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Blue Green Deployment Example

This is meant to be an example of Blue/Green deployments, done with nginx, unicorn and capistrano. Pretty simple.

How does it work

Nginx is serving up two Rails apps. One on port 80, the other on port 81. Port 80 is release, found at blue_green_deployment_release which is a symbolic link to either blue or green. Port 81 is staging, found at blue_green_deployment_staging which is always a symbolic link. Running cap production deploy always deploys to the staging environment (yes this is misleading) but it has a trick. It uses readlink to dynamically found out what staging is pointing too, and sets that as the deploy_to directory. If you don't do this, then the current symbolic link will eventually point to a directory at staging\releases which will be wrong when you swap symbolic links.

Ugh it's confusing, see the picture below.

After staging is production ready, you "deploy" production simply by running promote_release which swaps the symbolic links. Nginx does not need to be restarted - all requests will now go to new release environment. The old release enenvironment becomes staging. You'll want to deploy staging after this, to get it caught up.

Gotchas

I left my notes in, in a file called Notes.MD. But let me put in a few of the issues I had:

  • Getting Ruby installed via with chruby was a huge pain in the butt. See my notes for details.
  • Getting an environment variable for SECRET_KEY_BASE required changing /etc/environment. This is hacky and should be fixed.
  • You needed to make sure that the deploy user have sudo access to the ln command. Again see the ./Notes.md file.
  • I'm not sure this will work out of the box on a new machine, you may need to do some fiddling with directories. Again see the ./Notes.md file.