Dec 182010

Most of the Shuttle Tracking project is managed via an open-source repository on Github, providing a great platform for others to check out the project and see that code we’re using to track shuttles here at RPI.  The one caveat is that not all of the code can be released under an open source license.  Shuttle Tracking interfaces with an external data provider responsible for the in-vehicle modules and their API isn’t public.  We also have a lot of config options specific to RPI that wouldn’t make sense in a public sense like references to the CAS config, our hoptoad instance, and our Google analytics config.

To help manage these RPI-specific things I commit them to my  local ‘RPI’ branch.  This branch doesn’t get pushed to Github (because the world can’t see some of the “secrets”) but it provides version control over things and lets me easily test out the changes in my development copy.  We also use Capistrano for our deployment, it makes it very easy for me to push new code to production and (more importantly) rollback code when things are broken.  The problem with Capistrano, or my understanding of it, is that it doesn’t easily pull code from a less-than-public branches.

So, I wanted to get my RPI-specific changes to the production server, which can only pull from the public ‘master’ branch on Github.  To do this, I added some code to my Capistrano config/deploy.rb file to pull the RPI changes as well.  The following code generates a patch file, sends the patch to the production server, and applies the patch with the RPI changes.

desc "Apply RPI-specific patch"
task :apply_patch, :roles => :app do
  patch_contents = `git diff --no-prefix master..RPI`
  put(patch_contents, "#{release_path}/patch", :via => :scp)
  run "cd #{release_path} && patch -N -p0 < #{release_path}/patch"

You’ll need to add something in here to call this function, like:

after "deploy:update_code", "deploy:apply_patch"

Presto, now your production code will carry over changes committed to a local branch. To make sure things don’t get crazy with conflicts, I make a point of checking out the RPI branch and merging master in with it before deploying it. This gives me the opportunity to resolve any conflicts that might come up during the patch process before they actually happen. We can also, pretty easily, see what makes each release (running cap deploy or alike) specific to RPI by looking at the patch file in each release folder on the server.