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115 lines
4.0 KiB
ReStructuredText
115 lines
4.0 KiB
ReStructuredText
In Short
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========
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What is Channels?
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-----------------
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Channels extends Django to add :ref:`a new layer <what-are-channels>`
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that allows two important features:
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* WebSocket handling, in a way very :ref:`similar to normal views <websocket-example>`
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* Background tasks, running in the same servers as the rest of Django
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It allows other things too, but these are the ones you'll use to start with.
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How?
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----
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It separates Django into two process types:
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* One that handles HTTP and WebSockets
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* One that runs views, websocket handlers and background tasks (*consumers*)
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They communicate via a protocol called :doc:`ASGI <asgi>`, which is similar
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to WSGI but runs over a network and allows for more protocol types.
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Channels does not introduce asyncio, gevent, or any other async code to
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your Django code; all of your business logic runs synchronously in a worker
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process or thread.
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I have to change how I run Django?
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----------------------------------
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No, all the new stuff is entirely optional. If you want it, however, you'll
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change from running Django under a WSGI server, to running:
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* An ASGI server, probably `Daphne <http://github.com/django/daphne/>`_
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* Django worker servers, using ``manage.py runworker``
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* Something to route ASGI requests over, like Redis.
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Even when you're running on Channels, it routes all HTTP requests to the Django
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view system by default, so it works like before.
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What else does Channels give me?
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--------------------------------
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Other features include:
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* Easy HTTP long-poll support for thousands of clients at once
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* Full session and auth support for WebSockets
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* Automatic user login for WebSockets based on site cookies
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* Built-in primitives for mass triggering of events (chat, live blogs, etc.)
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* Zero-downtime deployment with browsers paused while new workers spin up
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* Optional low-level HTTP control on a per-URL basis
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* Extendability to other protocols or event sources (e.g. WebRTC, raw UDP, SMS)
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Does it scale?
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--------------
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Yes, you can run any number of *protocol servers* (ones that serve HTTP
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and WebSockets) and *worker servers* (ones that run your Django code) to
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fit your use case.
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The ASGI spec allows a number of different *channel layers* to be plugged in
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between these two components, with different performance characteristics, and
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it's designed to allow both easy sharding as well as the ability to run
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separate clusters with their own protocol and worker servers.
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Why doesn't it use my favourite message queue?
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----------------------------------------------
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Channels is deliberately designed to prefer low latency (goal is a few milliseconds)
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and high throughput over guaranteed delivery, which doesn't match some
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message queue designs.
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Some features, like :ref:`guaranteed ordering of messages <enforcing-ordering>`,
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are opt-in as they incur a performance hit, but make it more message queue like.
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Do I need to worry about making all my code async-friendly?
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-----------------------------------------------------------
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No, all your code runs synchronously without any sockets or event loops to
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block. You can use async code within a Django view or channel consumer if you
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like - for example, to fetch lots of URLs in parallel - but it doesn't
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affect the overall deployed site.
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What version of Django does it work with?
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-----------------------------------------
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You can install Channels as a library for Django >= 1.8. It has a few
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extra dependencies, but these will all be installed if you use ``pip``.
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Official project
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----------------
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Channels is not in the Django core as initially planned, but it's
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an official Django project since September 2016. More information about Channels
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being adopted as an official project are available on the
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`Django blog <https://www.djangoproject.com/weblog/2016/sep/09/channels-adopted-official-django-project/>`_.
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What do I read next?
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--------------------
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Start off by reading about the :doc:`concepts underlying Channels <concepts>`,
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and then move on to read our example-laden :doc:`Getting Started guide <getting-started>`.
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