actormodel

Actormodel (the actor model) is a mathematical model of concurrent computation that uses actors as the fundamental units of execution. An actor can perform three basic actions in response to a message: make local decisions, create new actors, and send a finite number of messages to known addresses. Communication in the actor model is asynchronous and occurs through immutable messages placed in a mailbox owned by the recipient actor. The model enforces encapsulation by avoiding shared mutable state, so concurrency is achieved by independent actors that interact only through message passing.

The actor model was introduced by Carl Hewitt in 1973 as a formal abstraction for concurrency in distributed systems. It was later formalized by Gul Agha and colleagues in the 1980s, who provided concrete operational semantics and proofs. Since then, it has influenced many programming languages and frameworks that emphasize isolation and fault tolerance.

Key concepts include actor identity, mailboxes, message envelopes, and dynamic creation. An actor processes messages one at a time, preserving internal state between messages, and may change its behavior for future messages. Messages can be delivered out of order or be delayed, making timing nondeterminism a natural aspect of the model. Some systems support reliable delivery, supervision trees, and failure handling patterns inspired by the actor model.

Common implementations include Erlang/OTP, Akka (for Scala and Java), Microsoft Orleans, and languages such as Pony and CAF. The model is particularly suited to scalable, distributed, and fault-tolerant applications, because components can fail independently and recover without global coordination.

Critics note potential performance overhead, difficulty reasoning about complex message flows, and the need for careful design to avoid livelocks or message loss in non-reliable networks.