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.

Migrating your app

The steps below run you through the process of migrating your Rails app from Heroku to Fly.

Provision and deploy Rails app 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 like Sidekiq.

fly launch

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:

fly deploy

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.

Transfer Heroku secrets

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 RAILS_MASTER_KEY, DATABASE_URL REDIS_URL, and 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

Transfer the Database

Set the 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 HEROKU_DATABASE_URL variable.

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.

Multiple processes & background workers

Heroku uses Procfiles to describe multi-process Rails applications. Fly describes multi-processes with the [processes] directive in the fly.toml file.

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 fly.toml file:

[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 fly.toml file:

[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.

fly deploy

You should see a web and worker process deploy.

Custom Domain & SSL Certificates

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.

In addition to supporting CNAME DNS records, Fly also supports A and AAAA records for those who want to point example.com (without the www.example.com) directly at Fly.

Cheat Sheet

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.

Commands

Fly commands are a bit different than Heroku, but you’ll get use to them after a few days.

Check out the Fly CLI docs for a more extensive inventory of Fly commands.

Deployments

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.

Release phase tasks

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".

Deploy via git

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.

Databases

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!

Heroku’s managed database

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.

Fly’s databases

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.

Pricing

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.

Free Plans

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.

Plans for Small Rails Apps

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.

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:

Plans for Medium to Large Rails Apps

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.

Here’s roughly the equivalent resources on Fly:

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.