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Mix tasks to deploy an Elixir release
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This module generates scripts which help deploy an Erlang release created with Elixir 1.9's native release support.

It supports deployment to the local machine, bare-metal servers or cloud servers using e.g. AWS CodeDeploy.

It generates scripts which can be run on the local machine or copied to a target machine to handle lifecycle tasks such as creating initial directory structure, unpacking release files, managing configuration, and starting/stopping,

It uses the mix_systemd library to generate a systemd unit file for the application, and shares conventions with it about naming files and systemd unit files.

Here is a complete example app which uses mix_deploy.


Add mix_deploy to the list of dependencies in mix.exs:

def deps do
    {:mix_systemd, "~> 0.6.0"},
    {:mix_deploy, "~> 0.6.0"},


The library gets standard information in mix.exs, e.g. the app name and version, then calculates default values for its configuration parameters.

By default, with no configuration, the scripts are set up for building and deploying on the same machine. The scripts deploy with the same OS user that runs the mix deploy.generate command, and run the app under an OS user with the same name as the app.

You can override these parameters using settings in config/config.exs, e.g.

config :mix_systemd,
  app_user: "app",
  app_group: "app"

config :mix_deploy,
  deploy_user: "deploy",
  deploy_group: "deploy",
  app_user: "app",
  app_group: "app"

The library tries to choose smart defaults, so you may not need to configure anything. See below for more options.


The deploy.init task copies template files into your project, then the deploy.generate task uses them to create the output files.

First, initialize templates under the rel/templates/deploy directory by running this command:

mix deploy.init

Next, generate the scripts based on your project's config:

MIX_ENV=prod mix deploy.generate
chmod +x bin/*

By default, mix deploy.generate creates scripts under a bin directory at the top level of your project. If you want to keep them separate, e.g. to create different files based on the environment, set output_dir_per_env: true in the config, and it will generate files under e.g. _build/prod/deploy.


This library generates the following scripts:

Systemd scripts

  • deploy-start: Start services
  • deploy-stop: Stop services
  • deploy-restart: Restart services
  • deploy-enable: Enable systemd units

Local deploy scripts

  • deploy-create-users: Create OS accounts for app and deploy users
  • deploy-create-dirs: Create dirs, e.g. /srv/foo/releases
  • deploy-copy-files: Copy files to target or staging directory
  • deploy-release: Deploy release, extracting to a timestamped dir under releases, then making a symlink
  • deploy-rollback: Rollback release, resetting the symlink to point to the last release

CodeDeploy deploy scripts

  • deploy-create-users: Create OS accounts for app and deploy users
  • deploy-clean-target: Delete target dir in preparation for install
  • deploy-extract-release: Extract release from tar to target current dir
  • deploy-set-perms: Set target file permissions so they can be used by deploy and/or app user

Build server scripts

  • deploy-stage-files: Copy output files to staging directory
  • deploy-sync-assets-s3: Sync priv/static files to S3 bucket for CloudFront CDN

Custom command scripts

  • deploy-migrate: Migrate database on target system by running a custom command. This runs under the app user account, not under sudo

  • deploy-remote-console: Launch a remote console for the app, setting up the environment properly. This runs interactively under the app user account, not under sudo

Environment setup scripts

These may be called by the systemd startup unit to get the config at runtime based on the environment.

  • deploy-runtime-environment-file: Create #{runtime_dir}/runtime-environment file on target from cloud-init metadata.
  • deploy-runtime-environment-wrap: Get runtime environment from cloud-init metadata, set environment vars, then launch main script. Probably better done with rel/
  • deploy-sync-config-s3: Sync config files from S3 bucket to app config dir
  • deploy-set-cookie-ssm: Get Erlang VM cookie from AWS SSM Parameter Store and write to file. Probably better done with rel/


The generated scripts are mostly straight bash, with minimal dependencies.

  • deploy-runtime-environment-file and deploy-runtime-environment-wrap use jq to parse the cloud-init JSON file.
  • deploy-sync-config-s3 uses the AWS CLI to copy files from an S3 bucket.
  • deploy-set-cookie-ssm uses the AWS CLI and jq to interact with Systems Manager Parameter Store.

To install jq on Ubuntu:

apt-get install jq

To install the AWS CLI from the OS package manager on Ubuntu:

apt-get install awscli


Deploy on local machine

With a local deploy, you check out the code on a server, build/test, then generate a release. You then run the scripts to set up the runtime environment, including systemd unit scripts, extract the release to the target dir and run it under systemd.

deploy-init-local is a convenience script which runs the other scripts to set up the system:

sudo bin/deploy-init-local

It does the following:

# Create users to run the app
sudo bin/deploy-create-users

# Create deploy dirs under /srv/foo and app dirs like /etc/foo
sudo bin/deploy-create-dirs

# Copy scripts used at runtime by the systemd unit
sudo cp bin/* /srv/foo/bin

# Copy and enable systemd unit files
sudo bin/deploy-copy-files
sudo bin/deploy-enable

After the initial setup, build a release as you normally would:

# Create release
MIX_ENV=prod mix release

Then deploy the release to the local machine:

# Extract release to target directory, creating current symlink

# Restart the systemd unit
sudo bin/deploy-restart

Roll back the release with the following:

sudo bin/deploy-restart

This library generates the scripts with paths and users based on the application configuration.

