Monday, September 22, 2008

Daemons in Ruby on Rails applications

I have created a Rails plug-in, daemonz, that automatically starts and stops the daemons associated with an application, without too much configuration. It works in both production and developer mode. Read below if you care about the motivation behind not using an existing tool.

The Story
In the life of every Ruby on Rails app, there comes a time when the boundaries of a single process are simply too narrow. It might be the need for speed-ups (memcached), full-text indexing (ferret), or background work (simple-daemon), or something entirely different. One thing is certain - daemons will start accompanying your applications.

My application, Movie Nights, desperately needed daemons for all the above reasons. I've been putting that off for as much as possible, because I didn't want to make my developers learn how to start all these processes, and I didn't want the hassle of having to remember all that. When I really had to do this, I knew there is only one right answer: automate.

So I looked into monit and god. They're awesome for the production environment, which didn't really help - I already have a deployment tool, which can handle daemons. I wanted something that would work under the development environment, and would automatically start/stop everything, and I accepted that I'll have to build my own.

Sadly, I don't have time to do this the right way (an integrated system that manages all the dependencies of a Rails app), so I settled for the quick way. Enter daemonz, a bandaid (not really a full solution) for the problem I've described above. Once it is configured, it knows how to start and stop the daemons in most non-trivial circumstances, e.g. having a cluster of app servers, or having the Rails framework brought up by tools. It's not a nice and elegant solution, but it works, and it took a few hours to code.

(If you're not a Rails programmer, hopefully you can at least enjoy the speed that Movie Nights has gained by performing all its Facebook API calls in a background process.)

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