doc: add a pattern doc for Dramatiq

docs/patterns/dramatiq.rst

@@ -0,0 +1,83 @@
+Dramatiq Background Tasks
+=========================
+
+Often when processing web requests there are things that can be moved
+out of the request-response cylce in order to conserve time.  An
+example of such a workload is sending an e-mail: not only can it take
+a while, but it can fail so it makes sense to move that sort of
+operation to a background queue where it can be processed
+asynchronously and retried if necessary.
+
+Dramatiq_ is a powerful distributed task processing library for
+Python 3.  This guide will show you how to use Dramatiq and Flask
+together.  We assume you've already read the Dramatiq `user guide`_.
+
+
+Installation
+------------
+
+Dramatiq is a separate Python package and it comes in two flavors::
+
+    $ pip install dramatiq[rabbitmq,watch]
+
+installs it with support for RabbitMQ as a task broker, and::
+
+    $ pip install dramatiq[redis,watch]
+
+installs it with support for Redis as a task broker.
+
+
+Configuration
+-------------
+
+There's nothing Flask-specific to set up, but you do need to tell
+Dramatiq what broker it should use.  For this example we're going to
+use RabbitMQ, so create an instance of ``RabbitmqBroker`` in a module
+called ``tasks.py`` and then set it as the default broker::
+
+    import dramatiq
+    import os
+
+    from dramatiq.brokers.rabbitmq import RabbitmqBroker
+
+    rabbitmq_url = os.getenv("RABBITMQ_URL", "amqp://127.0.0.1:5672")
+    broker = RabbitmqBroker(url=rabbitmq_url)
+    dramatiq.set_broker(broker)
+
+After that's done you can start writing actors.  In that same module::
+
+    @dramatiq.actor
+    def add(x, y):
+        print(f"The sum of {x} and {y} is {x + y}.")
+
+You can send that actor a message from your view by calling its
+``send`` method::
+
+    from . import tasks
+
+    @app.route("/", methods=["POST"])
+    def compute_sum():
+        tasks.add.send(request.form["x"], request.form["y"])
+        return "Deferred", 202
+
+
+Running workers
+---------------
+
+Since Dramatiq actors process message asynchronously, you'll have to
+run a separate set of processes -- called workers -- to actually
+process all the messages in the queue.  To do that, just invoke the
+``dramatiq`` command pointing it to where your tasks are defined::
+
+    $ env PYTHONPATH=. dramatiq tasks
+
+
+Next steps
+----------
+
+Check out `this repo`_ for an app that uses Dramatiq with Flask.
+
+
+.. _Dramatiq: https://dramatiq.io
+.. _user guide: https://dramatiq.io/guide.html
+.. _this repo: https://github.com/Bogdanp/flask_dramatiq_example

docs/patterns/index.rst

@@ -41,4 +41,5 @@ Snippet Archives <http://flask.pocoo.org/snippets/>`_.
    methodoverrides
    requestchecksum
    celery
+   dramatiq
    subclassing