By default, the scripts deploy the scripts as the same OS user that runs the mix deploy.generate command, and run the app under an OS user with the same name as the app.

You can override some variables using environment vars at execution time. For example, you can override the user accounts which own the files by setting the environment vars APP_USER, APP_GROUP, and DEPLOY_USER. Similarly, set DESTDIR and the copy script will add a prefix when copying files. This lets you copy files to a staging directory, tar it up, then extract it on a target machine.

For example:

mkdir -p ~/tmp/deploy
DESTDIR=~/tmp/deploy bin/deploy-create-dirs
DESTDIR=~/tmp/deploy bin/deploy-copy-files


Copy the scripts into the target machine, then run them as hooks for a deployment system such as AWS CodeDeploy.

Here is an example appspec.yml file:

version: 0.0
os: linux
  - source: bin
    destination: /srv/mix-deploy-example/bin
  - source: systemd
    destination: /lib/systemd/system
  - source: etc
    destination: /srv/mix-deploy-example/etc
    - location: bin/deploy-stop
      timeout: 300
    - location: bin/deploy-create-users
    - location: bin/deploy-create-dirs
    - location: bin/deploy-clean-target
    - location: bin/deploy-extract-release
    - location: bin/deploy-set-perms
    - location: bin/deploy-enable
    - location: bin/deploy-migrate
      runas: app
      timeout: 300
    - location: bin/deploy-start
      timeout: 3600
    - location: bin/validate-service
      timeout: 3600

Configuration options

The following sections describe common configuration options. See lib/mix/tasks/deploy.ex for the details of more obscure options.

If you need to make changes not supported by the config options, then you can check the templates into source control from rel/templates/deploy and make your own changes (contributions welcome!). You can also check in the generated scripts in the bin dir.

The list of templates to generate is in the templates config var. You can modify this list to remove scripts, and they won't be generated. You can also add your own scripts and they will be run as templates with the config vars defined.

templates: [


app_name: Elixir application name, an atom, from project app in mix.exs.

version: version from mix.exs project.

ext_name: External name, used for files and directories. Defaults to app_name with underscores converted to "-".

service_name: Name of the systemd service. Defaults to ext_name.

base_dir: Base directory where deploy files will go, default is /srv to follow systemd conventions.

deploy_dir: Directory where files will go, default is #{base_dir}/#{ext_name}


app_user: OS user account that the app should run under. Defaults to ext_name.

app_group: OS group account, defaults to ext_name.

deploy_user: OS user account that is used to deploy the app, e.g. own the files and restart it.

For security, this is separate from app_user, keeping the runtime user from being able to modify the source files.

This defaults to the user running the script, supporting local deploy. For remote deploy, set this to a user like deploy or the same as the app user.

deploy_group: OS group account, defaults to deploy_user.


restart_method: :systemctl | :systemd_flag | :touch, default :systemctl

The normal situation is that the app will be restarted using systemctl, e.g. systemctl restart #{service_name}.

sudo_deploy: Create an /etc/sudoers.d/#{ext_name} file allowing the deploy user to start/stop/restart the app using sudo. Default false.

sudo_app: Create an /etc/sudoers.d/#{ext_name} file allowing the app user user to start/stop/restart the app using sudo. Default false.

mix_systemd can generate an additional systemd unit file which watches for changes to a flag file and restarts the main unit. This allows updates to be pushed to the target machine by an unprivileged user account which does not have permissions to restart processes. touch the file #{flags_dir}/restart.flag and systemd will restart the unit. See mix_systemd for details.

Environment vars

The library sets a few common env vars:

  • mix_env: default Mix.env(), sets MIX_ENV.
  • env_lang: default en_US.UTF-8, used to set LANG.


Modern Linux defines a set of directories which apps use for common purposes, e.g. configuration or cache files. App files under /srv, configuration under /etc, transient files under /run, data under /var/lib. See systemd.exec for details.

Directories are named based on the app name, e.g. /etc/#{ext_name}. The dirs variable specifies which directories the app uses, by default:

dirs: [
  :runtime,       # App runtime files which may be deleted between runs, /run/#{ext_name}
                  # Used for RELEASE_TMP, RELEASE_MUTABLE_DIR, runtime-environment
  :configuration, # App configuration, e.g. db passwords, /etc/#{ext_name}
  # :state,       # App data or state persisted between runs, /var/lib/#{ext_name}
  # :cache,       # App cache files which can be deleted, /var/cache/#{ext_name}
  # :logs,        # App external log files, not via journald, /var/log/#{ext_name}
  # :tmp,         # App temp files, /var/tmp/#{ext_name}

More recent versions of systemd (after 235) will create these directories at start time based on the settings in the unit file. For earlier systemd versions, deploy-create-dirs will create them.

