Synchronous call, asynchronous cast

Two ways to message a GenServer. GenServer.call sends a request and blocks until the server replies, returning the value — a round trip. GenServer.cast sends a request and returns :ok at once, without waiting — fire and forget. call routes to handle_call, cast to handle_cast.

Choose based on whether the caller needs an answer. A read wants call; a fire-and-forget update wants cast. Select each and watch where it routes and what the client gets back.

Where each one routes A clean client API

Where each one routes

call makes the caller wait for the server's reply; cast lets it move on immediately. The cost of call is the wait: if the server does not reply within the timeout, the caller exits. Select each and follow both lanes.

The client request · select one

A cast is faster for the caller but tells you nothing — not even that the server is alive. A call confirms the work and returns a value, at the price of waiting. The choice is about whether you need the answer, not about speed alone.

A clean client API

Callers should not write GenServer.call and raw message tags themselves. Wrap them in named functions on the module, so the public surface reads as Tally.get/1 and Tally.inc/1 and the call-versus-cast choice stays an implementation detail.

defmodule Portal.Tally do
  # ... use GenServer, init, handle_call, handle_cast ...

  def get(pid), do: GenServer.call(pid, :get)   # sync: returns the count
  def inc(pid), do: GenServer.cast(pid, :inc)   # async: returns :ok
end

Tally.inc(tally)   # => :ok   (fire and forget)
Tally.inc(tally)   # => :ok
Tally.get(tally)   # => 2     (waits for the reply)

The idea

Ask and wait for an answer, or tell and move on without one.

In Elixir

call → handle_call → a reply; cast → handle_cast → :ok.

Next: supervisors and restart strategies — what keeps the server alive when it crashes.

References

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

Sources

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