Migrate from Heroku
This guide runs you through how to migrate a basic Rails application off of Heroku and onto Fly. It assumes you’re running the following services on Heroku:
- Puma web server
- Postgres database
- Redis in non-persistent mode
- Custom domain
- Background worker, like Sidekiq
If your application is running with more services, additional work may be needed to migrate your application off Heroku.
The steps below run you through the process of migrating your Rails app from Heroku to Fly.
From the root of the Rails app you’re running on Heroku, run
fly launch and
select the options to provision a new Postgres database, and optionally a Redis
database if you make use of Action Cable, caching, and popular third party gems
Creating app in ~/list Scanning source code Detected a Rails app ? Choose an app name (leave blank to generate one): list ? Select Organization: John Smith (personal) ? Choose a region for deployment: Ashburn, Virginia (US) (iad) Created app list in organization personal Admin URL: https://fly.io/apps/list Hostname: list.fly.dev Set secrets on list: RAILS_MASTER_KEY ? Would you like to set up a Postgresql database now? Yes For pricing information visit: https://fly.io/docs/about/pricing/#postgresql-clu ? Select configuration: Development - Single node, 1x shared CPU, 256MB RAM, 1GB disk Creating postgres cluster in organization personal . . . Postgres cluster list-db is now attached to namelist ? Would you like to set up an Upstash Redis database now? Yes ? Select an Upstash Redis plan Free: 100 MB Max Data Size Your Upstash Redis database namelist-redis is ready. . . . create Dockerfile create .dockerignore create bin/docker-entrypoint create config/dockerfile.yml Wrote config file fly.toml Your Rails app is prepared for deployment. Before proceeding, please review the posted Rails FAQ: https://fly.io/docs/rails/getting-started/dockerfiles/. Once ready: run 'fly deploy' to deploy your Rails app.
It is worth heeding the advice at the end of this: Before proceeding, please review the posted Rails FAQ: https://fly.io/docs/rails/getting-started/dockerfiles/.
After the application is provisioned, deploy it by running:
When that’s done, view your app in a browser:
fly apps open
There’s still work to be done to move more Heroku stuff over, so don’t worry if the app doesn’t boot right away. There’s a few commands that you’ll find useful to configure your environment:
fly logs- Read error messages and stack traces emitted by your Rails application.
fly ssh console --pty -C "/rails/bin/rails console"- Launches a Rails shell, which is useful to interactively test components of your Rails application.
To see all of your Heroku env vars and secrets, run:
heroku config -s | grep -v -e "RAILS_MASTER_KEY" -e "DATABASE_URL" -e "REDIS_URL" -e "REDIS_TLS_URL" | fly secrets import
This command exports the Heroku secrets, excluding
REDIS_TLS_URL, and imports them into Fly.
Verify your Heroku secrets are in Fly.
fly secrets list NAME DIGEST CREATED AT DATABASE_URL 24e455edbfcf1247a642cdae30e14872 14m29s ago LANG 95a7bb7a8d0ee402edde95bb78ef95c7 1m24s ago RACK_ENV fd89784e59c72499525556f80289b2c7 1m26s ago RAILS_ENV fd89784e59c72499525556f80289b2c7 1m26s ago RAILS_LOG_TO_STDOUT a10311459433adf322f2590a4987c423 1m25s ago RAILS_SERVE_STATIC_FILES a10311459433adf322f2590a4987c423 1m23s ago REDIS_TLS_URL b30fe87493e14d9b670dc0263dc935c9 1m25s ago REDIS_URL 4583a46e747696319573e8bfbd0db04d 1m21s ago SECRET_KEY_BASE 5afb43c2ddbba6c02ffa7e2834689692 1m22s ago
Any new data created by your Heroku app during this database migration won’t be moved over to Fly. Consider taking your Heroku application offline or place in read-only mode if you want to be confident that this migration will move over 100% of your Heroku data to Fly.
HEROKU_DATABASE_URL variable in your Fly environment.
fly secrets set HEROKU_DATABASE_URL=$(heroku config:get DATABASE_URL)
Alright, lets start the transfer remotely on the Fly instance.
fly ssh console
Then from the remote Fly SSH console transfer the database.
pg_dump -Fc --no-acl --no-owner -d $HEROKU_DATABASE_URL | pg_restore --verbose --clean --no-acl --no-owner -d $DATABASE_URL
You may need to upgrade your Heroku database to match the version of the source Fly database. Refer to Heroku’s Upgrading the Version of a Heroku Postgres Database for instructions on how to upgrade, then try the command above again.
After the database transfers unset the
fly secrets unset HEROKU_DATABASE_URL
Then launch your Heroku app to see if its running.
fly apps open
If you have a Redis server, there’s a good chance you need to set that up.
Heroku uses Procfiles to describe multi-process Rails applications. Fly describes multi-processes with the
[processes] directive in the
If your Heroku
Procfile looks like this:
web: bundle exec puma -C config/puma.rb worker: bundle exec sidekiq release: rails db:migrate
Move everything except for the
release: line to your
[processes] web = "bundle exec puma -C config/puma.rb" worker = "bundle exec sidekiq"
If you have a
release: line in your Heroku Procfile, that will listed separately in your
[deploy] release_command = "bin/rails db:migrate"
You will also want to prevent your release command from also being run during the deploy step. To do so, regenerate your dockerfile using:
$ bin/rails generate dockerfile --no-prepare
Next, under the
[[services]] directive, find the entry that maps to
internal_port = 8080, and add
processes = ["web"]. The configuration file should look something like this:
[[services]] processes = ["web"] # this service only applies to the web process http_checks =  internal_port = 8080 protocol = "tcp" script_checks = 
This associates the process with the service that Fly launches. Save these changes and run the deploy command.
