://docs.celeryproject.org/en/master/getting-started/first-steps-with-celery.html>`_ |
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guide in the official Celery documentation. |
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Installing Celery |
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----------------- |
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Celery is on the Python Package Index (PyPI), so it can be installed with |
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standard Python tools like :command:`pip` or :command:`easy_install`:: |
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$ pip install celery |
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Configuring Celery |
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------------------ |
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The first thing you need is a Celery instance, this is called the celery |
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application. It serves the same purpose as the :class:`~flask.Flask` |
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object in Flask, just for Celery. Since this instance is used as the |
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entry-point for everything you want to do in Celery, like creating tasks |
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and managing workers, it must be possible for other modules to import it. |
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For instance you can place this in a ``tasks`` module. While you can use |
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Celery without any reconfiguration with Flask, it becomes a bit nicer by |
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subclassing tasks and adding support for Flask's application contexts and |
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hooking it up with the Flask configuration. |
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This is all that is necessary to properly integrate Celery with Flask:: |
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from celery import Celery |
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def make_celery(app): |
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celery = Celery(app.import_name, broker=app.config['CELERY_BROKER_URL']) |
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celery.conf.update(app.config) |
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TaskBase = celery.Task |
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class ContextTask(TaskBase): |
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abstract = True |
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def __call__(self, *args, **kwargs): |
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with app.app_context(): |
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return TaskBase.__call__(self, *args, **kwargs) |
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celery.Task = ContextTask |
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return celery |
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The function creates a new Celery object, configures it with the broker |
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from the application config, updates the rest of the Celery config from |
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the Flask config and then creates a subclass of the task that wraps the |
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task execution in an application context. |
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Minimal Example |
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--------------- |
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With what we have above this is the minimal example of using Celery with |
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Flask:: |
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from flask import Flask |
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flask_app = Flask(__name__) |
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flask_app.config.update( |
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CELERY_BROKER_URL='redis://localhost:6379', |
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CELERY_RESULT_BACKEND='redis://localhost:6379' |
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) |
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celery = make_celery(flask_app) |
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@celery.task() |
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def add_together(a, b): |
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return a + b |
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This task can now be called in the background: |
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>>> result = add_together.delay(23, 42) |
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>>> result.wait() |
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65 |
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Running the Celery Worker |
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------------------------- |
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Now if you jumped in and already executed the above code you will be |
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disappointed to learn that your ``.wait()`` will never actually return. |
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That's because you also need to run celery. You can do that by running |
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celery as a worker:: |
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$ celery -A your_application.celery worker |
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The ``your_application`` string has to point to your application's package |
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or module that creates the `celery` object.
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