Introduction to Django Channels - ConFoo Vancouver 2016

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Eric Holscher ConFoo Vancouver December 7, 2016 Python & Django Go Async An introduction to Django Channels

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Who am I • Co-Founder of Read the Docs • Co-Founder of Write the Docs • Longtime member of the Django community

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Today • Understand the architecture of Channels • Understand the tradeoﬀs required • Know if it ﬁts your use case and organization • Know how to get started

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A bit of history first

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Django was created in 2005. Web was static pages rendered on the server side.

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Channels takes us from 2005 to 2015

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Channels adds ability for Django to handle H P/2, websockets, and other things outside a normal request/response cycle

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No content

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Architecture

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Channels

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Current Status • 1.0 imminent • Lives outside of Django itself • An oﬃcial project of the Django team • Will perhaps merge into core for Django 2.0

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A few different projects • ASGI • Daphne • Django Channels

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Terms • Interface Server • Channels • Messages • Routes • Consumers • Groups

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Interface Server • Holds connections open to the world • Speaks both H P and Websocket • Passes messages along the channels, doesn’t actually process anything

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Channels • First In First Out • At most once delivery • No guaranteed ordering • Messages expire a er a set time • ‘websocket.connect’, ‘http.request’, ‘chat.message’

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Messages • Send across the channels • Can have arbitrary content based on diﬀering protocols • Usually JSON

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# Sent on a ‘http.request’ channel { 'reply_channel': 'http.response!g23vD2x5', 'method': 'GET', 'http_version': '2', 'path': '/chat/socket/', 'query_string': 'foo=bar', 'headers': [['cookie', 'abcdef...']], } Messages

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Routing • Similar to Django URLConf • Allows for matching arbitrary things on the incoming messages • Can match against `path` on H P requests, or `address` on email sending

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routing = [ route( "websocket.receive", consumers.chat_receive, path=r"^/chat/socket/$", ), route( "email.receive", comment_response, address=r".*@example.com$" ) include("stats.routing", path="^/stats/"), ] Routing

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Consumer • Similar to a Django View • Takes a `message` and… does something

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# Consumer def ws_connect(message): message.reply_channel.send(message['text']) Consumer

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Groups • Contain sets of channels • Only allow add/remove/send — because distributed systems • Useful for keeping state and sending messages across sets of channels

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Architecture

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Basic Examples

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# Routing channel_routing = { 'http.request': consumers.http_consumer } # Message { 'reply_channel': 'http.response!g23vD2x5', 'method': 'GET', 'http_version': '2', 'path': '/chat/socket/', 'query_string': 'foo=bar', 'headers': [['cookie', 'abcdef...']], } # Consumer def http_consumer(message): response = HttpResponse('Hey there') message.reply_channel.send(response.channel_encode()) HTTP Request

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# Routing channel_routing = { 'websocket.connect': consumers.ws_connect, 'websocket.receive': consumers.ws_receive, 'websocket.disconnect': consumers.ws_disconnect, } # Message { 'text': 'Hello, world!', 'path': '/chat/socket/', 'reply_channel': 'websocket.send!9m12in2p', } # Consumer def ws_connect(message): message.reply_channel.send(message['text']) Websocket Request

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Major Features

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Authentication & Sessions • Slightly tricky • ‘websocket.connect’ has H P cookies & auth • There are two sessions, “http_sessions” & “channel_sessions”

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@channel_session_user_from_http def ws_add(message): Group( "chat-%s" % message.user.username[0] ).add(message.reply_channel) @channel_session_user def ws_message(message): Group("chat-%s" % message.user.username[0]).send({ "text": message['text'], }) Authentication & Sessions

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Generic Consumers • Similar to Generic Views • Abstracts common use cases for simpler logic • Ships with Websocket & JSON generic consumers currently

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class MyConsumer(JsonWebsocketConsumer): def connection_groups(self, **kwargs): return ["test"] def receive(self, content, **kwargs): """ Called when a message is received with decoded JSON content """ # Simple echo self.send(content) Generic Consumers

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MultiPlexing • Allows you to send multiple data types over a single connection • Uses ‘stream’ and ‘payload’ keys • Sends the payload to the mapped stream

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class Demultiplexer(WebsocketDemultiplexer): mapping = { "intval": "binding.intval", "stats": "internal.stats", } MultiPlexing

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Data Binding • Lets you keep client & server data models in sync • Update each side on change to either side in realtime • Uses Django serialization and multiplexing as standard wire format

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# Routing channel_routing = [ route_class(Demultiplexer, path="^/binding/"), route("binding.intval", IntegerValueBinding.consumer), ] # Consumer class IntegerValueBinding(WebsocketBinding): model = IntegerValue stream = "intval" fields = ["name", "value"] def group_names(self, instance, action): return ["intval-updates"] def has_permission(self, user, action, pk): return True Data Binding

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# Message (Client) { "payload": { "action": "update", "data": { "value": "56" }, "pk": "2" }, "stream": "intval" } # Message (Server) { "payload": { "action": "update", "data": { "name": "Cool", "value": 56 }, "model": "values.integervalue", "pk": 2 }, "stream": "intval" } Data Binding

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Additional niceties • Testing helpers • Optional guaranteed ordering • Zero Downtime Deployments

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Deploying

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# settings.py CHANNEL_LAYERS = { "default": { "BACKEND": "asgi_redis.RedisChannelLayer", "CONFIG": { "hosts": [("localhost", 6379)], }, "ROUTING": "myproject.routing.channel_routing", }, } # asgi.py import os from channels.asgi import get_channel_layer os.environ.setdefault("DJANGO_SETTINGS_MODULE", "my_project.settings") channel_layer = get_channel_layer() Configuration

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You need a Channel Layer, an Interface Server, and a pool of Workers

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# Run a Channel Layer redis-server # Run an Interface Server daphne my_project.asgi:channel_layer # Run the Workers ./manage.py runworker All services

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You should run more than 1 worker, as it’s single threaded

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Can limit the channels workers listen on, to split up long running and short running processes

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Websockets require a diﬀerent way of thinking about architecture. Channels embraces this

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Slackbot

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Usage of the same channel for Slack and IRC

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{ 'stream': ‘chat.mesage’, 'payload': { 'channel': 'general', 'user': 'eric', 'text', '!post general echo', } } Message

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Demo

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Recap

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Channels & ASGI is a swiss army knife of async

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Not just for Django, but a lot of the neat bits on top are

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Still relatively new, but deﬁnitely the future of realtime with Django

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Next steps are to get some real deployments in the world

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Resources • https://channels.readthedocs.io/ • https://github.com/andrewgodwin/ channels-examples/

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Image Credits • https://blog.heroku.com/ in_deep_with_django_channels_the_fut ure_of_real_time_apps_in_django

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Questions? • @ericholscher • [email protected] • Come talk to me around the conference