dynamic_host_user_dispatch.ex

# in your app supervisor
:my_app_user_routes = :ets.new(:my_app_user_routes, [:named_table, :bag, :public])

defmodule DynamicDispatch do

  def register(tab, host, path, plug, opts) do
    true = :ets.insert(tab, {host, path, plug, opts})
  end

  def unregister(tab, host, path, plug) do
    true = :ets.match_delete(my_app_user_routes, {host, path, plug, :_})
  end

  def init(tab), do: tab

  def call(conn, tab) do
    case :ets.match_object(tab, {conn.host, conn.requst_path, :_, :_}) do
      [{host, path, plug, opts}] -> plug.call(conn, plug.init(opts)
      [] -> conn
    end
  end
end

# in your existing router
forward "/", DynamicDispatch, :my_app_user_routes

# on app boot, pluck all routes from cold storage and register
for {host, path, plug, opts} <- user_routes_from_db do
  :ok = DynamicDispatch.register(:my_app_user_routes, host, path, plug, opts)
end


# register your router/controller/plug at any time based on user actions
DynamicDispatch.register(:my_app_user_routes, "foo.com", "/foo", FooController, :show)