Migrate from Heroku

This guide runs you through how to migrate a basic Elixir application off of Heroku and onto Fly.io. It assumes you’re running the following services on Heroku:

• Postgres database
• Custom domain
• Background worker, like Oban

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 Phoenix app from Heroku to Fly.

Provision and deploy Phoenix app to Fly

From the root of the Elixir app you’re running on Heroku, run fly launch and select the options to provision a new Postgres database.

When we run fly launch from the newly-created project directory, the launcher will:

• Ask you to select a deployment region
• Set secrets required by Phoenix (SECRET_KEY_BASE, for example)
• Run the Phoenix deployment setup task
• Optionally setup a Postgres instance in your selected region
• Deploy the application in your selected region

cd <app_name>
fly launch

Creating app in /Users/me/<app_name>
Scanning source code
Detected a Phoenix app
? App Name (leave blank to use an auto-generated name): <app_name>
? Select organization: flyio (flyio)
? Select region: mad (Madrid, Spain)
Created app <app_name> in organization soupedup
Set secrets on <app_name>: SECRET_KEY_BASE
Installing dependencies
Running Docker release generator
Wrote config file fly.toml
? Would you like to setup a Postgres database now? Yes
Postgres cluster <app_name>-db created
  Username:    postgres
  Password:    <password>
  Hostname:    &lt;app_name&gt;-db.internal
  Proxy Port:  5432
  PG Port: 5433
Save your credentials in a secure place, you will not be able to see them again!

Monitoring Deployment

1 desired, 1 placed, 1 healthy, 0 unhealthy [health checks: 2 total, 2 passing]
--> v0 deployed successfully

Connect to postgres
Any app within the flyio organization can connect to postgres using the above credentials and the hostname "&lt;app_name&gt;-db.internal."
For example: postgres://postgres:password@&lt;app_name&gt;-db.internal:5432

See the postgres docs for more information on next steps, managing postgres, connecting from outside fly:  https://fly.io/docs/reference/postgres/
Postgres cluster &lt;app_name&gt;-db is now attached to &lt;app_name&gt;

Would you like to deploy now? Yes
Deploying &lt;app_name&gt;

==> Validating app configuration
--> Validating app configuration done
Services
TCP 80/443 ⇢ 8080
Remote builder fly-builder-little-glitter-8329 ready
...

That’s it! Run fly apps open to see your deployed app in action.

Try a few other commands:

• fly logs - Tail your application logs
• fly status - App deployment details
• fly status -a <app_name>-db - Database deployment details
• fly deploy - Deploy the application after making changes

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 application.
• Connect via IEx

Transfer Heroku secrets

To see all of your Heroku env vars and secrets, run:

heroku config -s | grep -v -e "DATABASE_URL" | fly secrets import

This command exports the Heroku secrets, excludes DATABASE_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
MIX_ENV                       fd89784e59c72499525556f80289b2c7  1m26s ago
SECRET_KEY_BASE               5afb43c2ddbba6c02ffa7e2834689692  1m22s ago

Transfer the Database

Set the HEROKU_DATABASE_URL variable in your Fly.io environment.

fly secrets set HEROKU_DATABASE_URL=$(heroku config:get DATABASE_URL)

Alright, lets start the transfer remotely on the Fly.io 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.io 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.

Custom Domain & SSL Certificates

After you finish deploying your application to Fly.io and have tested it extensively, read through the Custom Domain docs and point your domain at it.

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

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.io and Heroku.

Commands

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

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

Deployments

By default Heroku deployments are kicked off via the git push heroku command. Fly.io works a bit differently by kicking of deployments via fly deploy—git isn’t needed to deploy to Fly.io. 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.io and GitHub Actions docs.

Deploy via git

Heroku’s default deployment technique is via git push heroku. Fly.io doesn’t require a git commit, just run fly deploy and the files on your local workstation will be deployed.

Fly.io can be configured to deploy on git commits with the following techniques with a GitHub Action.

Databases

Fly.io and Heroku have different Postgres database offerings. The most important distinction to understand about using Fly.io 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.io 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.io 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 Postgres

The most important thing you’ll want to be comfortable with using Fly.io’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.

See here for a more comprehensive guide for what’s required when running your Postgres databases on Fly.io.

Pricing

Heroku and Fly.io have very different pricing structures. You’ll want to read through the details on Fly.io’s pricing page before launching to production. The sections below serve as a rough comparison between Heroku’s and Fly.io’s plans as of August 2022.

Free Plans

Heroku will not offer free plans as of November 28, 2022.

Fly.io offers free usage for up to 3 full time VMs with 256MB of RAM, which is enough to run a Elixir app and Postgres database to get a feel for how Fly.io works.

Plans for Small Elixir 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.io 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.io looks like this:

Plans for Medium to Elixir Apps

There’s too many variables to compare Fly.io’s and Heroku’s pricing for larger Elixir applications depending on your needs, so you’ll definitely want to do your homework before migrating everything to Fly.io. 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.io offers 2 database instances to achieve similar levels of redundancy.