Merge pull request #4968 from pallets/docs-celery

rewrite celery docs, add example application

docs/patterns/celery.rst

@@ -1,105 +1,242 @@
-Celery Background Tasks
-=======================
+Background Tasks with Celery
+============================
 
-If your application has a long running task, such as processing some uploaded
-data or sending email, you don't want to wait for it to finish during a
-request. Instead, use a task queue to send the necessary data to another
-process that will run the task in the background while the request returns
-immediately.
+If your application has a long running task, such as processing some uploaded data or
+sending email, you don't want to wait for it to finish during a request. Instead, use a
+task queue to send the necessary data to another process that will run the task in the
+background while the request returns immediately.
+
+`Celery`_ is a powerful task queue that can be used for simple background tasks as well
+as complex multi-stage programs and schedules. This guide will show you how to configure
+Celery using Flask. Read Celery's `First Steps with Celery`_ guide to learn how to use
+Celery itself.
+
+.. _Celery: https://celery.readthedocs.io
+.. _First Steps with Celery: https://celery.readthedocs.io/en/latest/getting-started/first-steps-with-celery.html
+
+The Flask repository contains `an example <https://github.com/pallets/flask/tree/main/examples/celery>`_
+based on the information on this page, which also shows how to use JavaScript to submit
+tasks and poll for progress and results.
 
-Celery is a powerful task queue that can be used for simple background tasks
-as well as complex multi-stage programs and schedules. This guide will show you
-how to configure Celery using Flask, but assumes you've already read the
-`First Steps with Celery <https://celery.readthedocs.io/en/latest/getting-started/first-steps-with-celery.html>`_
-guide in the Celery documentation.
 
 Install
 -------
 
-Celery is a separate Python package. Install it from PyPI using pip::
+Install Celery from PyPI, for example using pip:
+
+.. code-block:: text
 
     $ pip install celery
 
-Configure
----------
 
-The first thing you need is a Celery instance, this is called the celery
-application.  It serves the same purpose as the :class:`~flask.Flask`
-object in Flask, just for Celery.  Since this instance is used as the
-entry-point for everything you want to do in Celery, like creating tasks
-and managing workers, it must be possible for other modules to import it.
+Integrate Celery with Flask
+---------------------------
 
-For instance you can place this in a ``tasks`` module.  While you can use
-Celery without any reconfiguration with Flask, it becomes a bit nicer by
-subclassing tasks and adding support for Flask's application contexts and
-hooking it up with the Flask configuration.
+You can use Celery without any integration with Flask, but it's convenient to configure
+it through Flask's config, and to let tasks access the Flask application.
 
-This is all that is necessary to integrate Celery with Flask:
+Celery uses similar ideas to Flask, with a ``Celery`` app object that has configuration
+and registers tasks. While creating a Flask app, use the following code to create and
+configure a Celery app as well.
 
 .. code-block:: python
 
-    from celery import Celery
+    from celery import Celery, Task
 
-    def make_celery(app):
-        celery = Celery(app.import_name)
-        celery.conf.update(app.config["CELERY_CONFIG"])
-
-        class ContextTask(celery.Task):
-            def __call__(self, *args, **kwargs):
+    def celery_init_app(app: Flask) -> Celery:
+        class FlaskTask(Task):
+            def __call__(self, *args: object, **kwargs: object) -> object:
                 with app.app_context():
                     return self.run(*args, **kwargs)
 
-        celery.Task = ContextTask
-        return celery
-
-The function creates a new Celery object, configures it with the broker
-from the application config, updates the rest of the Celery config from
-the Flask config and then creates a subclass of the task that wraps the
-task execution in an application context.
+        celery_app = Celery(app.name, task_cls=FlaskTask)
+        celery_app.config_from_object(app.config["CELERY"])
+        celery_app.set_default()
+        app.extensions["celery"] = celery_app
+        return celery_app
 
-.. note::
-    Celery 5.x deprecated uppercase configuration keys, and 6.x will
-    remove them. See their official `migration guide`_.
+This creates and returns a ``Celery`` app object. Celery `configuration`_ is taken from
+the ``CELERY`` key in the Flask configuration. The Celery app is set as the default, so
+that it is seen during each request. The ``Task`` subclass automatically runs task
+functions with a Flask app context active, so that services like your database
+connections are available.
 
