Help with Elixir app management

Questions / Help
, ,
1

I am building an Elixir app for prod. It has a :observer.start layout as given below

Screenshotfrom2018-02-1011-21-56.jpg1171×263 38.8 KB

My app’s flow is as follows.
Step 1: Workers under Elixir.AppName.MQTT.Supervisor receive some message.
Step 2: This message is sent to Elixir.AppName.Server
Step 3: Elixir.AppName.Server starts a worker dynamically under Elixir.AppName.on.Supervisor.
Step 4: The worker receives the message and acts accordingly
Let us call the worker, which is a GenServer, WORKER

My problem is that every time I have to perform some task in WORKERs I end up using send self(), message and having a corresponding handle_info in the WORKER.

I feel there is some issue with my current approach as I cannot utilize functions like GenServer.call or GenServer.cast

How should I remodel the application so that it makes use of other GenServer functions and callbacks?

2

Without code it is hard to tell, but let me ask You why You would want to call or cast inside the GenServer?

It’s the API side that use those functions.

The server side has access to all private functions, and knows its state, in which case would You need to make intra server calls?

3

I want to use call mainly because the worker is doing some CRUD operation in DB. I want to block the worker until the task is done.

By API side you mean something that a client(user or application) can use, right?

No intra-server calls have been planned.

Code structure is as follows

AppName.Server

defmodule AppName.Server do
    use GenServer

    def start_link do
        GenServer.start_link/3
    end

    def init/1 do
        send self(), :start_supervisors
        {:ok, state}
    end

    def handle_info/2 do
        # :start_supervisors is the message being handled
        child_spec = %{...}
        Supervisor.start_child(AppName.Supervisor, child_spec)
        {:noreply, state}
    end
end

AppName.on.Worker

defmodule AppName.on.Worker do
    use GenServer

    def start_link do
        # receives the message as argument
        GenServer.start_link/3
    end

    def init/1 do
        send self(), {:check_args, message_from_mqtt_as_map}
        {:ok, state}
    end

    def handle_info({:check_args, message_from_mqtt_as_map}, state) do
        case message_from_mqtt_as_map["action"] do
            :create ->
                do_create(message_from_mqtt_as_map)
                send self(), :some_task
            :remove ->
                do_remove(message_from_mqtt_as_map)
                send self(), :some_other_task
            :update ->
                do_update(message_from_mqtt_as_map)
                send self(), :yet_some_other_task
        end
        {:noreply, state}
    end

    def handle_info/2 do
        # other handle infos for :some_task, :some_other_task, :yet_some_other_task
    end

    # helper functions for the task
end
4

Maybe You can do like this?

# Maybe there is a typo in your module name :slight_smile: 

defmodule AppName.on.Worker do
    use GenServer

    # THIS IS API

    def start_link do
        # receives the message as argument
        GenServer.start_link/3
    end

    def some_task, do: GenServer.call ...

    # THIS IS SERVER SIDE

    def init/1 do
        send self(), {:check_args, message_from_mqtt_as_map}
        {:ok, state}
    end

    # Wherever You need it...
    def handle_call(:some_task, _from, _state) do
      perform_some_task()
    end

    def handle_info({:check_args, message_from_mqtt_as_map}, state) do
        case message_from_mqtt_as_map["action"] do
            :create ->
                do_create(message_from_mqtt_as_map)
                perform_some_task()
           ...
        end
        {:noreply, state}
    end

    # helper functions for the task
    defp perform_some_task, do: ...
end

BTW Why do You need to do this

send self(), :some_task

instead of a simple function call?

1 Like
5

send self(), :some_task is used so that the same task can be initialized by some other process.

Yes, the module name was obfuscated thus the typo and AppName.on.Worker should be AppName.On.Worker :sweat_smile:

As the some_task() cannot be called from within, so, from which process should I call AppName.On.Worker.some_task?