Capistrano: Doing things on rollback

Capistrano has the concept of a "rollback" that comes in really handy in case of errors. When you notice that your recent deploy was faulty, run cap deploy:rollback and you're back with the previous release. In case of an error during a deployment, Capistrano will rollback itself.

But, you may ask, how can it know how to revert all the custom stuff I'm doing during deployment? It can't. But you can tell it how to.

Capistrano 3 Show archive.org snapshot

Deploy and rollback are nearly identical, just some tasks are replaced:

deploy:updating → deploy:reverting
deploy:updated → deploy:reverted
deploy:finishing → deploy:finishing_rollback

Simply hook your custom rollback tasks to the respective Capistrano tasks.

Capistrano 2 Show archive.org snapshot

In your custom deploy task, use #on_rollback to define what needs to happen during a rollback. Example:

Copynamespace :slack do
  task :starting do
    on_rollback { slack.failed }
    slack_connect 'Deploy started'
  end

  task :failed do
    slack_connect 'Deploy failed'
  end
end

Note that there is a hidden requirement for on_rollback: it must be run inside a Capistrano transaction. Don't worry if you've never heard of this. As long as your custom task is hooked into the deploy task, it will work because deploy internally opens a transaction.