If you’re looking for a way to use feature flags in Phoenix projects, you might want to check out the laboratory package. It is an Elixir plug that allows you to turn features on or off in Phoenix or any plug based Elixir web apps.

It works by providing an endpoint that you can mount in your router, and you can access a page to turn on/off features. The feature state is stored in cookies, and you can check if a feature is enabled by checking Laboratory.enabled?(conn, :feature_name) .

Let’s try out adding a simple feature flag in a Phoenix project to see this in action. All we will do is show a subheading in the default Phoenix welcome page, if a subheading feature is enabled.

Generate a new Phoenix project. We won’t be needing ecto or brunch, so let’s skip those.

mix phoenix . new foobar -- no - brunch -- no - ecto

Add laboratory to mix.exs and run mix deps.get :

# mix.exs def deps do [ # other packages { :laboratory , "~> 0.0.1" }, ] end

Configure the :laboratory config inside config/dev.exs :

# config/dev.exs config :laboratory , features: [ { :subheading , "Sub heading" , "Subheading in the home page" }, ]

Mount the Laboratory plug inside the router.

# web/router.ex scope "/lab" , Laboratory do forward "/" , Router end

Now let’s just show a subheading in the welcome page if the feature flag is enabled.

<!-- web/templates/page/index.html.eex --> <%= if Laboratory . enabled? ( @conn , :subheading ) do %> <h3> Laboratory is enabled! </h3> <% end %>