Build, Deploy and Run an Elixir Application

Getting an application running on Fly is essentially working out how to package it as a deployable image and attach it to a database. Once packaged it can be deployed to the Fly infrastructure to run on the global application platform.

For this Getting Started article, we'll look at deploying an Elixir Phoenix application on Fly and connecting it to a PostgreSQL database.

The HelloElixir Application

Our example will be a basic "hello world" example using Phoenix. We'll assume that you already have Elixir and Phoenix installed.

Since most web applications require a database, let's get setup with a Postgres database as well.

All the code we will create can also be found in the hello_elixir Github repository. Just git clone to get a local copy if you want to focus on deployment (see the section "Install Flyctl and Login" for that). You can also use the repository as a reference.

Here's how we start the app.

mix hello_elixir

Running the Application

To run the application locally, we first need to create the database.

Tip: Make sure you have a local PostgreSQL server installed and running.

mix ecto.setup
The database for HelloElixir.Repo has been created

Now we can start the application.

mix phx.server
[info] Running HelloElixirWeb.Endpoint with cowboy 2.8.0 at (http)
[info] Access HelloElixirWeb.Endpoint at http://localhost:4000

Connect to localhost:4000 to confirm that you have a working Elixir application. Now to package it up for Fly.

Running Migrations in Production

We want our application to run database migrations when the application starts up. Locally in dev, we use mix ecto.migrate. In production when using an Elixir Release, we don't have the mix tool available to help out. Let's create a file called lib/hello_elixir/release.ex to help with this. This is just following the Phoenix guide on Deploying with Releases.

defmodule HelloElixir.Release do
  @moduledoc """
  Used for executing DB release tasks when run in production without Mix
  @app :hello_elixir

  def migrate do

    for repo <- repos() do
      {:ok, _, _} = Ecto.Migrator.with_repo(repo, &, :up, all: true))

  def rollback(repo, version) do
    {:ok, _, _} = Ecto.Migrator.with_repo(repo, &, :down, to: version))

  defp repos do
    Application.fetch_env!(@app, :ecto_repos)

  defp load_app do

We'll call this migrate function later.

Configure for Releases

Elixir applications can be compiled into a "release". This means we're not shipping our plain text code in a Docker container. This let's our application start up faster as well. To configure our simple hello_elixir application for releases, this is what we do.

Special Note on Fly Networking

Internally Fly uses IPv6 networking. This enables some cool features, but legacy Elixir applications need to be configured to work smoothly with it. Currently, even a newly generated Elixir application needs to be updated as well. These config changes tell Elixir, Phoenix, and the BEAM that we are using IPv6 addresses. The options look like inet6 and inet6_tcp. The next steps are how we configure that in our application.

Generate Release Config Files

We use the mix release.init command to create some sample files in the ./rel directory.

mix release.init
* creating rel/vm.args.eex
* creating rel/
* creating rel/env.bat.eex

We only need to configure rel/ This file gets turned into a shell script that the release uses to set ENV values used when we run any release commands. Here's the important parts.


ip=$(grep fly-local-6pn /etc/hosts | cut -f 1)
export ELIXIR_ERL_OPTIONS="-proto_dist inet6_tcp"

We configure the node to use a full node name when it runs. We get the Fly assigned IPv6 address and use that to name our node. Finally, we configure inet6_tcp for the BEAM as well.

Even if you don't care to cluster your nodes together, you still want to do this because it enables running an IEx shell in a running node.

Special Note on Clustering

To make clustering your Elixir applications easier on Fly, in the file, the RELEASE_NODE is named using the $FLY_APP_NAME and the IPv6 address. It will look something like this in practice.


While our hello_elixir app isn't clustered, it could easily be clustered using libcluster and a DNS strategy. So to keep things smooth and easy in case you want to go that route, then using the Fly app name as the node name works best.

If you want your application clustered, first focus on getting it deployed. After you have it deployed, there is an optional section at the end of this guide that walks you through getting it clustered.

