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Channels (Under The Hood)
Andrew Godwin
November 03, 2016
Programming
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6.8k
Channels (Under The Hood)
A talk I gave at Django Under The Hood 2016.
Andrew Godwin
November 03, 2016
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Transcript
None
Andrew Godwin Hi, I'm Django core developer Senior Software Engineer
at Used to complain about migrations a lot
It's magic.
It's magic.
The Problem 1
The Web is changing.
WebSockets
WebSockets WebRTC Long-polling MQTT Server-Sent Events
Python is synchronous. Django is synchronous.
Synchronous code is easier to write. Single-process async is not
enough.
Proven design pattern Not too hard to reason about
What could fit these constraints?
Loose Coupling 2
Not too tied to WebSockets Not too tied to Django
Well-defined, minimal interfaces
Easy to swap out or rewrite
The Message Bus HTTP Server Message Bus WSock Server Django
Project
What do you send? How do you send it?
ASGI
nonblocking send blocking receive add to group discard from group
send to group
JSON-compatible, dictionary-based messages onto named channels
Concrete Ideas 3
Develop using concrete examples
WebSocket connect receive disconnect accept/reject send
WebSocket websocket.connect websocket.send!abc1234 websocket.receive websocket.disconnect
At-most-once First In First Out Backpressure via capacity Not sticky
No guaranteed ordering No serial processing
HTTP & WS Channel Layer Django Worker HTTP & WS
Django Worker Django Worker Django Worker
{ "text": "Hello, world!", "path": "/chat/socket/", "reply_channel": "websocket.send!9m12in2p", }
Developed and spec'd HTTP WebSocket Rough drafts IRC Email Slack
Please, no. Minecraft Mainframe Terminal
{ "reply_channel": "http.response!g23vD2x5", "method": "GET", "http_version": "2", "path": "/chat/socket/", "query_string":
"foo=bar", "headers": [["cookie", "abcdef..."]], }
At-most-once First In First Out Backpressure via capacity Not sticky
No guaranteed ordering No serial processing
At-most-once First In First Out Backpressure via capacity Not sticky
No guaranteed ordering No serial processing
"order" key on receive messages Connection acceptance
Daphne HTTP/WebSocket Server Channels Django integration asgi-redis Redis backend asgi-ipc
Local memory backend asgiref Shared code and libs
Django-ish 4
It can take several tries to get a nice API.
Consumers based on Views Callable that takes an object Decorators
for functionality Class-based generics
@channel_session def chat_receive(message): name = message.channel_session["name"] message.reply_channel.send({"text": "OK"}) Group("chat").send({ "text":
"%s: %s" % (name, message["text"]), }) Message.objects.create( name=name, content=message["text"], )
Routing based on URLs List of regex-based matches Includes with
prefix stripping on paths More standardised interface
routing = [ route( "websocket.receive", consumers.chat_receive, path=r"^/chat/socket/$", ), include("stats.routing", path="^/stats/"),
route_class(ConsumerClass, path="^/v1/"), ]
Sessions are the only state Sessions hang off reply channels
not cookies Uses same sessions backends Available on the consumer's argument Can also access long-term cookie sessions
@enforce_ordering def receive_consumer(message): Log.objects.create(...)
session = session_for_reply_channel( message.reply_channel.name ) if not session.exists(session.session_key): try: session.save(must_create=True)
except CreateError: # Session wasn't unique raise ConsumeLater() message.channel_session = session
No Middleware New-style middleware half works No ability to capture
sends Decorators replace most cases
View/HTTP Django still there Can intermingle or just use one
type View system is just a consumer now
def view_consumer(message): replies = AsgiHandler()(message) for reply in replies: while
True: try: message.reply_channel.send(reply) except ChannelFull: time.sleep(0.05) else: break
Signals and commands runserver works as expected Signals for handling
lifecycle staticfiles configured for development
Beyond 5
Generalised async communication
Service messaging Security/CPU separation Sync & Async / Py2 &
Py3
Diversity of implementations More web servers More channel layers
More optimisation More efficient bulk sends Less network traffic on
receive
More maintainers More viewpoints, more time
1.0 coming soon Stable APIs for everything except binding
Thanks. Andrew Godwin @andrewgodwin channels.readthedocs.io github.com/andrewgodwin/channels-examples