I use Heroku for most of my hosting, and also use it a lot in my work at Rabid Technologies. Something I frequently run into is a migration not being run after a git push heroku master, often causing an embarrasing application error page until someone can get to running heroku run rake db:migrate.

Fortunately, Heroku supports release commands. These commands are run after each successful deployment, and also delay the restart of the dyno(s). This allows for any post-code-update steps to be taken before the application is updated for users.

Declaring release commands is very simple - a line simply needs to be added to the Procfile stating which commands should be run upon release. For example, this is the line I use to run migrations:

# Procfile
# ...
# release: bundle exec rake db:migrate

Adding this to your Procfile will change your post-receive hook output from Heroku slightly, as it will now output the status of the release command:

remote: -----> Launching...        
remote:  !     Release command declared: this new release will not be available until the command succeeds.        
remote:        Released v160        
remote:        https://myapp.herokuapp.com/ deployed to Heroku        
remote: 
remote: Verifying deploy... done.        
remote: Running release command....
remote: 
remote:    (0.6ms)  SELECT pg_try_advisory_lock(7931432920140052365)        
remote:    (1.2ms)  SELECT "schema_migrations"."version" FROM "schema_migrations" ORDER BY "schema_migrations"."version" ASC        
remote:   ActiveRecord::InternalMetadata Load (0.7ms)  SELECT  "ar_internal_metadata".* FROM "ar_internal_metadata" WHERE "ar_internal_metadata"."key" = $1 LIMIT $2  [["key", "environment"], ["LIMIT", 1]]        
remote:    (0.7ms)  BEGIN        
remote:    (0.5ms)  COMMIT        
remote:    (0.9ms)  SELECT pg_advisory_unlock(7931432920140052365)        
remote: Waiting for release.... done.

I find that this functionality is particularly useful for continuous deployment setup, as it only requires git to be installed to perform a Heroku deployment AND automatically run migrations. Without this functionality, it is slightly harder, as the deploy needs to happen in two steps - one to do the git push, and then another to do the heroku run. This process also requires the Heroku CLI to be installed and authenticated, which isn’t always the easiest thing in CI environments.