For security, we set permissions to 750, more restrictive than the systemd defaults of 755. You can configure them with e.g. configuration_directory_mode. See the defaults in lib/mix/tasks/deploy.ex.

systemd_version: Sets the systemd version on the target system, default 235. This determines which systemd features the library will enable. If you are targeting an older OS release, you may need to change it. Here are the systemd versions in common OS releases:

  • CentOS 7: 219
  • Ubuntu 16.04: 229
  • Ubuntu 18.04: 237

Additional directories

The library assumes a directory structure under deploy_dir which allows it to handle multiple releases, similar to Capistrano.

  • scripts_dir: deployment scripts which e.g. start and stop the unit, default bin.
  • current_dir: where the current Erlang release is unpacked or referenced by symlink, default current.
  • releases_dir: where versioned releases are unpacked, default releases.
  • flags_dir: dir for flag files to trigger restart, e.g. when restart_method is :systemd_flag, default flags.

When using multiple releases and symlinks, the deployment process works like this:

  1. Create a new directory for the release with a timestamp like /srv/foo/releases/20181114T072116.

  2. Upload the new release tarball to the server and unpack it to the releases dir

  3. Make a symlink from /srv/#{ext_name}/current to the new release dir.

  4. Restart the app.

If you are only keeping a single version, then you would simply deploy it to the /srv/#{ext_name}/current dir.

Runtime configuration

Configuration of an Elixir app can be split into three parts:

Build time settings are consistent for all servers, though they may have different options between e.g. staging and production. This is handled by config files config/config.exs and config/prod.exs, which result in an initial fixed application environment file in the release.

Environment / per machine / secrets settings depend on the environment the application is running in, e.g. the hostname of the db server and secrets like the db password. We store these external to the application release and load them from files or a configuration system like AWS Systems Manager Parameter Store or etcd.

This library generates scripts which can be called by systemd to get configuration. See mix_systemd for details.

deploy-sync-config-s3 syncs config files from an S3 bucket to the app config dir, e.g. /etc/foo. For example, we can use a config file in TOML format read at startup by the TOML configuration provider.

Conform is a similar way of making a machine-specific config file. Conform has been depreciated in favor of TOML.

deploy-set-cookie-ssm gets the Erlang VM cookie from AWS SSM Parameter Store and writes it to a file.

Runtime settings are dynamic and may change every time the application starts. For example, if we are running in an AWS auto scaling group, the IP address of the server normally changes every time it starts.

deploy-runtime-environment-file reads config from cloud-init and writes it to an environment file #{runtime_dir}/runtime-environment.

Similarly, deploy-runtime-environment-wrap gets cloud-init metadata, sets environment vars, then launches the main start script. This is probably best done by rel/ now.

If these are not flexible enough, you can make your own systemd unit which is set as a dependency of the app unit, so it will run first. Set runtime_environment_service to true here and configure it in mix_systemd.

Syncing config from S3

Here is a complete example of configuring an app from a config file which it pulls from S3 on startup.

We set up an ExecStartPre command in the systemd unit file which runs deploy-sync-config-s3 before starting the app. It runs the AWS cli command

aws s3 sync "s3://${CONFIG_S3_BUCKET}/${CONFIG_S3_PREFIX}" "${CONFIG_DIR}/"

CONFIG_S3_BUCKET is the source bucket, and CONFIG_S3_PREFIX is an optional path in the bucket. CONFIG_DIR is the app configuration dir on the target system, /etc/foo.

Set exec_start_pre in the mix_systemd config:

config :mix_systemd,
  app_user: "foo",
  app_group: "foo",
  exec_start_pre: [

For security, the app only has read-only access to its config files, and /etc/foo has ownership deploy:foo and mode 750. We prefix the command with "!" so it runs with elevated permissions, not as the foo user.

We need to set the CONFIG_S3_BUCKET variable in the environment so that deploy-sync-config-s3 can use it.

The systemd unit has EnvironmentFile commands which load environment variables from a series of files.


The systemd unit tries to load each in turn, and later files override earlier ones. The "-" on the front makes the file optional.

Settings in /srv/foo/current/etc/environment are the "build" environment, and /srv/foo/etc/environment are "deploy". Deploy settings take priority, as they may override the build defaults based on where we are deploying or what machine.

pre_build commands in the CodeBuild buildspec.yml file generates rel/etc/environment:

    - mkdir -p rel/etc
    - echo "CONFIG_S3_BUCKET=$CONFIG_S3_BUCKET" >> rel/etc/environment


In rel/config.exs an overlay adds the rel/etc/environment file to the release under etc/environment, which makes it show up as /srv/foo/current/etc/environment:

  set overlays: [
    {:mkdir, "etc"},
    {:copy, "rel/etc/environment", "etc/environment"},

You can also set variables in files/etc/environment and copy that to /srv/foo/etc/environment in e.g. a CodeDeploy appspec.yml:

  - source: bin
    destination: /srv/foo/bin
  - source: systemd
    destination: /lib/systemd/system
  - source: etc
    destination: /srv/foo/etc
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