View Source Deploying with Releases


What we'll need

The only thing we'll need for this guide is a working Phoenix application. For those of us who need a simple application to deploy, please follow the Up and Running guide.



Our main goal for this guide is to package your Phoenix application into a self-contained directory that includes the Erlang VM, Elixir, all of your code and dependencies. This package can then be dropped into a production machine.


Releases, assemble!

To assemble a release, you will need at least Elixir v1.9, however this guide assumes you are using Elixir v1.12 or later to take advantage of latest releases improvements:

$ elixir -v

If you are not familiar with Elixir releases yet, we recommend you to read Elixir's excellent docs before continuing.

Once that is done, you can assemble a release by going through all of the steps in our general deployment guide with mix release at the end. Let's recap.

First set the environment variables:

$ mix phx.gen.secret
$ export DATABASE_URL=ecto://USER:PASS@HOST/database

Then load dependencies to compile code and assets:

# Initial setup
$ mix deps.get --only prod
$ MIX_ENV=prod mix compile

# Compile assets
$ MIX_ENV=prod mix assets.deploy

And now run mix phx.gen.release:

$ mix phx.gen.release
==> my_app
* creating rel/overlays/bin/server
* creating rel/overlays/bin/server.bat
* creating rel/overlays/bin/migrate
* creating rel/overlays/bin/migrate.bat
* creating lib/my_app/release.ex

Your application is ready to be deployed in a release!

    # To start your system
    _build/dev/rel/my_app/bin/my_app start

    # To start your system with the Phoenix server running

    # To run migrations

Once the release is running:

    # To connect to it remotely
    _build/dev/rel/my_app/bin/my_app remote

    # To stop it gracefully (you may also send SIGINT/SIGTERM)
    _build/dev/rel/my_app/bin/my_app stop

To list all commands:


The phx.gen.release task generated a few files for us to assist in releases. First, it created server and migrate overlay scripts for conveniently running the phoenix server inside a release or invoking migrations from a release. The files in the rel/overlays directory are copied into every release environment. Next, it generated a release.ex file which is used to invoked Ecto migrations without a dependency on mix itself.

Note: If you are a docker user, you can pass the --docker flag to mix phx.gen.release to generate a Dockerfile ready for deployment.

Next, we can invoke mix release to build the release:

$ MIX_ENV=prod mix release
Generated my_app app
* assembling my_app-0.1.0 on MIX_ENV=prod
* using config/runtime.exs to configure the release at runtime

Release created at _build/prod/rel/my_app!

    # To start your system
    _build/prod/rel/my_app/bin/my_app start


You can start the release by calling _build/prod/rel/my_app/bin/my_app start, or boot your webserver by calling _build/prod/rel/my_app/bin/server, where you have to replace my_app by your current application name.

Now you can get all of the files under the _build/prod/rel/my_app directory, package it, and run it in any production machine with the same OS and architecture as the one that assembled the release. For more details, check the docs for mix release.

But before we finish this guide, there is one more feature from releases that most Phoenix application will use, so let's talk about that.


Ecto migrations and custom commands

A common need in production systems is to execute custom commands required to set up the production environment. One of such commands is precisely migrating the database. Since we don't have Mix, a build tool, inside releases, which are a production artifact, we need to bring said commands directly into the release.

The phx.gen.release command created the following release.ex file in your project lib/my_app/release.ex, with the following content:

defmodule MyApp.Release do
  @app :my_app

  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

Where you replace the first two lines by your application names.

Now you can assemble a new release with MIX_ENV=prod mix release and you can invoke any code, including the functions in the module above, by calling the eval command:

$ _build/prod/rel/my_app/bin/my_app eval "MyApp.Release.migrate"

And that's it! If you peek inside the migrate script, you'll see it wraps exactly this invocation.

You can use this approach to create any custom command to run in production. In this case, we used load_app, which calls Application.load/1 to load the current application without starting it. However, you may want to write a custom command that starts the whole application. In such cases, Application.ensure_all_started/1 must be used. Keep in mind, starting the application will start all processes for the current application, including the Phoenix endpoint. This can be circumvented by changing your supervision tree to not start certain children under certain conditions. For example, in the release commands file you could do:

defp start_app do
  Application.put_env(@app, :minimal, true)

And then in your application you check Application.get_env(@app, :minimal) and start only part of the children when it is set.



Elixir releases work well with container technologies, such as Docker. The idea is that you assemble the release inside the Docker container and then build an image based on the release artifacts.

If you call mix phx.gen.release --docker you'll see a new file with these contents:

# Find eligible builder and runner images on Docker Hub. We use Ubuntu/Debian instead of
# Alpine to avoid DNS resolution issues in production.
# This file is based on these images:
#   - - for the build image
#   - - for the release image
#   - - resource for finding needed packages
#   - Ex: hexpm/elixir:1.12.0-erlang-24.0.1-debian-bullseye-20210902-slim
ARG DEBIAN_VERSION=bullseye-20210902-slim


FROM ${BUILDER_IMAGE} as builder

# install build dependencies
RUN apt-get update -y && apt-get install -y build-essential git \
    && apt-get clean && rm -f /var/lib/apt/lists/*_*

# prepare build dir

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

# set build ENV
ENV MIX_ENV="prod"

# install mix dependencies
COPY mix.exs mix.lock ./
RUN mix deps.get --only $MIX_ENV
RUN mkdir config

# copy compile-time config files before we compile dependencies
# to ensure any relevant config change will trigger the dependencies
# to be re-compiled.
COPY config/config.exs config/${MIX_ENV}.exs config/
RUN mix deps.compile

COPY priv priv

# note: if your project uses a tool like,
# which customizes asset compilation based on what it finds in
# your Elixir templates, you will need to move the asset compilation
# step down so that `lib` is available.
COPY assets assets

# compile assets
RUN mix assets.deploy

# Compile the release
COPY lib lib

RUN mix compile

# Changes to config/runtime.exs don't require recompiling the code
COPY config/runtime.exs config/

COPY rel rel
RUN mix release

# start a new build stage so that the final image will only contain
# the compiled release and other runtime necessities

RUN apt-get update -y && apt-get install -y libstdc++6 openssl libncurses5 locales \
  && apt-get clean && rm -f /var/lib/apt/lists/*_*

# Set the locale
RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && locale-gen


WORKDIR "/app"
RUN chown nobody /app

# set runner ENV
ENV MIX_ENV="prod"

# Only copy the final release from the build stage
COPY --from=builder --chown=nobody:root /app/_build/${MIX_ENV}/rel/my_app ./

USER nobody

CMD /app/bin/server

Where my_app is the name of your app. At the end, you will have an application in /app ready to run as /app/bin/server.

A few points about configuring a containerized application:

  • If you run your app in a container, the Endpoint needs to be configured to listen on a "public" :ip address (like so that the app can be reached from outside the container. Whether the host should publish the container's ports to its own public IP or to localhost depends on your needs.
  • The more configuration you can provide at runtime (using config/runtime.exs), the more reusable your images will be across environments. In particular, secrets like database credentials and API keys should not be compiled into the image, but rather should be provided when creating containers based on that image. This is why the Endpoint's :secret_key_base is configured in config/runtime.exs by default.
  • If possible, any environment variables that are needed at runtime should be read in config/runtime.exs, not scattered throughout your code. Having them all visible in one place will make it easier to ensure the containers get what they need, especially if the person doing the infrastructure work does not work on the Elixir code. Libraries in particular should never directly read environment variables; all their configuration should be handed to them by the top-level application, preferably without using the application environment.