Runtime Config

Create the file config/runtime.exs.

import Config

if config_env() == :prod do
  secret_key_base =
    System.get_env("SECRET_KEY_BASE") ||
      raise """
      environment variable SECRET_KEY_BASE is missing.
      You can generate one by calling: mix phx.gen.secret

  app_name =
    System.get_env("FLY_APP_NAME") ||
      raise "FLY_APP_NAME not available"

  config :hello_elixir, HelloElixirWeb.Endpoint,
    server: true,
    url: [host: "#{app_name}", port: 80],
    http: [
      port: String.to_integer(System.get_env("PORT") || "4000"),
      # IMPORTANT: support IPv6 addresses
      transport_options: [socket_opts: [:inet6]]
    secret_key_base: secret_key_base

  database_url =
    System.get_env("DATABASE_URL") ||
      raise """
      environment variable DATABASE_URL is missing.
      For example: ecto://USER:PASS@HOST/DATABASE

  config :hello_elixir, HelloElixir.Repo,
    url: database_url,
    # IMPORTANT: Or it won't find the DB server
    socket_options: [:inet6],
    pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")

This expects to receive some ENV values at runtime. We're expecting SECRET_KEY_BASE with our Phoenix secret, our FLY_APP_NAME from Fly, and the DATABASE_URL for connecting to a Fly hosted Postgres database.

There are a few important config settings in config/runtime.exs that are worth covering in more detail.

The Phoenix Endpoint config needs to support IPv6 using the :inet6 option.

  config :hello_elixir, HelloElixirWeb.Endpoint,
    http: [
      # IMPORTANT: support IPv6 addresses
      transport_options: [socket_opts: [:inet6]]

Connecting to the Postgres instance also needs to support IPv6. This is part of the Repo config.

  config :hello_elixir, HelloElixir.Repo,
    # IMPORTANT: Or it won't find the DB server
    socket_options: [:inet6],

Also, you don't need to turn on TLS for connecting to the Postgres instance. Fly private networks operate over an encrypted WireGuard mesh, so traffic between application servers and PostgreSQL is already encrypted and there's no need to TLS.

Remove Non-Release Config

Our newly generated Phoenix app includes some boilerplate config that we need to remove because we're using releases. In our case, the config was moved to the config/runtime.exs file.

Delete the file config/prod.secret.exs.

Remove the last line of config/prod.exs that says import_config "prod.secret.exs".

Docker Setup


Here is a working Dockerfile that builds the application. The base image used for building the release is using hexpm/elixir. is the official package repository for Elixir. This image is kept up-to-date and the tags allow for easier selection of Elixir and Erlang versions. This one uses Alpine Linux but hexpm provides other base images you can use as well. See hexpm on Dockerhub for other options.

### Fist Stage - Building the Release
FROM hexpm/elixir:1.12.1-erlang-24.0.1-alpine-3.13.3 AS build

# install build dependencies
RUN apk add --no-cache build-base npm

# prepare build dir

# extend hex timeout

# install hex + rebar
RUN mix local.hex --force && \
    mix local.rebar --force

# set build ENV as prod

# Copy over the mix.exs and mix.lock files to load the dependencies. If those
# files don't change, then we don't keep re-fetching and rebuilding the deps.
COPY mix.exs mix.lock ./
COPY config config

RUN mix deps.get --only prod && \
    mix deps.compile

# install npm dependencies
COPY assets/package.json assets/package-lock.json ./assets/
RUN npm --prefix ./assets ci --progress=false --no-audit --loglevel=error

COPY priv priv
COPY assets assets

# NOTE: If using TailwindCSS, it uses a special "purge" step and that requires
# the code in `lib` to see what is being used. Uncomment that here before
# running the npm deploy script if that's the case.
# COPY lib lib

# build assets
RUN npm run --prefix ./assets deploy
RUN mix phx.digest

# copy source here if not using TailwindCSS
COPY lib lib

# compile and build release
COPY rel rel
RUN mix do compile, release

### Second Stage - Setup the Runtime Environment

# prepare release docker image
FROM alpine:3.13.3 AS app
RUN apk add --no-cache libstdc++ openssl ncurses-libs


RUN chown nobody:nobody /app

USER nobody:nobody

COPY --from=build --chown=nobody:nobody /app/_build/prod/rel/hello_elixir ./


CMD ["bin/hello_elixir", "start"]

This is a two-stage Dockerfile. There are two FROM commands. The first stage pulls in the source and builds the release. The second stage takes the prepared release and sets it up in a minimal Docker image. The final deployed image contains only our release.

Docker Ignore File

We add the file .dockerignore to the project with the following contents.


This keeps any natively compiled Elixir or Node packages from our development environments from causing a problem in the Linux container.

Install Flyctl and Login

We are ready to start working with Fly and that means we need flyctl, our CLI app for managing apps on Fly. If you've already installed it, carry on. If not, hop over to our installation guide. Once that's installed you'll want to login to Fly.

