TAKING Andrew Godwin @andrewgodwin CHANNELS ASYNC

Hi, I’m Andrew Godwin • Django core team member • Senior Software Engineer at • Does network programming for "fun"

2015: "django-onair"

2015: Channels 0.1

2017: Channels 1.0

Python 2.7 compatible Twisted for webserver Synchronous Django code

Webserver (Twisted) Django (Sync worker process) Clients Channel Layer (Redis or other) Webserver (Twisted) Django (Sync worker process)

Too many moving pieces No asyncio support Easy to shoot yourself in the foot

I was wrong.

2018: Channels 2.0

Asyncio-native Python 3.5 and up Supports async coroutines and sync threads

Webserver (Twisted) Django (Sync worker process) Clients Webserver (Twisted) Django (Sync worker process) Channel Layer (Redis or other)

75% rewrite

Django had to be partially async

WSGI/ASGI Handler (request translator) URL Routing (path to view mapping) Django Middleware (auth, sessions, etc.) Views (logic and presentation) ORM (database kerfuffling)

WSGI/ASGI Handler (request translator) URL Routing (path to view mapping) Django Middleware (auth, sessions, etc.) Views (logic and presentation) ORM (database kerfuffling) ASGI Routing (path/protocol/etc. mapping) ASGI Middleware (auth, sessions, etc.) Consumers (logic and presentation) synchronous

Async-native most of the way

But how?

sync_to_async async_to_sync

sync_to_async async_to_sync

Synchronous code has to run in threads ThreadPoolExecutor does most of the hard work

loop = asyncio.get_event_loop() future = loop.run_in_executor( self.threadpool, func, ) return await future

Calling the ORM Rendering templates Handing off to Django views

sync_to_async async_to_sync

Async code runs on the event loop We need to go find it! Or make our own.

# Make a future for the return information call_result = Future() # Use call_soon_threadsafe to schedule a synchronous callback on the # main event loop's thread if not (self.main_event_loop and self.main_event_loop.is_running()): # Make our own event loop and run inside that. loop = asyncio.new_event_loop() asyncio.set_event_loop(loop) try: loop.run_until_complete(self.main_wrap(args, kwargs, call_result)) finally: try: if hasattr(loop, "shutdown_asyncgens"): loop.run_until_complete(loop.shutdown_asyncgens()) finally: loop.close() asyncio.set_event_loop(self.main_event_loop) else: self.main_event_loop.call_soon_threadsafe( self.main_event_loop.create_task, self.main_wrap( args, kwargs, call_result, ), ) # Wait for results from the future. return call_result.result()

Make a Future Jump to the main thread and add the coroutine Tie the coroutine's end to triggering the Future Block the thread on the Future

Lets us provide async-native APIs And still let synchronous code use them

Both async and sync code are useful Channels lets you write your code as both

Async interfaces are separate to sync You can't provide both through one API

bit.ly/py-async-simp https://www.aeracode.org/2018/02/19/python-async-simplified/

ASGI Handler (ASGI-to-request translator) URL Routing (path to view mapping) Django Middleware (auth, sessions, etc.) Views (logic and presentation) ORM (database kerfuffling) ASGI Routing (path/protocol/etc. mapping) ASGI Middleware (auth, sessions, etc.) Consumers (logic and presentation) synchronous

How to make WSGI async?

ASGI: It's like WSGI but with an A in it Also asyncio and stuff like that

class Application: def __init__(self, scope): ... async def __call__(self, receive, send): event = await receive() ... await send(data)

"Turtles all the way down" Routing is an ASGI app. Middleware is too.

What does this mean for Django?

How much can we make async? Can we keep sync compatability?

What does an async ORM look like? Is it even a sensible endeavour?

Do we need to replace WSGI? Or does nobody want long-polling or websockets?

Multiple servers (daphne, uvicorn) Multiple frameworks (Django, …?)

Do we want to have everyone writing async? What's the balance? How do we be flexible enough?

What is Django?

Thanks. Andrew Godwin @andrewgodwin aeracode.org