class Faraday::RackBuilder

Returns an object that responds to `call` and returns a Response.

to the middleware stack.
the builder gets locked to ensure no further modifications are made
The builder is responsible for creating the app object. After this,

The "rack app" wrapped in middleware. All requests are sent here.

def app
  @app ||= begin
    lock!
    ensure_adapter!
    to_app
  end
end