-.. _migration guide: https://docs.celeryproject.org/en/stable/userguide/configuration.html#conf-old-settings-map.
+.. _configuration: https://celery.readthedocs.io/en/stable/userguide/configuration.html
 
-An example task
----------------
+Here's a basic ``example.py`` that configures Celery to use Redis for communication. We
+enable a result backend, but ignore results by default. This allows us to store results
+only for tasks where we care about the result.
 
-Let's write a task that adds two numbers together and returns the result. We
-configure Celery's broker and backend to use Redis, create a ``celery``
-application using the factory from above, and then use it to define the task. ::
+.. code-block:: python
 
     from flask import Flask
 
-    flask_app = Flask(__name__)
-    flask_app.config.update(CELERY_CONFIG={
-        'broker_url': 'redis://localhost:6379',
-        'result_backend': 'redis://localhost:6379',
-    })
-    celery = make_celery(flask_app)
+    app = Flask(__name__)
+    app.config.from_mapping(
+        CELERY=dict(
+            broker_url="redis://localhost",
+            result_backend="redis://localhost",
+            task_ignore_result=True,
+        ),
+    )
+    celery_app = celery_init_app(app)
+
+Point the ``celery worker`` command at this and it will find the ``celery_app`` object.
+
+.. code-block:: text
+
+    $ celery -A example worker --loglevel INFO
+
+You can also run the ``celery beat`` command to run tasks on a schedule. See Celery's
+docs for more information about defining schedules.
+
+.. code-block:: text
+
+    $ celery -A example beat --loglevel INFO
+
+
+Application Factory
+-------------------
+
+When using the Flask application factory pattern, call the ``celery_init_app`` function
+inside the factory. It sets ``app.extensions["celery"]`` to the Celery app object, which
+can be used to get the Celery app from the Flask app returned by the factory.
+
+.. code-block:: python
+
+    def create_app() -> Flask:
+        app = Flask(__name__)
+        app.config.from_mapping(
+            CELERY=dict(
+                broker_url="redis://localhost",
+                result_backend="redis://localhost",
+                task_ignore_result=True,
+            ),
+        )
+        app.config.from_prefixed_env()
+        celery_init_app(app)
+        return app
+
+To use ``celery`` commands, Celery needs an app object, but that's no longer directly
+available. Create a ``make_celery.py`` file that calls the Flask app factory and gets
+the Celery app from the returned Flask app.
+
+.. code-block:: python
+
+    from example import create_app
+
+    flask_app = create_app()
+    celery_app = flask_app.extensions["celery"]
+
+Point the ``celery`` command to this file.
+
+.. code-block:: text
+
+    $ celery -A make_celery worker --loglevel INFO
+    $ celery -A make_celery beat --loglevel INFO
+
 
-    @celery.task()
-    def add_together(a, b):
+Defining Tasks
+--------------
+
+Using ``@celery_app.task`` to decorate task functions requires access to the
+``celery_app`` object, which won't be available when using the factory pattern. It also
+means that the decorated tasks are tied to the specific Flask and Celery app instances,
+which could be an issue during testing if you change configuration for a test.
+
+Instead, use Celery's ``@shared_task`` decorator. This creates task objects that will
+access whatever the "current app" is, which is a similar concept to Flask's blueprints
+and app context. This is why we called ``celery_app.set_default()`` above.
+
+Here's an example task that adds two numbers together and returns the result.
+
+.. code-block:: python
+
+    from celery import shared_task
+
+    @shared_task(ignore_result=False)
+    def add_together(a: int, b: int) -> int:
         return a + b
 
-This task can now be called in the background::
+Earlier, we configured Celery to ignore task results by default. Since we want to know
+the return value of this task, we set ``ignore_result=False``. On the other hand, a task
+that didn't need a result, such as sending an email, wouldn't set this.
+
+
+Calling Tasks
+-------------
+
+The decorated function becomes a task object with methods to call it in the background.
+The simplest way is to use the ``delay(*args, **kwargs)`` method. See Celery's docs for
+more methods.
+
+A Celery worker must be running to run the task. Starting a worker is shown in the
+previous sections.
+
+.. code-block:: python
+
+    from flask import request
 
-    result = add_together.delay(23, 42)
-    result.wait()  # 65
+    @app.post("/add")
+    def start_add() -> dict[str, object]:
+        a = request.form.get("a", type=int)
+        b = request.form.get("b", type=int)
+        result = add_together.delay(a, b)
+        return {"result_id": result.id}
 
-Run a worker
-------------
+The route doesn't get the task's result immediately. That would defeat the purpose by
+blocking the response. Instead, we return the running task's result id, which we can use
+later to get the result.
 
