Specify your dependencies with Capistrano

There is this one cool trick with Capistrano: the deploy:check task. I’ve known about it for a while; it does a great job at sanity-checking your deployment environment: existence of directories, write permissions etc. What I learned only recently is that you can specify your own, custom dependencies.

Straight from Capistrano docs:

You can define your own dependencies, as well, using the `depend' method:

  depend :remote, :gem, "tzinfo", ">=0.3.3"
  depend :local,  :command, "svn"
  depend :remote, :directory, "/u/depot/files"

Let’s give it a shot, will we? I’ve got a number of dependencies on one of my apps:

depend :remote, :gem, "slave", ">=1.2"            # because of BackgrounDRb
depend :remote, :gem, "hpricot", ">=0.6"
depend :remote, :gem, "feed-normalizer", ">=1.4"  # I'm fetching feeds
depend :remote, :gem, "soap4r", ">=1.5"           # querying the Clearforest service

The above, of course, goes into your config/deploy.rb. Let’s check it out:

~/projects/myapp $ cap deploy:check

Seconds later, you might be presented with the following:

The following dependencies failed. Please check them and try again:
--> gem `slave' >=1.2 could not be found

So awesome. Now you can employ this little fella for the task :)

It’s also useful to check presence of certain directories. I’m running apps on edge mostly, so naturally I have a single Rails checkout on each box that I symlink from vendor/rails. Let’s teach Capistrano how to do it:

# (irrelevant parts of config/deploy.rb recipe excluded)
set :deploy_to, "/home/mislav/rails/#{application}/"

# the svn checkout of the framework (always kept fresh)
set :framework_path, "/home/mislav/rails/edge/"

# tell 'deploy:check' task to look for it
depend :remote, :directory, framework_path

after 'deploy:update_code' do
  run "ln -nfs #{framework_path} #{release_path}/vendor/rails" 
end

Sweet. Now take it from here ;)