You should see a
worker process deploy.
After you finish deploying your application to Fly and have tested it extensively, read through the Custom Domain docs and point your domain at Fly.
Old habits die hard, especially good habits like deploying frequently to production. Below is a quick overview of the differences you’ll notice initially between Fly and Heroku.
Fly commands are a bit different than Heroku, but you’ll get use to them after a few days.
|Tail log files||
Check out the Fly CLI docs for a more extensive inventory of Fly commands.
By default Heroku deployments are kicked off via the
git push heroku command. Fly works a bit differently by kicking of deployments via
fly deploy—git isn’t needed to deploy to Fly. The advantage to this approach is your git history will be clean and not full of commits like
git push heroku -am "make app work" or
git push heroku -m "ok it will really work this time".
To achieve the desired
git push behavior, we recommend setting up
fly deploy as the final command in your continuous integration pipeline, as outlined for GitHub in the Continuous Deployment with Fly and GitHub Actions docs.
Heroku has a
release: rake db:migrate command in their Procfiles to run tasks while the application is deployed. Rails 7.1 will include a
bin/rails db:prepare in the list of commands to be run on deploy in their
bin/docker-entrypoint file. Fly.io supports both approaches.
If you don’t want to run migrates by default per release, delete the prequite but leave the
:release task. You’ll be able to manually run migrations on Fly via
fly ssh console -C "/app/bin/rails db:migrate".
Heroku’s default deployment technique is via
git push heroku. Fly doesn’t require a git commit, just run
fly deploy and the files on your local workstation will be deployed.
Fly can be configured to deploy on git commits with the following techniques with a GitHub Action.
Fly and Heroku have different Postgres database offerings. The most important distinction to understand about using Fly is that it automates provisioning, maintenance, and snapshot tasks for your Postgres database, but it does not manage it. If you run out of disk space, RAM, or other resources on your Fly Postgres instances, you’ll have to scale those virtual machines from the Fly CLI.
Contrast that with Heroku, which fully manages your database and includes an extensive suite of tools to provision, backup, snapshot, fork, patch, upgrade, and scale up/down your database resources.
The good news for people who want a highly managed Postgres database is they can continue hosting it at Heroku and point their Fly instances to it!
One command is all it takes to point Fly apps at your Heroku managed database.
fly secrets set DATABASE_URL=$(heroku config:get DATABASE_URL)
This is a great way to get comfortable with Fly if you prefer a managed database provider. In the future if you decide you want to migrate your data to Fly, you can do so pretty easily with a few commands.
The most important thing you’ll want to be comfortable with using Fly’s database offering is backing up and restoring your database.
As your application grows, you’ll probably first scale disk and RAM resources, then scale out with multiple replicas. Common maintenance tasks will include upgrading Postgres as new versions are released with new features and security updates.
You Postgres, now what? is a more comprehensive guide for what’s required when running your Postgres databases on Fly.
Heroku and Fly have very different pricing structures. You’ll want to read through the details on Fly’s pricing page before launching to production. The sections below serve as a rough comparison between Heroku and Fly’s plans as of August 2022.
Please do your own comparison of plans before switching from Heroku to Fly. The examples below are illustrative estimates between two very different offerings, which focuses on the costs of app & database servers. It does not represent the final costs of each plan. Also, the prices below may not be immediately updated if Fly or Heroku change prices.
Heroku will not offer free plans as of November 28, 2022.
Fly offers free usage for up to 3 full time VMs with 256MB of RAM, which is enough to run a tiny Rails app and Postgres database to get a feel for how Fly works.
Heroku’s Hobby tier is limited to 10,000 rows of data, which gets exceeded pretty quickly requiring the purchase of additional rows of data.
|App Dyno||512MB RAM||$7/mo|
Fly’s pricing is metered for the resources you use. Database is billed by the amount of RAM and disk space used, not by rows. The closest equivalent to the Heroku Hobby tier on Fly looks like this:
|App Server||1GB RAM||~$5.70/mo|
|Database Server||256MB RAM / 10Gb disk||~$3.44/mo|
There’s too many variables to compare Fly and Heroku’s pricing for larger Rails applications depending on your needs, so you’ll definitely want to do your homework before migrating everything to Fly. This comparison focuses narrowly on the costs of app & database resources, and excludes other factors such as bandwidth costs, bundled support, etc.
|App Dyno||2.5GB RAM||$250/mo||8||$2,000/mo|
|Database||61GB RAM / 1TB disk||$2,500/mo||1||$2,500/mo|
Here’s roughly the equivalent resources on Fly:
|App Server||4GB RAM / 2X CPU||~$62.00/mo||8||~$496/mo|
|Database Server||64GB RAM / 500GB disk||~$633/mo||2||~$1,266/mo|
Again, the comparison isn’t realistic because it focuses only on application and database servers, but it does give you an idea of how the different cost structures scale on each platform. For example, Heroku’s database offering at this level is redundant, whereas Fly offers 2 database instances to achieve similar levels of redundancy.