Config Files As Programs

While working on a few recent tasks, I've hit upon a small but powerful technique: My configuration files are actually programs in their own right.

Expression languages of some form are de rigueur in modern frameworks. Especially in server code, where you need to deploy the same code base to different servers and have them point to different databases, mail servers, or whatever. In Spring, for instance, you can write something like ${project.database.url} and it will assume you mean to use not the literal string but the value of the project.database.url property, which is probably different in your dev environment and your production environment.

But my technique is vastly more powerful. I'm sure others have thought of it, too, but it sort of snuck up on me.

The first time I used this trick was when I put my work's SQL into Ruby scripts. I did that for readability's sake, but as I added more queries, I began to do the things one does does with programs. I refactored two similar queries into a method that took a differentiating argument and returned the query. I declared constants to hold common values. I started to ponder a Ruby module that I could import for various utility functions. The end result from the Java code's view was a set of query definitions. But within the query definition itself, I had normal programming tools at my disposal.

On a personal project (which I hope to write about soon), I decided to use YAML for the config files. I've gotten used to the format for config files, thanks to my work with AppEngine, and Ruby can easily work with it. My original idea for the config file was just fixed dates and times when I wanted my script to wake up and record an Internet radio stream. But then I realized that I wanted to record some streams on every weekday or on the first Sunday of the month.

So I thought, what if, before I feed the file to the YAML parser, I first run it through ERB, Ruby's powerful templating system? As long as the YAML parser sees something that looks like YAML, it doesn't matter how it gets there, right?

Right. I defined a method in my config file — in my config file! — that would return the next weekday after the given date. Then, when defining the value for a field, I called that method to calculate the value. My "YAML" file looks something like: start_date: <%next_weekday.strftime('%Y-%m-%d')%> 18:00. Which won't give you anything useful from the YAML parser. But run it through the ERB parser, and the YAML parser sees start_date: 2010-09-28 18:00, which is perfectly valid YAML syntax and is what you intend for the value to be.