Hi, I'm
Andrew Godwin
Django core developer
Senior Software Engineer at
Used to complain about migrations a lot
It's magic.
It's magic.
1 The Problem
The Web is changing.
WebSockets
Long-polling
WebRTC
WebSockets
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?
2 Loose Coupling
Not too tied to WebSockets
Not too tied to Django
Well-defined, minimal interfaces
Easy to swap out or rewrite
The Message Bus
HTTP Django
Server Project
Message Bus
WSock
Server
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
3 Concrete Ideas
Develop using concrete examples
WebSocket
connect
accept/reject
receive
send
disconnect
WebSocket
websocket.connect
websocket.receive
websocket.disconnect
websocket.send!abc1234
At-most-once
First In First Out
Backpressure via capacity
Not sticky
No guaranteed ordering
No serial processing
HTTP Django Django
& WS Worker Worker
Channel Layer
HTTP Django Django
& WS Worker 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 Channels
HTTP/WebSocket Server Django integration
asgi-redis asgi-ipc
Redis backend Local memory backend
asgiref
Shared code and libs
4 Django-ish
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
5 Beyond
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
