Existing Elixir Apps

If you have an existing Elixir app that you want to move over to Fly, this guide walks you through the initial deployment process and shows you techniques you can use to troubleshoot issues you may encounter in a new environment.

Provision Elixir and Postgres Servers

To configure and launch your Elixir app, you can use fly launch and follow the wizard.

When you run fly launch from the newly-created project directory, the launcher provides some defaults for your new app, and gives you the option to tweak the settings.


cd my-app-name
fly launch

You’ll get a summary of the defaults chosen for your app:

Organization: MyOrgName              (fly launch defaults to the personal org)
Name:         my-app-name            (derived from your directory name)
Region:       Secaucus, NJ (US)      (this is the fastest region for you)
App Machines: shared-cpu-1x, 1GB RAM (most apps need about 1GB of RAM)
Postgres:     <none>                 (not requested)
Redis:        <none>                 (not requested)

? Do you want to tweak these settings before proceeding? Yes
Opening https://fly.io/cli/launch/bea626e2d179a083a3ba622a367e24ec ...

Type y at the prompt to open the Fly Launch page, and make the following changes to your app config:

  • Change the default app name and region, if needed.
  • For Databases, select Fly Postgres, give the Postgres database app a name (for example, your app name with -db appended) and choose a configuration.

Once you confirm your settings, you can return to the terminal, where the launcher will:

  • Run the Phoenix deployment setup task
  • Build the image
  • Set secrets required by Phoenix (SECRET_KEY_BASE, for example)
  • Deploy the application in your selected region
Waiting for launch data... Done
Created app 'my-app-name' in organization 'personal'
Admin URL: https://fly.io/apps/my-app-name
Hostname: my-app-name.fly.dev
Set secrets on my-app-name: SECRET_KEY_BASE
Creating postgres cluster in organization personal
Creating app...
Setting secrets on app my-app-name-db...
Provisioning 1 of 1 machines with image flyio/postgres-flex:15.3@sha256:44b698752cf113110f2fa72443d7fe452b48228aafbb0d93045ef1e3282360a6
Waiting for machine to start...
Machine 2865550c7e96d8 is created
==> Monitoring health checks
  Waiting for 2865550c7e96d8 to become healthy (started, 3/3)

Postgres cluster my-app-name-db created
  Username:    postgres
  Password:    EChe3BrhCjsPQEI
  Hostname:    my-app-name-db.internal
  Flycast:     fdaa:2:45b:0:1::1d
  Proxy port:  5432
  Postgres port:  5433
  Connection string: postgres://postgres:EChe3BrhCjsPQEI@my-app-name-db.flycast:5432

Save your credentials in a secure place -- you won't be able to see them again!

Connect to postgres
Any app within the MyOrgName organization can connect to this Postgres using the above connection string

Now that you've set up Postgres, here's what you need to understand: https://fly.io/docs/postgres/getting-started/what-you-should-know/
Checking for existing attachments
Registering attachment
Creating database
Creating user

Postgres cluster my-app-name-db is now attached to my-app-name
The following secret was added to my-app-name:
Postgres cluster my-app-name-db is now attached to my-app-name
Generating rel/env.sh.eex for distributed Elixir support
Preparing system for Elixir builds
Installing application dependencies
Running Docker release generator
Wrote config file fly.toml
Validating /Users/anderson/test-elixir-gs/hello_elixir2/fly.toml
✓ Configuration is valid
==> Building image
Remote builder fly-builder-black-pine-7645 ready
Remote builder fly-builder-black-pine-7645 ready
==> Building image with Docker
--> docker host: 20.10.12 linux x86_64


--> Pushing image done
image: registry.fly.io/my-app-name:deployment-01HPMGHTG8XSYH3ZCV82SF5CEZ
image size: 126 MB

Watch your deployment at https://fly.io/apps/my-app-name/monitoring

Provisioning ips for my-app-name
  Dedicated ipv6: 2a09:8280:1::2a:bc0b:0
  Shared ipv4:
  Add a dedicated ipv4 with: fly ips allocate-v4

Running my-app-name release_command: /app/bin/migrate

 ✔ release_command 784eee4c294298 completed successfully
This deployment will:
 * create 2 "app" machines

No machines in group app, launching a new machine
Creating a second machine to increase service availability
Finished launching new machines
NOTE: The machines for [app] have services with 'auto_stop_machines = true' that will be stopped when idling

Checking DNS configuration for my-app-name.fly.dev

Visit your newly deployed app at https://my-app-name.fly.dev/

Make sure to note your Postgres credentials from the output.

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 postgres-database-app-name - Database deployment details
  • fly deploy - Deploy the application after making changes

Troubleshooting your initial deployment

Since this is an existing Elixir app, its highly likely it might not boot because you probably need to configure secrets or other service dependencies. Let’s walk through how to troubleshoot these issues so you can get your app running.

View log files

If your application didn’t boot on the first deploy, run fly logs to see what’s going on.

fly logs

This shows the past few log file entries and tails your production log files.

Open an IEx Session

It can be helpful to open a IEx Session to run commands and diagnose production issues.

To do this, we will login with SSH to our application VM. There is a one-time setup task for using SSH. Follow the instructions.

fly ssh issue --agent

With SSH configured, let’s open a console.

fly ssh console --pty -C "/app/bin/hello_elixir remote"
Connecting to hello_elixir.internal... complete
Erlang/OTP 23 [erts-11.2.1] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:1]

Interactive Elixir (1.11.2) - press Ctrl+C to exit (type h() ENTER for help)

You have a live IEx shell into your application!

Deploy your application after changes

Deploying your application is done with the following command:

fly deploy

This will take a few seconds as it uploads your application, builds a machine image, deploys the images, and then monitors to ensure it starts successfully. Once complete visit your app with the following command:

fly apps open

If all went well, you’ll see your Elixir application homepage.