The GenServer behaviour

A GenServer is a behaviour (F3.06) that abstracts the receive loop (F3.07). You implement callbacks — init/1 sets the starting state, handle_call/3 answers synchronous requests, handle_cast/2 handles asynchronous ones — and the framework runs the loop, the mailbox, and the replies. The state you return threads into the next callback.

The portal's hand-written tally becomes a GenServer. Each callback returns a tagged tuple that tells the framework what to reply and what the next state is. Select a callback and read its return and its effect on the state.

The callbacks and their returns The whole module

The callbacks and their returns

Each callback receives the current state and returns a tuple. init/1 returns {:ok, state}; handle_call/3 returns {:reply, value, state}; handle_cast/2 returns {:noreply, state}. Select one and see what it returns and how the state moves.

The callback · select one

The return tuple is the whole contract with the framework: its tag decides whether the caller gets a reply, and its last element becomes the state handed to the next message.

The whole module

use GenServer brings in the loop and default callbacks; you add the ones you care about, each marked @impl true. A thin client API wraps the raw GenServer calls so callers never touch the PID protocol directly.

defmodule Portal.Tally do
  use GenServer

  # client API
  def start_link(n), do: GenServer.start_link(__MODULE__, n)

  # callbacks
  @impl true
  def init(n), do: {:ok, n}

  @impl true
  def handle_call(:get, _from, count), do: {:reply, count, count}

  @impl true
  def handle_cast(:inc, count), do: {:noreply, count + 1}
end

F3.07 · the hand-written loop

You wrote receive, the reply send, and the recursive loop(state) by hand.

In Elixir

use GenServer runs the loop; you fill in init, handle_call, and handle_cast.

Next: synchronous call, asynchronous cast — the two ways a client reaches these callbacks.

References

Primary sources for this lesson, and where it connects in the course.

Sources

GenServer — Elixir documentation — the stateful server behaviour and its callbacks.
Mix & OTP: GenServer — Elixir documentation — building one step by step.
Supervisor — Elixir documentation — fault tolerance and restarts.