Used to implement Bayeux extensions, which can be used to filter or change incoming messages and the responses sent back to the client.

For example:

defmodule AuthenticationExtension do
  use Pixie.Extension

  def incoming %Event{message: %{ext: %{username: u, password: p}=message}}=event do
    case User.authenticate username, password do
      :ok ->
        %{event | message: %{message | ext: nil}}
      :error ->
        %{event | response: %{response | error: "Authentication Failed"}}
    end
  end

  def incoming %Event{}=event do
    %{event | response: %{response | error: "Authentication Failed"}}
  end

Note that you must always provide a “catch all” function head that matches all other events and returns them - otherwise the runtime will start raising exceptions and generally make you feel sad.

You can dynamically add your extension to the extension stack at runtime using YourExtension.register and YourExtension.unregister