An Introduction to Channels

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An Introduction to Django Channels Scott Burns @scottsburns

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We’re hiring! Let’s talk after?

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WSGI • Web Server Gateway Interface • Deﬁnes the communication between applications/frameworks and servers. • PEP 333 — Dec. 2003 • Py3 addressed in PEP 3333 • Request→Response

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Django • “Batteries Included” Framework • Object-Relational Mapper • Templating • URL Routing • Signals, etc • Large 3rd-party ecosystem https://www.djangoproject.com/community/logos/

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Quick Demo

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Client (Browser, cURL, Native, etc) HTTP WSGI time Application Server

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WSGI Server (Gunicorn, uWSGI, etc) WSGI App Really… WSGI App WSGI App

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Client time Application HTTP 2

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Client time Application WebSockets …

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Challenges • Python async is improving quickly • Building a websocket-capable server is doable with autobahn, etc • How to actually build applications though?

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Challenges, cont. • Once you have servers with active web sockets, what next? • Message sockets on different machines? • Broadcast to many sockets (again, across many machines)? • Dead sockets? • Distributed systems are hard!

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Real application ? ?

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Channels • Adds a messaging layer between Django and interface servers • Moves the (difﬁcult, error prone) async code to the interface, letting application developers continue to write synchronous code (yay!)

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Interface Server (Daphne) ChannelLayer Worker Worker Worker Clients

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Glossary • Consumers: handler functions for messages • Analogous to Django views • Interface servers: async servers to which clients connect (and stay connected?) • channels.ChannelLayer: transport layer for moving messages between interfaces and consumers • These differ based on the deployment strategy

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Channel Named ﬁrst-in, ﬁrst-out task queue Group Named, broadcast-able sets of channels

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send(“channel_name”, {“type”: “message”}) receive_many([“channel_one”, “channel_two”]) group_add(“group”, “channel_name”) group_discard(“group”, “channel_name”) send_group(“group”, {“type”: “message”})

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HTTP View Request Response

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Messages Consumer Message Message Message

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$ pip install channels INSTALLED_APPS = [ …, channels, …, ]

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Let’s build a chat

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One room, all clients receive all messages

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One view to serve the page & assets

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Consume on… • websocket.connect: add the (response) channel to the chat Group. • websocket.receive: broadcast the message to connected sockets in the Group. • websocket.disconnect: remove the particular socket from the Group.

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Demo

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Deployments

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Interface Server (Daphne) ChannelLayer Worker Worker Worker Clients Interface Server (Daphne) Worker Worker Worker Machine 1 Machine 2

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Interface Server (Daphne) ChannelLayer Worker Worker Worker Clients Interface Server (Daphne) Worker Worker Worker Machine 1 Machine 2 ChannelLayer

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Interface ChannelLayer Wor Wor Wor ChannelLayer Interface ChannelLayer Wor Wor Wor Interface ChannelLayer Wor Wor Wor (RedisLocalChannelLayer)

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Philosophically • It’s not just about WebSockets • Generalized eventing for Django • Many Protocols • Multi-framework backends?

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Links • Andrew Godwin’s 2016 PyCon Talk • Channels docs (channels.readthedocs.org) are fabulous • http://www.aeracode.org/2016/6/16/philosophy- channels/

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Questions?