Elixir event handling -- am I failing at "let it crash?"

1_explanation.md

I have a genserver that is taking messages from a websocket and passing them to an event handler. The event handler receives a map from the genserver and takes action where appropriate. So for example, in response to a new user create event, the event handler would check if the user's email address comes from a suspicious domain, and if so, take some actions to reduce abuse.

So in event_handler.ex you can see that when a new message comes in for a user create event, I pass the message to UserHandler.handle_suspicious_domain. That function returns the same map which I pass to UserHandler.handle_suspicious_ip. But I'm not sure if that's idomatic since I'm not transforming the map, I'm just passing it from function to function.

Then you can see in user_handler.ex that I take the domain name through a set of transformations and then make a decision. Looking at line 25 you can see that I have a function that does nothing and only exists to prevent a crash. That seems wasteful and also runs contrary to let it crash.

Any ideas on how to make this more idiomatic?

Hypothesis

It might be better in EventHandler to spawn a task for each of those steps steps(handle_suspicious_domain and handle_suspicious_ip) and then the task can crash if any part of the function doesn’t go exactly right. Does that seem like a better approach?

event_handler.ex

defmodule EventHandler do
  # behaviors and other message handling happens above
  def handle_cast({:message, msg}, state) do
    case is_a(msg) do
      {"user", "create"} ->
        Logger.debug "User create event detected."
        msg
        |> UserHandler.handle_suspicious_domain
        |> UserHandler.handle_suspicious_ip
      {"thing1", "create"} ->
        Logger.debug "Found a thing1 create event"
      {"thing2", "create"} ->
        Logger.debug "Found a thing2 create event"
      _ -> {:skip, "not implemented"}
    end

    {:noreply, state}
  end
end

user_handler.ex

defmodule UserHandler do
  @doc """
  Receives a map corresponding to an api message. Extracts the username
  and domain and checks if that domain is in the database table of known-
  bad domains and takes action if necessary.
  """
  @spec handle_suspicious_domain(map()) :: map()
  def handle_suspicious_domain(msg) do
    msg                                             # %{"_internal" => ...}
    |> get_username                                 # "[email protected]"
    |> get_domain                                   # "example.com"
    |> lookup_domain                                # true
    |> act_on_suspicious_domain(get_username(msg))  # :acknowledged

    # Return the msg so it can move onto the next step in the chain.
    msg
  end
  
  def act_on_suspicious_domain(true, username) do
    Logger.warn "#{username} came from a suspicious domain"
    # TODO -- add some actions here that I don't want to make public.
    :acknowledged
  end
  
  # The sole purpose of this function is to prevent a crash in the event that
  # the domain is NOT suspicious.
  def act_on_suspicious_domain(false, _username) do
    :acknowledged
  end
end