Router based on host

dacz

I’m building an app that would handle request for bare domain as well as for subdomains. Each subdomain is client space.

I have very different requirements for bare domain routing and for subdomain routing (same paths with very different functions). So I want to split router in two, one for root and one for subdomains (all subdomains have the same routing).

After researching I arrived to using forward function in the main router to forward to my plug:

defmodule AppWeb.Plugs.RouterSelector do
  @behaviour Plug

  @domain_init AppWeb.RouterDomain.init([])
  @root_init AppWeb.RouterRoot.init([])

  @impl true
  def init(opts), do: opts

  @impl true
  def call(%Plug.Conn{} = conn, _opts) do
    if conn.assigns.has_domain do
      AppWeb.RouterDomain.call(conn, @domain_init)
    else
      AppWeb.RouterRoot.call(conn, @root_init)
    end
    |> Plug.Conn.halt()
  end
end

Each of the routers called in this plug are “regular ones”.

I’m not sure if that is the right way to do it. And another thing that puzzles me it that I needed to add this |> Plug.Conn.halt() at the end of if block. Without it it told me about double rendering (I expected that the normal router called here will halt the request once done).

Is it the correct way to do it? And why (if) is there the explicit halt?

Thanks!

13 comments

/phoenix #router

0 779 13

2025-04-10 19:44:10 UTC

Marked As Solved

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BartOtten

You could have one with hosts: topdomain.com and one scope without hosts set which is a catchall (the hosts field is converted to _ in the route match pattern)

Make sure the toplevel goes first.

Ps. Hosts should be a list of binaries, mobile editing is…less than ideal.

Post #9

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BartOtten

Seems you should be able to define two pipelines with different plugs and two scopes using these different pipelines (and even different controllers).

Post #2

acrolink

Usually for things like this I use nginx in front. Actually, I always use nginx in front.

Post #1

BartOtten

Aa for the halt:

Halt stops the pipeline from proceeding following plugs. It does not halt the request.

So we are in pipeline land. Which clears things up.

Even though a sub phoenix router will send a response to finish the user request (after it halts), the parent plug pipeline that called the phoenix router does not receive that halt instruction; only the response.

So without the explicit halt in the router selector plug, the plug pipeline in the toplevel router will continue to run. Causing the double render error.

This is why the parent router selector plug must also send a halt.

So in lists:
Subrouter pipeline
Plug a
Plug b
Plug render < halt and return conn

Toplevel router pipeline
Plug a
Plug b
Plug RouterSelelector. < halt and return conn
Plug render # it conn reaches me I am second render

Post #6