Toggle is a stupid-simple feature flag library for Elixir.

It provides a simple API for enabling and disabling feature flags in your application.

Installation

Add toggle to your list of dependencies in mix.exs:

def deps do
  [
    {:toggle, "~> 0.1.0"}
  ]
end

Usage

In order to use Toggle, you need to add an Ecto Migration to your application to set up the database table Toggle will use to store data.

Currently, Toggle supports SQLite3 and Postgres as database backends.

You can create migrations using the Toggle.Migrations.SQLite or Toggle.Migrations.Postgres helper modules:

defmodule MyApp.Repo.Migrations.CreateFlagsTables do
  use Ecto.Migration

  def up do
    # Replace `Toggle.Migrations.SQLite` with `Toggle.Migrations.Postgres` if you are using Postgres
    Toggle.Migrations.SQLite.up(table: "toggle_flags")
  end

  def down do
    # Replace `Toggle.Migrations.SQLite` with `Toggle.Migrations.Postgres` if you are using Postgres
    Toggle.Migrations.SQLite.down(table: "toggle_flags")
  end
end

After you have run your migrations, you can start using Toggle in your application.

iex> Toggle.enabled?("my_feature")
false
iex> Toggle.enable!("my_feature")
:ok
iex> Toggle.enabled?("my_feature")
true

You can also enable flags specifically for some arbitrary resource:

iex> Toggle.disable!("my_feature")
:ok
iex> Toggle.enable!("my_feature", user_id: 123)
:ok
iex> Toggle.enabled?("my_feature")
false
iex> Toggle.enabled?("my_feature", user_id: 123)
true

Resources are key-value pairs that can be used to enable or disable a flag for a specific entity. The expected data type for both the key and value is an atom, string, or integer.

Today, toggle only supports boolean flags, but we plan to add support for more flag types in the future.

Whether or not a flag is enabled is determined by the following rules:

• If a flag is enabled for a resource, that resource's value takes precedence over the global value.
• If a flag is disabled for a resource, that resource's value takes precedence over the global value.
• Otherwise, the global value is used.

Caching

Querying flags on every request can be expensive, so Toggle provides a caching layer to speed up flag lookups.

Whenever a flag is created, updated, or deleted, we update a cache with the latest flag data using Cachex under the :toggle_cache namespace.

Whenever the cache is mutated, Toggle.Cache will asynchronously publish the changes to all other connected nodes using Erlang's :erpc module. In the future, we plan to add support for other pubsub strategies.

Using Toggle.enabled?/1, Toggle.enabled?/2, Toggle.disabled?/1, or Toggle.disabled?/2 will automatically use the cache if it is available.

The cache has a default ttl of one minute.

Configuration

Toggle aims to be as simple as possible, and as such, it does not require any configuration.

The only possible configuration at the time of writing is:

• :flag_table_name - The name of the database table used to store flags. Defaults to "toggle_flags".
• :cache_ttl - The time-to-live for cached flags. Defaults to 1 minute.

See Toggle.Comptime for more information.

Further Reading

The core API for Toggle is provided by the Toggle module. You can use the Toggle module to enable and disable flags, and check if a flag is enabled or disabled.

For more complex querying, please see the Toggle.Flags module, the Toggle.Flags.Flag schema, and the libraries this application is built on: EctoModel and EctoHooks.

The Toggle.Flags.Flag schema implements the EctoModel.Queryable behaviour, which lends itself well to fluent-api style querying.

For example, you are able to query flags using the Toggle.Flags.Flag.query/2 function:

iex> MyApp.Repo.one(Toggle.Flags.Flag.query(name: "my_flag", disabled: true))
%Toggle.Flags.Flag{name: "my_flag", enabled: false}

Additionally, please feel free to query the Toggle.Flags.Flag schema directly.

Future Work

• Add support for more flag types such as :percentage and :datetime.
• Add support for deprecating flags and emitting deprecation warnings after a certain date.
• Add support for hooks that can be run before or after a flag is enabled or disabled.
• Pluggable pub/sub strategies.
• Add a Phoenix.LiveView interface for managing flags in real-time.
  • Including metrics and analytics for flag usage.