This page is the top Google result for “phoenix conditional router forward”, so I’ll just leave a decently thorough answer that will hopefully help whoever bumps into this thread. (I’ll be honest: It’s probably gonna be me in 6 months! )

To forward a request to one of several different routers in Phoenix, do this:

1. Forward the matching path in your router:

lib/your_project_web/router.ex

  scope "/" do
    pipe_through :api

    forward "/", YourProjectWeb.Plugs.ConditionalRequestForwarder
  end

2. Create a plug that matches and dispatches to the appropriate router based on your condition:

lib/your_project_web/plugs/conditional_request_forwarder.ex

defmodule YourProjectWeb.Plugs.ConditionalRequestForwarder do
  @behaviour Plug

  def init(opts), do: opts

  def call(%Plug.Conn{} = conn, _opts) do
    case conn.params["some_param"] do
      nil -> YourProjectWeb.SomeRouter.call(conn, [])
      _ -> YourProjectWeb.SomeOtherRouter.call(conn, [])
    end
  end
end

And that’s how you can conditionally forward a request to one of several possible routers!