How Your Ruby Code Is Deployed on Engine Yard Cloud

General concepts

There are two components that facilitate code deployment:

  • Client-side component. This component connects (via SSH) to the application master of your environment and ensures that the server-side component is installed and is the correct version. The client-side component then invokes the server-side component on the application master to coordinate code deployment.
  • Server-side component. This component ensures that it is installed on all other instances in the environment. Server-side then manages and coordinates the code deployment process whenever the client-side component requests a deployment.

1. Code fetch

  • On the application master, the source code is fetched from the git repository, then the chosen branch is checked out into /data/<appname>/shared/cached-copy. The application master then copies this (via rsync) to all the other instances in your environment.
  • On each instance, the contents of /data/<appname>/shared/cached-copy are copied to /data/<appname>/releases/<timestamp>.

2. Gem bundling

After your code is copied to all instances in your environment, the server-side component checks for the existence of a Gemfile.

  • If a Gemfile is found, the bundler gem is installed and the command bundle install is run on each application instance.
  • If you need a specific version of bundler, specify it in your Gemfile.
  • We suggest you generate a Gemfile.lock to ensure consistency across deployment environments.

3. Default configuration

After your application gems are installed, the server-side component generates and symlinks configuration files and directories for your application.

  • Configuration files and directories necessary for your application are generated or symlinked and put into place. Examples of these files include: config/database.yml, config/mongrel_cluster.yml, config/newrelic.yml, the log directory and more.

4. Database migration

After default configuration has been setup, your database migrations are run with appropriate deploy hooks.

  • The deploy hook deploy/before_migrate.rb is run.
  • The migration command specific to your application is run (see the --migrate option).
  • Finally, the deploy hook deploy/after_migrate.rb is run.

After migrations and their deploy hooks have been executed, your application symlink is created.

  • The deploy hook deploy/before_symlink.rb is run.
  • Your application symlink is created.
    /data/<appname>/current points to /data/<appname>/releases/<timestamp>
  • Finally, the deploy hook deploy/after_symlink.rb is run.

6. Restart application servers

After your application code is in place and has been appropriately symlinked, restart your application servers.

  • The deploy hook deploy/before_restart.rb is run.
  • Your application servers in your environment are now restarted. For example, if your environment is using Passenger, the server-side component will touch the file /data/<appname>/current/tmp/restart.txt to tell Passenger to restart.
  • Finally, the deploy hook deploy/after_restart.rb is run.

7. Final cleanup

If there are more than three previous releases in /data/<appname>/releases, all but the most recent three are removed. Your deployment is now finished.

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