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Handling Configuration

One of the things you may have heard about deploying releases is that configuration is hard. This was never exactly completely true, but in previous iterations of the release tooling, it was certainly more difficult to deal with some edge cases; particularly dealing with applications that abuse the application env for configuration, rather than using parameterized processes and functions. This pain was due to the fact that Mix’s config file, config.exs, was not usable directly in releases.

Background

Releases instead required a sys.config file, essentially a static form of configuration defined in Erlang syntax. In order to build releases for Elixir applications, older tooling compiled the results of evaluating the Mix config file to sys.config format, and bundled that into the release.

This meant that some constructs for runtime configuration, namely dynamic things, like System.get_env/1 would be evaluated at build time, not runtime, which was a source of confusion. Older tools introduced REPLACE_OS_VARS, an environment variable you export at runtime which instructed the tool to replace occurances of ${VAR} with the value from the system environment in the sys.config file.

The problem with REPLACE_OS_VARS is that it’s essentially a hack, and as such had limitations - you could only use the ${VAR} syntax in strings, because otherwise Mix couldn’t evaluate the config. The result is that you could only do “dynamic” configuration with string values, and not values of other types.

This was generally only a problem with applications that were not well behaved with regard to configuration - but sometimes you can’t choose your dependencies, and so a solution was still needed.

Included Applications

One approach to working around the problem of configuring an application needing dynamic configuration at boot is adding it to :included_applications. Applications in this list are not automatically started by the runtime, instead it is up to your application to start them as part of your supervision tree. Doing so allows you to perform some work before that application is started, namely configuration.

The downside of :included_applications is that an application can only be included once, and any applications which depend on it being started must also be included, or they will never start and your application will never start, leading to a chicken and egg problem where you can’t start the included application because your app can’t start because the included application isn’t started!

In general this approach should be reserved for when it is the only solution, or when your application truly needs to control the lifecycle of the included application. Usually this only applies to applications within your stack, not dependencies.

Alternative Tools

There were a variety of tools which were born out of the need to work around runtime configuration of poorly designed applications. One of which I wrote, called Conform. Libraries like this worked as release plugins, manipulating the sys.config file via hooks at runtime to inject custom configuration.

While such tools work, they are inherently fragile, as they needed to do orchestration via shell scripts to properly handle replacing configs without destroying originals, playing nice with upgrades, and integrating with some of the release tooling’s implicit behavior. Ideally we wanted something that was integrated at the same level as the sys.config mechanism itself.

Config Providers

In 2.0+, Distillery provides a framework for loading configuration at runtime from sources other than sys.config, called Config Providers. Providers are just modules which implement a behavior, and are configured via rel/config.exs. You can have no config providers (using just sys.config) or many providers (running in order and applying configuration on top of the last).

Config providers are executed prior to boot, and the resulting application env is then persisted to a final sys.config, which is then used by the release itself.

Config providers can be used by adding the provider module and its configuration to rel/config.exs like so:

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environment :prod do
  set config_providers: [
    {Toml.Provider, [path: "${RELEASE_ROOT_DIR}/config.toml"]}
  ]
  set overlays: [
    {:copy, "config/defaults.toml", "config.toml"}
  ]
end

The above (using the provider from toml) instructs Distillery to modify the boot script for the release to execute the provider with Toml.Provider.init([path: "..."]). If you are wondering how to get a config file in the release, the above example also shows how you might use an overlay to copy a default config file into the release, to a location which will not overwrite a previous config when the release is extracted, but will be handy when configuring the release for the first time.

Any provider module must implement the Mix.Releases.Config.Provider behavior. See its documentation for details.

This configuration framework supports a variety of interesting configuration scenarios, such as reading from files like .toml, .yaml, or .json; using :httpc to request configuration from a metadata endpoint like etcd or Consul.

Mix Config Provider

Out of the box, Distillery also includes a provider for Mix.Config, which allows you to use config.exs files with a release. You have to opt in to this provider, because Mix.Config blends compile-time and runtime configuration options into one file, and many legacy configs make assumptions about the environment the config is evaluated in (namely that a Mix project is available, or that commands like git can be invoked and have meaningful output). You can use it like so:

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environment :prod do
  set config_providers: [
    {Mix.Releases.Config.Providers.Elixir, ["${RELEASE_ROOT_DIR}/etc/config.exs"]}
  ]
  set overlays: [
    {:copy, "rel/config/config.exs", "etc/config.exs"}
  ]
end

You may notice above that I’m pulling in a Mix config from rel/config/config.exs, this is the recommended way to deal with runtime configuration using Mix.Config; create a dedicated config which handles fetching runtime configuration only, and place it in rel/config. Use the standard config/*.exs files for compile-time configuration (of course, feel free to use dev.exs or it’s equivalent for development runtime config).

You can copy configs straight from config/, but I would avoid doing so.

Alternative Configuration

There are some additional methods for providing configuration, either for the VM or for applications:

Option 1: erl_opts

One approach is to provide configuration via :erl_opts:

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environment :prod do
  set erl_opts: "-kernel sync_nodes_mandatory '[foo@hostname, bar@hostname]'"
end

This works well for simple config values, but is unwieldy for complex values (such as the example above).

Option 2: vm.args

Another approach is to set config flags via vm.args:

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environment :prod do
  set vm_args: "rel/prod.vm.args"
end
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# prod.vm.args
-kernel sync_nodes_mandatory '[foo@${HOSTNAME}, bar@${HOSTNAME}]'

This approach is a much better way of handling arguments to the VM during boot. And with REPLACE_OS_VARS=true exported in the runtime environment, values can even be dynamic, as shown in the example above, where ${HOSTNAME} would be replaced with the value of the system HOSTNAME environment variable.

