rebar3_fly

A rebar3 plugin for deploying Erlang/OTP applications to Fly.io.

Scaffolds all the files you need for a production deployment — Dockerfile, release config, fly.toml — and handles the deploy.

Installation

Add to your rebar.config:

{project_plugins, [
    rebar3_fly
]}.

You also need flyctl installed:

curl -L https://fly.io/install.sh | sh

Quick start

# Scaffold deployment files
rebar3 fly init

# Create the Fly app
fly launch --no-deploy

# (Optional) Create and attach Postgres
fly postgres create --name myapp-db --region arn
fly postgres attach myapp-db

# Deploy
rebar3 fly deploy

Commands

rebar3 fly init

Generates all files needed for a Fly.io deployment:

Existing files are never overwritten.

After running, add the printed relx config to your rebar.config.

rebar3 fly deploy

Builds the Docker image locally and deploys to Fly.io.

If a .tool-versions file exists, the erlang and rebar versions are passed as Docker build args (ERLANG_VERSION, REBAR_VERSION).

rebar3 fly status

Shows the current status of your Fly.io app.

What it generates

Dockerfile

A multi-stage Dockerfile that:

1. Builder stage — uses the official erlang Docker image, compiles deps first (cached layer), then copies source and builds a release
2. Runtime stage — minimal Debian image with only the libraries needed to run ERTS

The runtime image is automatically selected based on your OTP version:

This matters because OTP 28 requires GLIBC 2.38+, which is only available in Debian Trixie.

fly.toml

Default configuration:

• Region: arn (Stockholm)
• VM: shared-cpu-1x, 1 GB RAM
• Internal port: 8080 (HTTPS enforced)
• Auto-stop/start machines enabled

Fly.io and IPv6

Fly.io uses IPv6 for internal networking. If your app connects to Fly Postgres, you need to pass inet6 as a socket option to your database driver.

For example, with kura (which uses pgo):

config() ->
    #{
        pool => my_repo,
        hostname => <<"my-db.internal">>,
        port => 5433,
        database => <<"my_app">>,
        username => <<"postgres">>,
        password => <<"secret">>,
        pool_size => 10,
        socket_options => [inet6]
    }.

Or parse DATABASE_URL from the environment (set automatically by fly postgres attach):

init(Config) ->
    case os:getenv("DATABASE_URL") of
        false -> Config;
        Url -> parse_database_url(Url)
    end.

See the Fly Postgres guide for a complete example.

License

MIT