README

This experiment depends on internals of ``aiohttp`` which tend to change often and increase in complexity. It isn't maintained any longer.

django-c10k-demo is an experimental playground for high concurrency in Django with WebSockets.

It was originally written to handle 10 000 concurrent real-time connections to Django, hence the name.

It combines several interesting concepts: the C10k problem, the WebSocket protocol, the Django web framework, and Python's built-in asynchronous IO support.

Running the code

Prerequisites

• Install Python 3.4 or 3.3.
• Install aiohttp, Django, and websockets, most likely with virtualenv and pip. If you're using Python 3.3, install asyncio.
• Clone this repository.
• Configure your OS to allow lots of file descriptors. On OS X: sudo sysctl -w kern.maxfiles=40960 kern.maxfilesperproc=20480
• Open two shells and bump their file descriptor limit: ulimit -n 10240

Game of Life demo

This demo is a distributed Game of Life. One client manages the life of one cell. Clients connect to a server through a WebSocket and register to receive updates from their neighbors. The server coordinates the startup sequence and relays messages between clients, but it doesn't know anything about the rules of the game; all the logic is handled by the clients!

• In the first shell, start the server: python manage.py runserver
• In the second shell, start the workers: python manage.py gameoflife
• In a browser, go to http://localhost:8000/

gameoflife shouldn't display anything. runserver should display an increasing number of workers connected and then an increasing number of workers subscribed.

The page in the browser registers to receive updates from all clients, and updates in real time as soon as the workers start running. Alive cells are dark, dead cells are light. Their hue shifts slightly at each step to show how the grid updates.

gameoflife accepts a number of options to configure the game:

• The size of the grid is 32. You can change it with -s.
• The initial state is random with one cell alive out of four on average. You can load a pattern from a file with -p. See gameoflife/patterns/ for some common examples.
• When a pattern is provided, it's centered on the grid. You can disable this behavior with -C.
• The grid is cyclic: the left side is connected to the right side and the top to the bottom. You can disable this behavior with -W, for example to test guns.
• The workers run forever, unless you specify a number of steps with -n.
• The workers make at most one step per second — this only matters on small grids since the game won't run that fast on larger grids. You can adjust the speed limit with -l.

C10k demo

This is the original demo in this project. It handles 10 000 concurrent real- time connections to Django.

• In the first shell, start the server: python manage.py runserver
• In the second shell, start the clients: python manage.py testecho

runserver shouldn't display anything and testecho should show the number of connections, peaking at 10000 clients are connected!.

The connections are established over a period of two minutes. Once connected, each client repeats the following sequence three times: wait one minute, send a message, and read the reply of the server. Clients also receive a welcome and a goodbye message from the server. The entire demo takes five minutes if your system is fast enough.

If you don't reach 10 000 connections, it means that some clients finish their sequence and disconnect before all the clients are connected, because your system is too slow. If you see exceptions, it means that your OS isn't tuned correctly for such benchmarks. Decreasing CLIENTS or increasing DELAY in testecho may help in both cases.

Under the hood

Here are the underlying components in no particular order, with some hints on their quality and reusability.

WebSocket API for Django

Here's an example of a WebSocket echo server in Django:

from c10ktools.http import websocket

@websocket
def handler(ws):
    yield from ws.send((yield from ws.recv()))

WebSocket handlers are hooked in the URLconf like regular HTTP views. Arguments can be captured in the URLconf and passed to the handlers.

This doesn't allow sharing an URL between a regular HTTP view and a WebSocket handler, but I'm happy with this limitation as it's probably a good practice to keep them separate anyway.

Inside a WebSocket handler, you can use yield from ws.recv() and yield from ws.send() freely. You can also call yield from ws.send() outside the handler.

The @websocket decorator should only be applied to coroutines. It takes care of closing the WebSocket connection when the handler terminates.

Hook for the upgrade to WebSocket

The API described above requires the upgrade from HTTP to WebSocket to happen after Django's URL dispatcher has routed the request to a view. As a consequence, the upgrade must be performed within the framework of WSGI.

PEP 3333 predates real-time on the web and PEP 3156 doesn't propose to update it. This point might be addressed by a future version of the standard (PEP 3356 anyone?) In the meantime our only choice is to bastardize WSGI, steering away from compliance — sorry Graham.

The WebSocket opening handshake is completed by sending a HTTP response. This is achieved with WSGI, but it isn't compliant because the response includes hop-by-hop headers, Upgrade and Connection.

The switch to the WebSocket protocol is performed in close(). In asyncio terms, the transport is disconnected for the HTTP protocol and reconnected to the WebSocket protocol. Then a task is started to run the WebSocket handler and close the connection when it terminates. This design is very debatable:

• This isn't an intended use case for the close() method.
• The protocol transplant relies on non-standard variables in environ.
• It abuses private APIs of asyncio and of aiohttp which aren't quite stable.

Asynchronous development server

django-c10k-demo takes advantage of aiohttp's WSGI support to adapt Django's built-in developement server to run on top of asyncio.

This component can be used independently by adding the 'c10ktools' application to INSTALLED_APPS. It monkey-patches the django-admin.py runserver command to run on top of the asyncio event loop.

Asynchronous production server

django-c10k-demo works with aiohttp's gunicorn worker class:

$ gunicorn -k aiohttp.worker.AsyncGunicornWorker c10kdemo.wsgi

Of course, this stack is experimental. It's unlikely to ever become "production-ready". Use it at your own risk!