A simple SSL enabled Comet style Pub/Sub server with Node.js
I’ve been meaning to have a play with Node.js for a while, but up until now I haven’t had a good reason. Running through a ‘Hello World’ example is all well and good, but I’m the kind of person who actually needs to do something when learning a new technology; otherwise I’ll just pick it up for an afternoon and then put it down and forget about it.
So I decided to try writing a simple SSL enabled Comet style pub/sub server (for reasons that may become apparent in future posts) and it turned out to be really easy; taking just over 100 lines of code (including comments and liberal spacing).
The complete code’s at the bottom of this post, so if that’s what your after, feel free to skip ahead.
Otherwise, I’m just going to quickly run through some of the steps it took me to get there:
1. Learning the basics of Node.js
For this I turned to the The Node Beginner Book, which I’d really recommend. It’s a nice advanced introduction for developers familiar with at least one object-orientated programming language and completely new to Node.js. It skips over the simple stuff, like data types and basic programming flows, and takes you through developing ‘a complete web application which allows the users of this application to view web pages and upload files’; introducing a lot of the important Node.js concepts along the way.
2. Figuring out how to implement SSL with Node.js
With a basic understanding of Node.js under my belt, I then decided to approach what I thought would be the most tricky part: adding SSL to the server. I’ve had issues with this before in different languages and so was expecting some pain; however it was pretty simple.
It turned out the most tricky part was generating the certificates and this was still pretty easy. All I needed to do was install openssl and then issue the following command line commands:
One slight gotcha was that, as a Windows user, the second command (openssl req…) kept failing for me with the error message: “Unable to load config info from /usr/local/ssl/openssl.cnf”. A quick tip pointed out that all you need to do to solve this is specify the config path yourself, by using -config. E.g.
3. Figuring out how to do Comet with Node.js
Again it turns out this is really simple. Simple (at least for the moment) seems to be a word I’m associating a lot with Node.js. To implement a long polling Comet approach, the two key elements appear to be:
Set it so that the incoming connection doesn’t expire - by calling req.connection.setTimeout(0);
Store the response objects for the incoming connections somewhere and then use them to send the data later on.
Here’s a relatively simple example (based on one I found here) that causes each request to ‘/comet’ to wait up to 5 seconds before it gets a reply:
Here I’ve used a repeating method (called every 5 seconds using setTimeout(...)) to trigger sending the data; however in a real implementation this would be triggered by some kind of event (e.g. a publish being received).
4. Putting it all together
The final step was pulling all this together to create my pub/sub server; allowing applications to:
Subscribe - by sending a GET request to '/s?t=<topic>'. The connection will then remain open until a publish for that topic comes in, at which point the message is sent and the connection closed.
Publish - by sending a POST request to '/p?t=<topic>' with the content of the message as the body of the post, encoded in utf-8.
Here’s the full code, including copious comments, so hopefully you can understand what I’ve done:
Now, this is my first piece of Node programming, so please be gentle; however I don’t think I’ve done anything too stupid. First of all, it works and does what I want. Secondly, I think it should be relatively scalable (though I’ve not tested this). And thirdly, due to the single threaded nature of Node.js (Yay!) there shouldn’t be any real concurrency issues.
The one thing I am missing, is some kind of logic to purge dead connections, but I guess you can’t have everything :)