Actors, Mailboxes & Supervision

The Actor Model on the JVM

Akka is a JVM toolkit for concurrent, distributed, resilient applications built on the actor model — independent, isolated units (actors) communicating exclusively via async messages, each owning its state privately with no shared mutable memory. This sidesteps classic shared-memory concurrency bugs by design, trading manual locks/mutexes for a higher-level, message-passing abstraction.

Mailboxes & Sequential Processing

Each actor has a mailbox — a queue holding incoming messages until the actor's dispatcher schedules it to process the next one. An actor handles messages sequentially, one at a time, which is exactly why its internal state never needs explicit locking: concurrency comes from having many actors, not multiple threads touching one actor's state simultaneously.

ActorSystem — the Runtime Container

The ActorSystem is the top-level entry point, managing the actor hierarchy's lifecycle, scheduling, and the underlying thread pools (dispatchers) that execute actors.

Supervision — "Let It Crash"

override val supervisorStrategy = OneForOneStrategy() {
  case _: ArithmeticException => Resume
  case _: NullPointerException => Restart
  case _: Exception => Escalate
}

Directly inspired by Erlang/OTP: a parent actor supervises children and decides how to respond to a failure (restart, stop, resume, or escalate), delegating fault handling rather than requiring every actor to defensively handle every possible error internally.