Later papers show a remarkable continuity of terminology and concepts for example the Agha etc paper from 1993 shows clearly that the understanding and terminology of the Actor model that I have summarized is accurate 1 2 The Actor Model of Computation In this subsection we discuss the principles underlying the actor model of computation compare it to other models and illustrate programming in our actor language See 1 for more discussion of the actor model and for many examples of programming using them 1 2 1 The Actor Philosophy The early work on Actors 12 13 14 3 4 5 16 17 proposed some fundamental principles Every object is an actor this includes messages and numbers No actor can be operated on looked at taken apart or modified in anyway except by sending a message to that actor requesting it perform the operation itself The only thing that happens in an actor system is an event which consists of an actor receiving a message An actor can send many messages at the same time but can only receive them one at a time In other words the arrival order is linear while the structure of events is only partially ordered by the notion of one event preceding another Actors have a set of acquaintances other actors it knows about and can send messages to This set can increase in time since the actor may create new acquaintances and it may also hear about them in messages it receives These are the only ways the set of acquaintances can increase The key features of actor based computation that have evolved from these guiding principles are Message passing is fundamental Mail delivery is guaranteed i e fairness There is arrival order non determinism in message delivery Actors are history sensitive shared objects Resource allocation and interconnection topology are dynamic Blissex