Application servers such as Django and Ruby-on-Rails have been developed without intention to create long living connections. Therefore these frameworks are not a good fit for web applications, which shall react on asynchronous events initiated by the server. One feasible solution for clients wishing to be notified for events is to continuously poll the server using an XMLHttpRequest (Ajax). This however produces a lot of traffic, and depending on the granularity of the polling interval, it is not a viable solution for real time events such as chat applications or browser based multiplayer games.
Web application written in Python usually use WSGI as the communication layer between the webserver and themselves. WSGI is a stateless protocol which defines how to handle requests and making responses in a simple way abstracted from the HTTP protocol, but by design it does not support non-blocking requests.
The WSGI protocol can not support websockets¶
In Django, the web server accepts an incoming request, sets up a WSGI dictionary which then is passed to the application server. There the HTTP headers and the payload is created and immediately afterwards the request is finished and flushed to the client. This processing typically requires a few dozen milliseconds. The throughput, such a server can handle, is the average response time multiplied by the number of concurrent workers. Each worker requires its own thread/process and a good rule of thumb is to configure not more than twice as many workers as the number of cores available on that host. Otherwise you will see a decrease in overall performance, caused by too many context switches, for which the scheduler of the operating system is responsible.
Due to this workflow, it is almost impossible to add support for long term connections, such as websockets, on top of the WSGI protocol specification. Therefore most websocket implementations go for another approach. The websocket connection is controlled by a service running side by side with the default application server. Here, a webserver with support for long term connections, dispatches the requests from the clients.
A webserver able to dispatch websocket requests is the NGiNX server. Normal requests are sent to Django using the WSGI protocol, whereas the long living websocket connections are passed over to a special service responsible only for that.
Here, Django communicates with Node.JS using a RESTful API. This however is hard to maintain because it pulls in two completely different technologies. In alternative proposals, other Python based asynchronous event frameworks such as Tornado or Twisted are used. But they all look like makeshift solutions, since one has to run a second framework side by side with Django. This makes the project dependent on another infrastructure and thus harder to maintain. Moreover, having to run two concurrent frameworks can be quite embarrassing during application development, specially while debugging code.
While searching for a simpler solution, I found out that uWSGI offers websockets right out of the box. With Redis as a message queue, and a few lines of Python code, one can bidirectionally communicate with any WSGI based framework, for instance Django. Of course, here it also is prohibitive to create a new thread for each open websocket connection. Therefore that part of the code runs in one single thread/process for all open connections in a cooperative concurrency mode using the excellent gevent and greenlet libraries.
This approach has some advantages:
- It is simpler to implement.
- The asynchronous I/O loop handling websockets can run
- inside Django with
./manage.py runserver, giving full debugging control.
- as a stand alone HTTP server, using uWSGI.
- using NGiNX or Apache (>= 2.4) as proxy in two decoupled loops, one for WSGI and one for websocket HTTP in front of two separate uWSGI workers.
- inside Django with
- The whole Django API is available in this loop, provided that no blocking calls are made. Therefore the websocket code can access the Django configuration, the user and the session cache, etc.
Using Redis as a message queue¶
One might argue that all this is not as simple, since an additional service – the Redis data server – must run side by side with Django. Websockets are bidirectional but their normal use case is to trigger server initiated events on the client. Although the other direction is possible, it can be handled much easier using Ajax – adding an additional TCP/IP handshake.
Here, the only “stay in touch with the client” is the file descriptor attached to the websocket. And since we speak about thousands of open connections, the footprint in terms of memory and CPU resources must be brought down to a minimum. In this implementation, only one open file handle is required for each open websocket connection.
Productive webservers require some kind of session store anyway. This can be a memcached or a Redis data server. Therefore, such a service must run anyway and if we can choose between one of them, we shall use one with integrated message queuing support. When using Redis for caching and as a session store, we practically get the message queue for free.
One of the nice features of Redis is its infinite scalability. If one Redis server can’t handle its workload, interconnect it with another one and all events and messages are mirrored across this network. Since django-websocket-redis can be deployed multiple times and as self-contained Django applications, this configuration can scale infinitely, just interconnect the Redis servers to each other.
On the main entry point of your site, add a loadbalancer capable of proxying the websocket protocol. This can be any OSI level 4 loadbalancer such as the Linux Virtual Server project, or if you prefer OSI level 7, the excellent HAProxy.