WSGI, Twisted and Server Sent Events

Old-school web applications were easy to create. Big powerful frameworks like Django give you a lot of tools you can leverage. One weak point of all those WSGI framework is that they didn't integrate well with anything that broke outside the usual request-response cycle.

The usual approach nowadays is to use WebSockets for real-time communication between browser clients and web servers. The usual way to do that would be to use a server capable of handling many concurrent connections, and use a message bus from the WSGI app to communicate to that service. That is a lot of moving parts.

In my use case where I build a lot of intranet applications, deploying and maintaining all this infrastructure is a very big burden, so the result is usually to not even explore this kind of functionality.

However, given that I deploy on Twisted, I wanted to explore what kind of cool things I could build on it.

Enter SSE

Server-Sent Events aren't that new - they have just been shadowed by WebSockets. They are a simple data format that is send from the server to the client via a plain HTTP connection. The Javascript API is quite simple, and it even handles retries for you. It's compatible with a lot of recent browsers, but I haven't really done a lot of research on it.

Sample Code

Here is a very simple WSGI app (using It just has a form and a form POST handler.

And a basic twisted server to run it:

And a SSE-savvy twisted.web resource:

And a very simple index.html

How it works

The WSGI app just calls some Python code. Through crochet we ensure that it gets back a useful result (though in this case, we just throw it away). We use a plain POST to send data to the server. Converting that to an AJAX request is left as an exercise to the reader. The SSE handler is a singleton that keeps track of all the listeners that are connected to it, and broadcasts messages to it.

Does it scale?

It should! I have no experience running Twisted Web under heavy load but it's more than enough for intranet-style apps (even when I have 50 machines hitting some API endpoints quite frequently). If someone wants to run some testing, please get in touch.

What's next?

I would like to make this a bit more reusable, with some better discovery than the current "inject a global function into the namespace". Also, Django integration is something I'd like to investigate. And why not try if the same approach can be extended to web sockets as well?