-If you jumped in and already executed the above code you will be
-disappointed to learn that ``.wait()`` will never actually return.
-That's because you also need to run a Celery worker to receive and execute the
-task. ::
 
-    $ celery -A your_application.celery worker
+Getting Results
+---------------
+
+To fetch the result of the task we started above, we'll add another route that takes the
+result id we returned before. We return whether the task is finished (ready), whether it
+finished successfully, and what the return value (or error) was if it is finished.
+
+.. code-block:: python
+
+    from celery.result import AsyncResult
+
+    @app.get("/result/<id>")
+    def task_result(id: str) -> dict[str, object]:
+        result = AsyncResult(id)
+        return {
+            "ready": result.ready(),
+            "successful": result.successful(),
+            "value": result.result if result.ready() else None,
+        }
+
+Now you can start the task using the first route, then poll for the result using the
+second route. This keeps the Flask request workers from being blocked waiting for tasks
+to finish.
+
+The Flask repository contains `an example <https://github.com/pallets/flask/tree/main/examples/celery>`_
+using JavaScript to submit tasks and poll for progress and results.
+
+
+Passing Data to Tasks
+---------------------
+
+The "add" task above took two integers as arguments. To pass arguments to tasks, Celery
+has to serialize them to a format that it can pass to other processes. Therefore,
+passing complex objects is not recommended. For example, it would be impossible to pass
+a SQLAlchemy model object, since that object is probably not serializable and is tied to
+the session that queried it.
+
+Pass the minimal amount of data necessary to fetch or recreate any complex data within
+the task. Consider a task that will run when the logged in user asks for an archive of
+their data. The Flask request knows the logged in user, and has the user object queried
+from the database. It got that by querying the database for a given id, so the task can
+do the same thing. Pass the user's id rather than the user object.
+
+.. code-block:: python
 
-The ``your_application`` string has to point to your application's package
-or module that creates the ``celery`` object.
+    @shared_task
+    def generate_user_archive(user_id: str) -> None:
+        user = db.session.get(User, user_id)
+        ...
 
-Now that the worker is running, ``wait`` will return the result once the task
-is finished.
+    generate_user_archive.delay(current_user.id)

examples/celery/README.md

@@ -0,0 +1,27 @@
+Background Tasks with Celery
+============================
+
+This example shows how to configure Celery with Flask, how to set up an API for
+submitting tasks and polling results, and how to use that API with JavaScript. See
+[Flask's documentation about Celery](https://flask.palletsprojects.com/patterns/celery/).
+
+From this directory, create a virtualenv and install the application into it. Then run a
+Celery worker.
+
+```shell
+$ python3 -m venv .venv
+$ . ./.venv/bin/activate
+$ pip install -r requirements.txt && pip install -e .
+$ celery -A make_celery worker --loglevel INFO
+```
+
+In a separate terminal, activate the virtualenv and run the Flask development server.
+
+```shell
+$ . ./.venv/bin/activate
+$ flask -A task_app --debug run
+```
+
+Go to http://localhost:5000/ and use the forms to submit tasks. You can see the polling
+requests in the browser dev tools and the Flask logs. You can see the tasks submitting
+and completing in the Celery logs.

examples/celery/make_celery.py

@@ -0,0 +1,4 @@
+from task_app import create_app
+
+flask_app = create_app()
+celery_app = flask_app.extensions["celery"]

examples/celery/pyproject.toml

@@ -0,0 +1,11 @@
+[project]
+name = "flask-example-celery"
+version = "1.0.0"
+description = "Example Flask application with Celery background tasks."
+readme = "README.md"
+requires-python = ">=3.7"
+dependencies = ["flask>=2.2.2", "celery[redis]>=5.2.7"]
+
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"