When I first started deploying my sites to Heroku, I’d have to run migrations. After a bit of code hammering, I was pretty happy to push my code, then run migrations in a one off dyno (
heroku run rails db:migrate), then maybe restart my dynos (
Then I was shown release tasks and it blew my cotton socks off!
What are release tasks?!
Release tasks run after your app has been built, but just before it goes out to the web. If any of your tasks fail, the deploy will be cancelled, keeping the old version of your app in place.
This is awesome! It means you can have zero downtime deploys.
Creating release tasks
When you deploy, Heroku looks for a
release command in your Procfile. So if you’d like to run migrations on every deploy, you can update your
Procfile to have this line:
release: bundle exec rails db:migrate
However, I like to store my commands in a shell script, then have them run only if they’re enabled by environmental variables.
To do this as well, create a
release-tasks.sh file, and make sure it has executable permissions:
touch release-tasks.sh && chmod 0777 release-tasks.sh
Then append this line to your
release: bash ./release-tasks.sh
Running tasks based on environmental variables
It’s pretty advantageous to be able to control which commands are able to run based on your environment setup, e.g. you might want to notify external APIs of a production deploy, but not when you’ve built a review app. Alternatively you might just not want to seed production.
To do this, you can set a “Config Variable” (environmental variable) in your Heroku app settings, with the name
RUN_MIGRATIONS_DURING_RELEASE with the value of
true. Then when that variable is true, you can ask your
release-tasks.sh to run a command around that.
Here is my current
release-tasks.sh file which I normally add to my Rails 5 projects:
#!/bin/bash echo "Running Release Tasks" if [ "$RUN_MIGRATIONS_DURING_RELEASE" == "true" ]; then echo "Running Migrations" bundle exec rails db:migrate fi if [ "$SEED_DB_DURING_RELEASE" == "true" ]; then echo "Seeding DB" bundle exec rails db:seed fi if [ "$CLEAR_CACHE_DURING_RELEASE" == "true" ]; then echo "Clearing Rails Cache" bundle exec rails r "Rails.cache.clear" fi echo "Done running release-tasks.sh"