Suppose I have a simple genserver that mantains a simple :queue as it's state. Items are continually being added with handle_cast. Every 5 seconds I process the queue with Process.send_after. That call gets handled with a call back to handle_info with the current state. The queue gets processed and emptied then a new empty queue is applied as the current state of the genserver.

My question is this:
What happens when calls come into genserver while the queue is being processed? Since I return a new empty queue to handle_info {:noreply, :queue.new} will that write over items that were being added while I was processing the queue? Or will the genserver casts be themselfs queued up then allowed to finish once handle_info is done?
Basically I am concerned about missing items during handle_info.

Code:

defmodule TcpClient.Queue do
use GenServer
require Logger

def start_link do
queue = :queue.new()
GenServer.start_link(__MODULE__, queue, name: {:global, :tcp_queue})
end

def init(queue) do
Logger.debug("Starting up Queue")
schedule_work()
{:ok, queue}
end

def enqueue(msg) do
Logger.debug("Item Added")
GenServer.cast(whereis(), {:enqueue, msg})
end

defp schedule_work() do
Process.send_after(self(), :work, 1 * 1 * 300)
end

def handle_cast({:enqueue, msg}, state) do
{:noreply, :queue.in(msg, state)}
end

def handle_info(:work, queue) do
case :queue.is_empty(queue) do
true ->
Logger.debug("No items to Process")
nil

false ->
Logger.debug("Processing Queue")

:queue.to_list(queue)
|> Enum.map(&TcpClient.Repo.add_message(&1))

queue = :queue.new()
end

schedule_work()
{:noreply, queue}
end

def whereis() do
:global.whereis_name(:tcp_queue)
end
end


1 Answer
1

Incoming messages are being put into process mailbox and are not processed until the process has returned from previous handle_***. You are risking to overflow your process’ mailbox, not to miss some messages.

handle_***

To prevent this, GenStage was explicitly created by Elixir Core team to fight back pressure.

GenStage

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