FeatureFlag

The What

FeatureFlag allows you to write functions that can have their behavior changed by setting/modifying a config value.

For instance, using FeatureFlag, you can define a function like so:

defmodule MyApp do
  use FeatureFlag

  def math(x, y), feature_flag do
    :add -> x + y
    :multiply -> x * y
    :subtract -> x - y
  end
end

This function will do one of three things depending on its feature flag value. You must set this value in your config like:

config :feature_flag, :flags, %{
  # the key here is {module_name, function_name, arity}
  {MyApp, :add, 2} => :add
}

At this point, the function would behave like so:

iex> MyApp.math(3, 4)
7

At runtime, you can change feature flag value using FeatureFlag.set/2, like so:

FeatureFlag.set({MyApp, :add, 2}, :multiply)

And now the function would behave like so:

iex> MyApp.math(3, 4)
12

Boolean feature flags

If a function’s feature flag will only ever be true or false, you can definie your function like so:

defmodule MyApp do
  use FeatureFlag

  def get(key), feature_flag do
    get_from_cache(key)
  else
    get_from_db(key)
  end
end

Use Case

The goal of this library is to make Elixir less pure provide an elegant and consistent mechanism for changing what a function does depending on a value that can easily be modified (i.e. a configuration value).

This could very easily be done in plain Elixir via a simple case statement:

defmodule MyApp do
  def math(x, y) do
    case Application.fetch_env!(:my_app, :math) do
      :add -> x + y
      :multiply -> x * y
      :subtract x - y
    end
  end
end

There’s nothing wrong with this approach, and really no need to reach for anything else.

However, beyond removing a marginal amount of code, FeatureFlag provides a consistent interface for defining functions with this config-based branching.

Usage

Add FeatureFlag as a dependency in your mix.exs file.

def deps do
  [
    {:feature_flag, "~> 0.1.4"}
  ]
end

Run mix deps.get, then define your function:

defmodule MyApp
  use FeatureFlag

  def get(key), feature_flag do
    :old_database ->
      get_from_old_database(key)

    :new_database ->
      get_from_new_database(key)

    :get_from_newer_database ->
      get_from_newer_database(key)
  end
end

If you attempt to compile now, it will fail, because you need to explictly declare the feature flag value for this function in your config:

# config/{dev,test,prod}.exs

config :feature_flag, :flags, %{
  {MyApp, :get, 1} => :old_database
}

Then you’re done! Initially, this function will simply execute the :old_database block. You can change this at runtime by running:

FeatureFlag.set({MyApp, :get, 1}, :new_database)

Mentions

I’d like to thank the following people who contributed to this project either via code and/or good ideas:

• @evuez
• @zph

I’d also like to thank @Packlane for giving me time to work on and share this software.