Last updated 2nd June 2022


Web PaaS supports building and deploying applications written in Elixir. There is no default flavor for the build phase, but you can define it explicitly in your build hook. Web PaaS Elixir images support both committed dependencies and download-on-demand. The underlying Erlang version is 22.0.7.

Supported versions


To specify an Elixir container, use the type property in your .platform.app.yaml.

type: 'elixir:1.9'

Web PaaS variables

Web PaaS exposes relationships and other configuration as environment variables. Most notably, it allows a program to determine at runtime what HTTP port it should listen on and what the credentials are to access other services.

To get the PORT environment variable (the port on which your web application is supposed to listen) you would:

String.to_integer(System.get_env("PORT") || "8888")

Some of the environment variables are in JSON format and are base64 encoded. You would need to import a JSON parsing library such as JSON or Poison to read those. (There is an example for doing this to decode the PLATFORM_RELATIONSHIPS environment variable in the section below.)

Tip: Remember config/prod.exs is evaluated at build time and has no access to runtime configuration. Use config/releases.exs to configure your runtime environment.

Building and running the application

If you are using Hex to manage your dependencies, it is necessary to specify a set of environment variables in your .platform.app.yaml file that define the MIX_ENV and SECRET_KEY_BASE, which can be set to the Web PaaS-provided PLATFORM_PROJECT_ENTROPY environment variable:

        MIX_ENV: 'prod'

Include in your build hook the steps to retrieve a local Hex and rebar, and then run mix do deps.get, deps.compile, compile on your application to build a binary.

    build: |
        mix local.hex --force
        mix local.rebar --force
        mix do deps.get --only prod, deps.compile, compile

The above build hook works for most cases and assumes that your mix.exs file is located at your app root.

Assuming mix.exs is present at your app root and your build hook matches the above, you can then start it from the web.commands.start directive.

The start command must run in the foreground, so you should set the --no-halt flag when calling mix run.

The following basic .platform.app.yaml file is sufficient to run most Elixir applications.

name: app

type: elixir:1.9

        MIX_ENV: 'prod'

    build: |
        mix local.hex --force
        mix local.rebar --force
        mix do deps.get --only prod, deps.compile, compile

        start: mix run --no-halt
            allow: false
            passthru: true

Note that there is still an Nginx proxy server sitting in front of your application. If desired, certain paths may be served directly by Nginx without hitting your application (for static files, primarily) or you may route all requests to the Elixir application unconditionally, as in the example above.


The recommended way to handle Elixir dependencies on Web PaaS is using Hex. You can commit a mix.exs file in your repository and the system downloads the dependencies in your deps section using the build hook above.

  defp deps do
      {:platformshconfig, "~> 0.1.0"}

Accessing Services

{{% config-reader %}} Web PaaS Config Reader library {{% /config-reader%}}

If you are building a Phoenix app for example, it would suffice to add a database to .platform/services.yaml and a relationship in .platform.app.yaml. Put the lib in your deps and, assuming you renamed the prod.secret.exs to releases.exs per the Phoenix guide, change:




See Web PaaS Config Reader Documentation for the full API.

Accessing Services Manually

The services configuration is available in the environment variable PLATFORM_RELATIONSHIPS.

Given a relationship defined in .platform.app.yaml:

    postgresdatabase: "dbpostgres:postgresql"

Assuming you have in mix.exs the Poison library to parse JSON:

  defp deps do
      {:poison, "~> 3.0"}

And assuming you use ecto you could put in config/config.exs:

relationships = Poison.decode!(Base.decode64!(System.get_env("PLATFORM_RELATIONSHIPS")))
[postgresql_config | _tail] = relationships["postgresdatabase"]

config :my_app, Repo,
  database: postgresql_config["path"],
  username: postgresql_config["username"],
  password: postgresql_config["password"],
  hostname: postgresql_config["host"]

and setup Ecto during the deploy hook:

deploy: |
    mix do ecto.setup

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