Runtime Configuration

config.exs & sys.config

There are differences in how you approach runtime configuration of your application when using releases vs regular Mix projects. You still use your config.exs, but there are caveats which apply:

  • With releases, at runtime there is no longer any Mix project information, as your application is indistinguishable from any other Erlang application. This means your application should never use anything from the Mix.* namespace.
  • Related to the above, it is not possible to use Mix tasks, generally speaking, you can include the :mix application in a release, but whether it works is uncertain at best, due to the fact that Mix is designed to be used in conjunction with the Mix project structure, and with a mix.exs available. Neither of which are true in releases. There is ongoing work on making this possible, but at this time it is a caveat. Look into Custom Commands in the meantime.
  • With Mix projects, your configuration is evaluated at runtime, so you can use functions such as System.get_env/1 to conditionally change configuration based on the runtime environment. With releases, config.exs is evaluated at build time, and converted to a sys.config file. Using functions like System.get_env/1 will result in incorrect configuration. Instead, you must configure your application so that if you need to fetch environment information at runtime, you can do so. See the Configuration Conventions section below for more information.

Configuration Conventions

NOTE: It is no longer recommended to use the {:system, "VAR"} convention, as it has inconsitent use in the community, and instead we’d rather push people towards an approach which works every time, rather than only sometimes.

Instead the recommend approach to configuration is as follows:

  • Provide default configuration in config/config.exs
  • Provide configuration values known in advance for a given environment in config/<env>.exs
  • For any runtime configuration, either set those values in the Application start/2 callback, prior to starting your supervisor tree, or in the init/1 callbacks of individual supervisors/workers in your application. You can use Elixir code to load configuration from wherever, and validate it closest to where it’s used. It is recommended that you create your own Config module which boxes up common validation/loading logic so that you can easily get config values at runtime without cluttering up your start/2 or init/1 callbacks.

The above approach works for both release and deploy-source deployments, and so works the same everywhere. In addition, it keeps configuration close to where it’s cared about, and enables you to pick up configuration changes when you restart parts of the application without needing to restart the whole release.

Configuring Dependencies

If you have dependencies which require runtime configuration, you should add them to included_applications and start them as part of your supervisor tree, as shown below.

defmodule MyApplication do
  use Application

  def start(_type, _args) do
    import Supervisor.Spec

    # Do your configuration here
    Application.put_env(:some_dep, :some_key, some_val)

    children = [
      # You need to replace `:some_dep` here with the name of the module
      # containing `:some_dep`'s application module. For example, our application
      # is called `:myapp`, but our application module is `MyApplication`, so we would
      # use `MyApplication` in the supervisor specification here, not `:myapp`.
      # You can easily determine this by looking at the `mix.exs` of the dependency, or the
      # `.app` file in an Erlang application.
      supervisor(:some_dep, [:normal, []], function: :start),
    Supervisor.start_link(children, opts)

Configuration Tools

It may be the case that you are providing release packages to end users, who will need to configure the application. These end users may even be your own internal ops team. They may or may not be familiar with Erlang terms, and thus sys.config is a very poor configuration experience. In these situations you may want to consider an alternative configuration tool I wrote, called conform. It was designed specifically for use with releases, and with ease of use for end-users as the ultimate goal.

As a developer, you define a schema which maps simple configuration settings contained in an init-style .conf file, into the specific structures required for your application’s configuration. To see an example of what these look like, take a look here.

When you deploy your application, users simply modify the .conf file as needed, and run the release, conform handles converting the configuration into runtime configuration for the release using the schema, and you can access that configuration via Application.get_env/2 as usual.


This file is how you configure the Erlang runtime for your release. By default, it sets up the VM in distributed mode with the name set to <release_name>@, and the cookie set to <release_name>. This is how we are able to remote shell to the node once it’s started.

For a complete list of flags you can use in vm.args, see here.

However, as is often the case, you may want to dynamically configure the name of the node (when clustering) and/or the cookie (for security), as well as other settings based on values provided via environment variables. You can do so by setting REPLACE_OS_VARS=true and then using ${VAR_NAME} in the vm.args file.

If you are uncertain where the default vm.args is located, you may find it under releases/<version>/vm.args.

You may also provide your own config directory where your custom vm.args, sys.config, and potentially other configuration files will be loaded from, by setting RELEASE_CONFIG_DIR=path/to/files. By default this will be set to the root directory of the release, i.e. the folder to which you extracted the tarball. If vm.args or sys.config cannot be found in RELEASE_CONFIG_DIR, it will fall back to using the ones under the releases/<version> directory.


The run_erl command is responsible for running a release on Unix systems, capturing STDERR and STDOUT so that all output can be logged as well as allowing monitoring and remote debugging of a running release.

Several environment variables are useful in configuring run_erl, for example to customize logging output. To specify environment variables to apply to run_erl, you can add a line like set run_erl_env: "RUN_ERL_LOG_MAXSIZE=10000000 RUN_ERL_LOG_GENERATIONS=10" in your release configuration.

This configuration can also be specified in the RUN_ERL_ENV environment variable at the time of running mix release.

For a complete list of environment variables respected by run_erl, see here.