For Cloud Foundry to automatically invoke a Rake task while a Ruby or Ruby on Rails app is deployed, you must do the following:

• Include the Rake task in your app.
• Configure the application start command using the command attribute in the application manifest.

Use the following example to invoke a Rake database migration task at application startup:

1. Create a file with the Rake task name and the extension .rake, and store it in the lib/tasks directory of your application.

2. Add the following code to your rake file:

   namespace :cf do
       desc "Only run on the first application instance"
       task :on_first_instance do
           instance_index = JSON.parse(ENV["VCAP_APPLICATION"])["instance_index"] rescue nil
       exit(0) unless instance_index == 0
       end
   end

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   This Rake task limits an idempotent command to the first instance of a deployed application.

3. Add the task to the manifest.yml file with the command attribute, referencing the idempotent command rake db:migrate chained with a start command.

   applications:
   - name: my-rails-app
     command: bundle exec rake cf:on_first_instance db:migrate && rails s -p $PORT

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