Watch Phoenix.PubSub.broadcast/2 calls from your context with :dbg

troubleshooting.exs

{:ok, _} =
  :dbg.tracer(
    :process,
    {fn
       {:trace, pid, :call, {mod, fun, args} = call}, stacks ->
         mfa = Exception.format_mfa(mod, fun, Enum.count(args))
         stack = Map.get(stacks, pid, [])

         if match?({Phoenix.PubSub, :broadcast, _}, call) do
           Pretty.inspect(stack, label: "broadcast stack")
         end

         Map.put(stacks, pid, [mfa | stack])

       {:trace, pid, :exception_from, {mod, fun, arity}, {class, value}}, stacks ->
         mfa = Exception.format_mfa(mod, fun, arity)
         Pretty.inspect({class, value}, label: "exception_from")

         case Map.get(stacks, pid) do
           [^mfa | tail] -> Map.put(stacks, pid, tail)
           stack -> IO.warn("bad exception_from " <> inspect(%{mfa: mfa, stack: stack}))
         end

       {:trace, pid, :return_from, {mod, fun, arity}, _retval}, stacks ->
         mfa = Exception.format_mfa(mod, fun, arity)

         case Map.get(stacks, pid) do
           [^mfa | tail] -> Map.put(stacks, pid, tail)
           stack -> IO.warn("bad return_from " <> inspect(%{mfa: mfa, stack: stack}))
         end
     end, %{}}
  )

:dbg.p(:all, [:call])
:dbg.p(:all, [:call, :return_to])
:dbg.tp(MyApp.Context, [{:_, [], [{:return_trace}, {:exception_trace}]}])
:dbg.tp(Phoenix.PubSub, :broadcast, [{:_, [], [{:return_trace}, {:exception_trace}]}])

on_exit(&:dbg.stop_clear/0)