Launch the App on Fly

To launch an app on fly, run fly launch in the directory with your source code. This creates and configures a fly app for you by inspecting your source code, then prompts you to deploy.

fly launch
$ fly launch
Scanning source code
Detected Dockerfile app
? Select region: sea (Seattle, Washington (US))
Created app icy-leaf-7381 in organization personal
Wrote config file fly.toml
? Would you like to deploy now? No

Don't deploy it just yet. We're going to adjust the generated fly.toml file first.

The fly launch command scans your source code to determine how to build a deployment image as well as identify any other configuration your app needs, such as secrets and exposed ports.

After your source code is scanned and the results are printed, you'll be prompted for an organization. Organizations are a way of sharing applications and resources between Fly users. Every Fly account has a personal organization, called personal, which is only visible to your account. Let's select that for this guide.

Next, you'll be prompted to select a region to deploy in. The closest region to you is selected by default. You can use this or change to another region. You can find the list of supported regions here.

At this point, flyctl created an app for you and wrote your configuration to a fly.toml file. You'll then be prompted to build and deploy your app. Once complete, your app will be running on fly.

Inside fly.toml

The fly.toml file now contains a default configuration for deploying your app. In the process of creating that file, flyctl has also generated a Fly-side application slot with a new name. In this case, it is icy-leaf-7381. If we look at the fly.toml file we can see the name in there:

app = "icy-leaf-7381"

kill_signal = "SIGINT"
kill_timeout = 5


  auto_rollback = true

  http_checks = []
  internal_port = 8080
  protocol = "tcp"
  script_checks = []

    hard_limit = 25
    soft_limit = 20
    type = "connections"

    handlers = ["http"]
    port = 80

    handlers = ["tls", "http"]
    port = 443

    grace_period = "1s"
    interval = "15s"
    restart_limit = 6
    timeout = "2s"

The flyctl command will always refer to this file in the current directory if it exists, specifically for the app name/value at the start. That name is used to identify the application to the Fly platform. The rest of the file contains settings to be applied to the application when it deploys.

We'll have more details about these properties as we progress, but for now, it's enough to say that they mostly configure which ports the application will be visible on.

Customizing fly.toml

Elixir applications need a little customization to the generated fly.toml file.

app = "icy-leaf-7381"

kill_signal = "SIGTERM"
kill_timeout = 5


  release_command = "/app/bin/hello_elixir eval HelloElixir.Release.migrate"

  internal_port = 4000
  protocol = "tcp"

    hard_limit = 25
    soft_limit = 20

    handlers = ["http"]
    port = 80

    handlers = ["tls", "http"]
    port = 443

    grace_period = "30s" # allow some time for startup
    interval = "15s"
    restart_limit = 6
    timeout = "2s"

There are two important changes here:

  • We added the [deploy] setting. This tells Fly that on a new deploy, run our database migrations.
  • The kill_signal is set to SIGTERM. An Elixir node does a clean shutdown when it receives a SIGTERM from the OS.

Some other values were tweaked as well. Check that internal_port matches your application.

Preparing to Deploy

We're almost there! Before we can deploy our new app, we need to setup a few things in our Fly account first. Namely, we want to provide the needed secrets and we need a database!

Setting our Secrets on Fly

Elixir has a mix task that can generate a new Phoenix key base secret. Let's use that.

mix phx.gen.secret

It generates a long string of random text. Let's store that as a secret for our app. When we run this command in our project folder, flyctl uses the fly.toml file to know which app we are setting the value on.


Creating our Fly Postgres Database

fly postgres create
? App name: hello-elixir-db
Automatically selected personal organization: Mark Ericksen
? Select region: sea (Seattle, Washington (US))
? Select VM size: shared-cpu-1x - 256
? Volume size (GB): 10
Creating postgres cluster hello-elixir-db in organization personal
Postgres cluster hello-elixir-db created
  Username:    <USER>
  Password:    <PASSWORD>
  Hostname:    hello-elixir-db.internal
  Proxy Port:  5432
  PG Port: 5433
Save your credentials in a secure place, you won't be able to see them again!

