Channels (Under The Hood)

Transcript

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   None
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   Andrew Godwin Hi, I'm Django core developer Senior Software Engineer

   at Used to complain about migrations a lot
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   It's magic.

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   It's magic.

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   The Problem 1

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   The Web is changing.

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   WebSockets

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   WebSockets WebRTC Long-polling MQTT Server-Sent Events

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   Python is synchronous. Django is synchronous.

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    Synchronous code is easier to write. Single-process async is not

    enough.
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    Proven design pattern Not too hard to reason about

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    What could fit these constraints?

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    Loose Coupling 2

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    Not too tied to WebSockets Not too tied to Django

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    Well-defined, minimal interfaces

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    Easy to swap out or rewrite

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    The Message Bus HTTP Server Message Bus WSock Server Django

    Project
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    What do you send? How do you send it?

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    ASGI

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    nonblocking send blocking receive add to group discard from group

    send to group
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    JSON-compatible, dictionary-based messages onto named channels

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    Concrete Ideas 3

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    Develop using concrete examples

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    WebSocket connect receive disconnect accept/reject send

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    WebSocket websocket.connect websocket.send!abc1234 websocket.receive websocket.disconnect

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    At-most-once First In First Out Backpressure via capacity Not sticky

    No guaranteed ordering No serial processing
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    HTTP & WS Channel Layer Django Worker HTTP & WS

    Django Worker Django Worker Django Worker
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    { "text": "Hello, world!", "path": "/chat/socket/", "reply_channel": "websocket.send!9m12in2p", }

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    Developed and spec'd HTTP WebSocket Rough drafts IRC Email Slack

    Please, no. Minecraft Mainframe Terminal
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    { "reply_channel": "http.response!g23vD2x5", "method": "GET", "http_version": "2", "path": "/chat/socket/", "query_string":

    "foo=bar", "headers": [["cookie", "abcdef..."]], }
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    At-most-once First In First Out Backpressure via capacity Not sticky

    No guaranteed ordering No serial processing
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    At-most-once First In First Out Backpressure via capacity Not sticky

    No guaranteed ordering No serial processing
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    "order" key on receive messages Connection acceptance

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    Daphne HTTP/WebSocket Server Channels Django integration asgi-redis Redis backend asgi-ipc

    Local memory backend asgiref Shared code and libs
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    Django-ish 4

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    It can take several tries to get a nice API.

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    Consumers based on Views Callable that takes an object Decorators

    for functionality Class-based generics
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    @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"], )
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    Routing based on URLs List of regex-based matches Includes with

    prefix stripping on paths More standardised interface
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    routing = [ route( "websocket.receive", consumers.chat_receive, path=r"^/chat/socket/$", ), include("stats.routing", path="^/stats/"),

    route_class(ConsumerClass, path="^/v1/"), ]
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    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
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    @enforce_ordering def receive_consumer(message): Log.objects.create(...)

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    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
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    No Middleware New-style middleware half works No ability to capture

    sends Decorators replace most cases
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    View/HTTP Django still there Can intermingle or just use one

    type View system is just a consumer now
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    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
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    Signals and commands runserver works as expected Signals for handling

    lifecycle staticfiles configured for development
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    Beyond 5

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    Generalised async communication

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    Service messaging Security/CPU separation Sync & Async / Py2 &

    Py3
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    Diversity of implementations More web servers More channel layers

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    More optimisation More efficient bulk sends Less network traffic on

    receive
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    More maintainers More viewpoints, more time

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    1.0 coming soon Stable APIs for everything except binding

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    Thanks. Andrew Godwin @andrewgodwin channels.readthedocs.io github.com/andrewgodwin/channels-examples