Discussion about uses for Agent Processes

Oh I’d love to hear some good use cases too. I sort of feel guilty not using Agent at all. I usually grab GenServer instead without much thinking.

Yeah, same here, I always have ‘some’ functionality associated with shared state like that, so a GenServer has always seemed better, otherwise for shared state without functionality then I always use ETS/Mnesia instead… ^.^;

I have noticed one place that I tend to use it. I’ll share that later, so as not to bias the answers of others just yet…

I find it interesting that not many folks are chiming in with answers yet. I wonder if that signifies that they don’t get a ton of usage.

For myself, I have found them useful in testing other processes. I’ve rigged processes to pull pre-arranged values from them and stored values representing work happening inside of a process that I could later write assertions against.

The one time I tried to use them in an application, I grew to regret it and later replaced them with GenServer processes, for the functionality reasons noted by @OvermindDL1.

Oops, I did forget another decent usage I found for them!

I recorded the events of a simulation as they happened, then fetched and cleared all changes in the drawing code (using get_and_update()). That worked fine.

I’ve been wondering the same thing myself. When do people actually use agent?

I’d thought about using them as a low TTL query cache

I don’t think I ever used Agent in one of my programs. That said, the use cases in the wild, I saw, are:

• accumulation of the commands during a Migration, so they can be properly reversed on rollback: https://github.com/elixir-ecto/ecto/blob/master/lib/ecto/migration/runner.ex
• mix config: elixir/lib/mix/lib/mix/config.ex at main · elixir-lang/elixir · GitHub

I also don’t use Agents for the production runtime code. I did use them on few occasions in tests.

I’m using an agent like KV.bucket: http://elixir-lang.org/getting-started/mix-otp/agent.html. In the following function I’m using an agent as “accumulator”. If there’s a handier solution (I’m not as experienced as most posters) I would like to hear.

 @doc """
    Validate the input `body` in the taskform, task identified with `element_id', `socket` is third param
  """
  @spec validate_task(map, String.t, Socket.t) :: :ok
  def validate_task(body, element_id, socket) do
    Agent.start_link(fn -> [] end, name: :error_accumulator)

    traverse = fn(data, trav) ->
      case data do
        [_|_] ->
          Enum.map(data, &(trav.(&1,trav)))
        %{"key" => name, "validate" => validation, "label" => label} ->
          result = validate_item(body |> Map.fetch!(name),
                                 label,
                                 validation)
          if result != "", do: Agent.update(:error_accumulator, &List.insert_at(&1, 0, result))
        %{"components" => components} ->
           trav.(components,trav)
        %{"columns" => columns} ->
           trav.(columns,trav)
        %{"rows" => rows} ->
           trav.(rows, trav)
        _ ->
           data
      end
    end

    WorkflowAgent.get_user_tasks(socket.assigns.agent_pid)
    |> Stream.filter(fn task -> Map.fetch!(task, :id) === element_id end)
    |> Enum.at(0)
    |> Map.fetch!(:formfields)
    |> Enum.at(0)
    |> Map.fetch!("components")
    |> traverse.(traverse)

    errors = Agent.get(:error_accumulator, &(&1))

    Agent.stop(:error_accumulator, :normal)

    case errors do
      [] -> push socket, "cleanupFormIO", %{}
            :ok
      _  -> push socket, "alert",  %{message: "<br>" <> (errors |> List.foldl(" ", fn(x, acc) -> x <> acc end)), type: "error"}
            :errors_found
    end
  end