Elixir

Upsun Fixed 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. Upsun Fixed Elixir images support both committed dependencies and download-on-demand. The underlying Erlang version is 22.0.7.

Supported versions

You can select the major and minor version.

Patch versions are applied periodically for bug fixes and the like. When you deploy your app, you always get the latest available patches.

Specify the language

To use Elixir, specify elixir as your app’s type:

myapp:
  type: 'elixir:<VERSION_NUMBER>'

For example:

myapp:
  type: 'elixir:1.18'

Built-in variables

Upsun Fixed 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.)

Building and running the application

If you are using Hex to manage your dependencies, you need to specify the MIX_ENV environment variable:

variables:
  env:
    MIX_ENV: 'prod'

The SECRET_KEY_BASE variable is generated automatically based on the PLATFORM_PROJECT_ENTROPY variable. You can change it.

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.

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

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 following basic app configuration is sufficient to run most Elixir applications.

name: myapp

type: 'elixir:1.18'

variables:
  env:
    MIX_ENV: 'prod'

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

web:
  commands:
    start: mix phx.server
  locations:
    /:
      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.

Dependencies

The recommended way to handle Elixir dependencies on Upsun Fixed 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"}
    ]
  end

Accessing Services

You can get all information about a deployed environment, including how to connect to services, through environment variables. Your app can access these variables.

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:

System.get_env("DATABASE_URL")

to

Platformsh.Config.ecto_dsn_formatter("database")

See 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:

# Relationships enable an app container's access to a service.
# The example below shows simplified configuration leveraging a default service
# (identified from the relationship name) and a default endpoint.
# See the Application reference for all options for defining relationships and endpoints.
relationships:
  postgresql:

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

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

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["postgresql"]

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:

hooks:
  deploy: |
    mix do ecto.setup    

Configuration reader

While you can read the environment directly from your app, you might want to use the Elixir configuration reader library. It decodes service credentials, the correct port, and other information for you.