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GenServer.reply/2

As explained in Elixir in Action, Third Edition

GenServer.reply/2 sends a deferred reply from a GenServer process to the caller of a synchronous request.

When a GenServer performs database work, doing that work inside the server process can create a scalability bottleneck. A synchronous handler can spawn a worker, return {:noreply, state}, and defer the response until the worker finishes. section 7.3.4

def handle_call({:get, key}, caller, state) do
  spawn(fn ->                                   #1
    data = case File.read(file_name(key)) do
      {:ok, contents} -> :erlang.binary_to_term(contents)
      _ -> nil
    end
 
    GenServer.reply(caller, data)               #2
  end)
 
  {:noreply, state}                             #3
end

A runnable handle_call/3 pattern shows the worker reading data, sending it with GenServer.reply/2, and letting the server accept further requests. section 7.3.4

The pattern works as follows:

  • Use the second handle_call/3 argument to retain the caller PID and unique request identifier. section 7.3.4
  • Spawn a worker for the database operation, then call GenServer.reply/2 with the result. section 7.3.4
  • Keep the server process responsive, but bound concurrency separately when simultaneous clients could overwhelm disk I/O. section 7.3.4

How Elixir in Action, Third Edition covers it

How deep it goes
Explains the deferred-reply sequence through handle_call/3, worker spawning, caller data, and a later GenServer.reply/2 call.
Examples
Uses a {:get, key} handler that reads a file in a spawned process and replies with decoded data.
What it assumes
Assumes familiarity with GenServer callbacks, synchronous calls, spawned processes, and caller information.
What you can do after
Apply it via GenServer by returning {:noreply, state} and replying from the spawned worker.
How it connects
Connects GenServer.reply/2 with handle_call/3, process spawning, synchronous calls, and concurrency pooling.
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This explainer was generated by Manning's knowledge graph from the text of Elixir in Action, Third Edition. The section links point to the parts of the book it draws on.