The downside is that it still relies on some degree of static information, e.g. the nodes in the sync_nodes_mandatory list above are known in advance in the example, but what about when the node names are not known?

Option 3: Runtime Generation

If none of the above are workable solutions, you are likely in a situation where you will need to construct vm.args or sys.config dynamically as part of your deployment, or using boot hooks like pre_configure. You would do something like the following:

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environment :prod do
  # The template for vm.args
  set vm_args: "rel/prod.vm.args"
  # The hook to mutate the template
  set pre_configure_hooks: "rel/hooks/pre_configure.d"
end
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# prod.vm.args
-kernel sync_nodes_mandatory '[${SYNC_NODES_MANDATORY}]'
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#!/usr/bin/env bash
# hooks/pre_configure.d/generate_vm_args.sh

export SYNC_NODES_MANDATORY

if [ -z "$HOSTNAME" ]; then
  echo 'Expected $HOSTNAME to be set in the environment, unable to configure mandatory nodes!'
  exit 1
fi

# If no nodes defined, set it to an empty list
if [ -z "$NODES"]; then
  SYNC_NODES_MANDATORY=""
else
  # Expects $NODES to be something like 'foo,bar,baz'
  for node in `echo "$NODES" | sed -e s/,/\n/g`; do
    if [ -z "$SYNC_NODES_MANDATORY" ]; then
      # If no nodes defined yet, adding the first
      SYNC_NODES_MANDATORY="\\'${node}@${HOSTNAME}\\'"
    else
      # Otherwise append an element to the list with ,
      SYNC_NODES_MANDATORY="${SYNC_NODES_MANDATORY},\\'${node}@${HOSTNAME}\\'"
    fi
  done
fi

This setup above will generate, at runtime, a final vm.args that looks something like the following, given NODES="foo,bar,baz" and HOSTNAME=host.local:

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-kernel sync_nodes_mandatory '[\'foo@host.local\',\'bar@host.local\',\'baz@host.local\']'

Option 4: Configuration Management

Last, but not least, using configuration management systems like Salt, Puppet, Chef, Ansible, etc., are great ways to handle generating the vm.args or other configuration files during deployment. They typically have better templating tools so that you don’t have to write something from scratch in shell.

If this is an option for you, I would start here before reaching for any of the above options.

Wrap up

As you can see, there are a variety of ways to configure your release at runtime. In general, you should reach for config providers for app config, and vm.args for VM config - if those approaches aren’t viable, one of these others may work for you.

Configuring Applications

Lastly, I want to provide some general guidance on designing your applications to be easily configurable when used as a component in a larger system.

Rule 1: Parameters, not environment

If you need some piece of configuration, and you are wondering how best to handle getting it to where it is needed, always try to pass configuration as parameters first, rather than using the application environment (e.g. Application.get_env/2).

Non-OTP Applications

For non-OTP applications, this is simple, have all your API functions which require configuration, accept a list of options as a parameter, and pass it down the stack to where it’s needed.

You can make this parameter default to an empty list, or some predetermined set of defaults with opts \\ [] or opts \\ @defaults, like so:

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defmodule Log do
  @default_device :standard_io
  @default_opts [device: @default_device]

  def info(msg, opts \\ @default_opts) do
    device = Keyword.get(opts, :device, @default_device)
    IO.puts(device, msg)
  end
end

In the example above, users of this API can use Log.info(msg) for the most part, unless they need to change the defaults. In cases where you can’t accept a default, require the extra parameter, or use Keyword.fetch!/2 to raise an error if an option was not provided.

OTP Applications

For OTP applications, parameterization starts at the root of your application, in the application callback module, namely in the start/2 callback:

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defmodule MyApp do
  use Application

  def start(_type, _args) do
    Supervisor.start_link(__MODULE__, [], name: __MODULE__.Supervisor)
  end

  def init(_) do
    all = Application.get_all_env(:myapp)
    config = [
      port: get_in(all, [__MODULE__.Component, :port]) || System.get_env("PORT"),
      # ...
    ]
    children = [
      {__MODULE__.Component, [config]},
      # ...
    ]
    Supervisor.init(children, strategy: :one_for_one)
  end
end

In the above example, our application will fetch configuration from the application environment, but then build up the actual configuration which will be passed to children of the root supervisor by first looking for a value in the application env, then falling back to other methods. Once constructed, this configuration is passed down the supervision tree, from parent to child, starting at the root.

This approach can be extracted into a MyApp.Config module, where you can put all the logic for fetching configuration and converting it into it’s final form, keeping your application callback module clean.

An additional benefit of this approach is that in your tests, you can start many copies of __MODULE__.Component with different configuration, allowing you to test that component in parallel. This requires taking an approach to designing your processes so that they are not relying on singletons or other global state, but the very first step is taking configuration as parameters.

“A la carte” OTP Applications

This approach to designing applications is similar to what I just described, but your application callback module does not actually start any children. Instead, users of your application start things like __MODULE__.Component within their own supervision tree, and take over responsibility for providing configuration via parameters.

You may still have need for some global state, or singleton processes, which do not require configuration, and are invisible to users of your application. In these cases your application callback module will start only these children, and leave the rest up to users of the application. You can see this approach in action in the Postgrex library, for example.

run_erl

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.