Monitoring Deployment

2 desired, 2 placed, 2 healthy, 0 unhealthy [health checks: 6 total, 6 passing]
--> v0 deployed successfully

Connect to postgres
Any app within the personal organization can connect to postgres using the above credentials and the hostname "hello-elixir-db.internal."
For example: postgres://<USER>:<PASSWORD>@hello-elixir-db.internal:5432

See the postgres docs for more information on next steps, managing postgres, connecting from outside fly:

We can take the defaults which select the lowest values for CPU, size, etc. This is perfect for getting started.

Attach our App to the Database

We can use flyctl to attach our app to the database which also sets our needed DATABASE_URL ENV value.

fly postgres attach --postgres-app hello-elixir-db
Postgres cluster hello-elixir-db is now attached to icy-leaf-7381
The following secret was added to icy-leaf-7381:

We can see the secrets that Fly is using for our app like this.

fly secrets list
NAME            DIGEST                           DATE
DATABASE_URL    830d8769ff33cba6c8b29d1cd6a6fbac 1m10s ago
SECRET_KEY_BASE 84c992ac7ef334c21f2aaecd41c43666 9m20s ago

Looks like we're ready to deploy!

Deploying to Fly

To deploy changes to your app, just run just run:

fly deploy

First, flyctl builds our Dockerfile and pushes it to a Fly container registry.

This will lookup our fly.toml file, and get the app name icy-leaf-7381 from there. Then flyctl will start the process of deploying our application to the Fly platform. Flyctl will return you to the command line when it's done.

Viewing the Deployed App

Now the application has been deployed, let's find out more about its deployment. The command fly status will give you all the essential details.

fly status
  Name     = icy-leaf-7381
  Owner    = personal
  Version  = 3
  Status   = running
  Hostname =

Deployment Status
  ID          = 9762642f-baa4-e4df-c683-13f2ce26a6bc
  Version     = v3
  Status      = successful
  Description = Deployment completed successfully
  Instances   = 1 desired, 1 placed, 1 healthy, 0 unhealthy

f617e72a 3       sea    run     running 1 total, 1 passing 0        1m34s ago

Connecting to the App

The quickest way to browse your newly deployed application is with the fly open command.

fly open

Your browser will be sent to the displayed URL. Fly will auto-upgrade this URL to a HTTPS secured URL.

Arrived at Destination

You have successfully built, deployed, and connected to your first Elixir application on Fly. Check out the following bonus sections to go further!

Bonus Sections

With your application up and running, there are some additional things you can do to go further. Using some flyctl commands, we can easily do some powerful things with our application.

These bonus tips cover:

  • Getting an IEx shell into your running node. This helps you manage and work with your running system.
  • Clustering multiple Elixir nodes together. Say "Hello!" to the power of Distributed Computing!
  • Scaling your application out to more machines and even distant regions (with or without clustering).

What is the IP Address?

If you want to know what IP addresses the app is using, try fly ips list:

fly ips list
TYPE ADDRESS                              CREATED AT
v4                      24m5s ago
v6   2a09:8280:1:ce56:c80f:5071:6e94:6688 24m5s ago

Getting an IEx Shell into a Running Node

As mentioned before, Elixir supports getting a IEx shell into a running production node. How cool is that? We already took the steps to configure rel/, so this step should be pretty easy.

There are a couple prerequisites, we first need to establish an SSH Shell to our machine on Fly.

This step sets up a root certificate for your account and then issues a certificate.

fly ssh establish
fly ssh issue

With SSH configured, let's open a console.

fly ssh console
Connecting to icy-leaf-7381.internal... complete
/ #

If all has gone smoothly, then you have a shell into the machine! Now we just need to launch our remote IEx shell. The Dockerfile was configured to pull our application into /app. So our command for the hello_elixir app looks like this:

app/bin/hello_elixir remote
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)

Awesome! A running IEx shell into our node. You can safely disconnect it using CTRL+C, CTRL+C.

Clustering Your Application

Elixir and the BEAM have the incredible ability to be clustered together and pass messages seamlessly between nodes. Fly makes clustering easy! This extra (and totally optional) portion of the guide walks you through clustering your Elixir application.

There are 2 parts to getting clustering quickly setup on Fly.

  • Installing and using libcluster
  • Scaling our application to multiple instances

Adding libcluster

The widely adopted library libcluster helps here.

Libcluster supports multiple strategies for finding and connecting with other nodes. The strategy we'll use is DNSPoll which was added in version 3.2.2 of libcluster, so make sure you're using that version or newer.

