Using Ruby's Autoload Method To Configure Your App Just-in-Time

Reading The Ruby Programming Language was a great experience — like revisiting a country I thought I knew intimately, but with expert tour guides who showed me whole new landscapes. It's also a good primer on what's changing in Ruby 1.9.

One of my favorite discoveries was Ruby's autoload method. Using autoload, you can associate a constant with a filename to be loaded the first time that constant is referenced, like so:

autoload :BCrypt, 'bcrypt'
autoload :Digest, 'digest/sha2'

The first time the interpreter encounters the constant BCrypt, it will load the file 'bcrypt' from Ruby's current load path, which it assumes will contain the definition of that constant. (Note that autoload takes the name of the constant, in symbol form, not the constant itself).

Here's an example of how useful it can be. OtherInbox uses beanstalkd in a few places where we haven't yet migrated to SQS. I was loading the beanstalk client with a Rails initializer, 'config/initializers/beanstalk.rb':

require 'beanstalk-client'
BEANSTALK = Beanstalk::Pool.new(['localhost:11300'])

Making this initial connection on our production server takes five seconds or more each time I restarted the app or dropped into the console. That doesn't sound like much but when you're doing that a few times a day, it starts to add up. So I moved the beanstalk code out of the initializer and into 'lib/etc/load_beanstalk.rb'. I placed all of my autoloads in a single initializer, 'config/initializers/autoload.rb'. For beanstalk, the statement is:

autoload :BEANSTALK, 'etc/load_beanstalk'

Now, the app starts more quickly, and even better, this library doesn't get loaded into memory by parts of the app that don't need it.