Serialized state with GenServer

The idea

A GenServer provides a process that owns state and handles requests through callbacks. A synchronous call expects a reply, while a cast sends an asynchronous request without that direct reply contract. The callback returns both its response behavior and the next state, making state transitions explicit. Clients interact through a small API rather than reading a shared mutable variable.

Operations that must be atomic should be expressed as one request. A client-side read followed by a separate write permits another request to interleave between them. Server callbacks are serialized, so lengthy blocking work can delay all clients; move suitable independent work elsewhere while retaining a clear ownership protocol. Test state transitions and request ordering instead of relying on timing guesses.

defmodule CounterServer do
  use GenServer
  def init(n), do: {:ok, n}
  def handle_call(:next, _from, n), do: {:reply, n + 1, n + 1}
end
{:ok, pid} = GenServer.start_link(CounterServer, 0)
IO.inspect(GenServer.call(pid, :next))
IO.inspect(GenServer.call(pid, :next))

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Each call increments and replies in one serialized callback, and the returned next state is used by the following request.