After installing libcluster, add it to the application like this:

defmodule HelloElixir.Application do
  use Application

  def start(_type, _args) do
    topologies = Application.get_env(:libcluster, :topologies) || []

    children = [
      # ...
      # setup for clustering
      {Cluster.Supervisor, [topologies, [name: HelloElixir.ClusterSupervisor]]}

    # ...

  # ...

Our next step is to add the topologies configuration to the file config/runtime.exs.

  app_name =
    System.get_env("FLY_APP_NAME") ||
      raise "FLY_APP_NAME not available"

  config :libcluster,
    debug: true,
    topologies: [
      fly6pn: [
        strategy: Cluster.Strategy.DNSPoll,
        config: [
          polling_interval: 5_000,
          query: "#{app_name}.internal",
          node_basename: app_name

REMEMBER: Deploy your updated app so the clustering code is available. Ex: fly deploy

This configures libcluster to use the DNSPoll strategy and look for other deployed apps using the $FLY_APP_NAME on the .internal private networks.

This assumes that your rel/ file is configured to name your Elixir node using the $FLY_APP_NAME.

Before it can be clustered, we have to have multiple instances. We'll do that next!

Running Multiple Instances

There are two ways to run multiple instances.

  1. Scale our application to have multiple instances in one region.
  2. Add an instance to another region (multiple regions).

Both approaches are valid and our Elixir application doesn't change at all for the approach you choose!

Let's first start with a baseline of our single deployment.

fly status
f9014bf7 26      sea    run     running 1 total, 1 passing 0        1h8m ago

Scaling in a Single Region

Let's scale up to 2 instances in our current region.

fly scale count 2
Count changed to 2

Checking on the status we can see what happened.

fly status
eb4119d3 27      sea    run     running 1 total, 1 passing 0        39s ago
f9014bf7 27      sea    run     running 1 total, 1 passing 0        1h13m ago

We now have two instances in the same region! That was easy.

Let's make sure they are clustered together. We can check the logs:

fly logs
app[eb4119d3] sea [info] 21:50:21.924 [info] [libcluster:fly6pn] connected to :"icy-leaf-7381@fdaa:0:1da8:a7b:ac2:f901:4bf7:2"

But that's not as rewarding as seeing it from inside a node. From an IEx shell, we can ask the node we're connected to, what other nodes it can see.

fly ssh console
/app/bin/hello_elixir remote
iex(icy-leaf-7381@fdaa:0:1da8:a7b:ac2:f901:4bf7:2)1> Node.list

I included the IEx prompt because it shows the IP address of the node I'm connected to. Then getting the Node.list returns the other node. Our two instances are connected and clustered!

Scaling to Multiple Regions

Fly makes it super easy to run instances of your applications physically closer to your users. Through the magic of DNS, they will be directed to their nearest region of your application. You can read about regions here and see the list of regions to choose from.

Starting back from our baseline of a single instance running in sea which is Seattle, Washington (US), I'll add the region ewr which is Parsippany, NJ (US). This puts an instance on both coasts of the US.

fly regions add ewr
Region Pool:
Backup Region:

Looking at the status right now shows that we're only in 1 region because our count is set to 1.

fly status
cdf6c422 29      sea    run     running 1 total, 1 passing 0        58s ago

Let's add a 2nd instance and see it deploy to ewr.

fly scale count 2
Count changed to 2

Now our status shows we have two instances spread across 2 regions!

fly status
0a8e6666 30      ewr    run     running 1 total, 1 passing 0        16s ago
cdf6c422 30      sea    run     running 1 total, 1 passing 0        6m47s ago

Let's ensure they are clustered together.

fly ssh console
/app/bin/hello_elixir remote
iex(icy-leaf-7381@fdaa:0:1da8:a7b:ac2:cdf6:c422:2)1> Node.list

We have two instances of our application deployed to the West and East coasts of the North American continent and they are clustered together! Our users will automatically be directed to the server nearest them. That is so cool!

Elixir has the built-in super power of Distributed Computing and with very little effort, you can deploy clustered and globally distributed applications yourself!


Some problems are harder to diagnose because they deal with Elixir releases or Docker build problems. Typically, you don't run the application that way locally, so you only encounter those problems when it's time to deploy.

Here are a few tips to help diagnose and identify problems.

  • Run mix release locally on your project.
  • Build the Dockerfile locally to verify it builds correctly. docker build .
  • config/runtime.exs for the :prod config isn't normally run during local use. Carefully review it.
  • fly logs to check server logs.