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Basics of ACTOR programming?
In computer science, the Actor model is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent digital computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and determine how to respond to the next message received. The Actor model originated in 1973.  It has been used both as a framework for a theoretical understanding of computation, and as the theoretical basis for several practical implementations of concurrent systems.
what is actor programming?
Actor only was released on the Microsoft Windows 2.1 and 3.0 operating system. Actor used perhaps the first pure object-oriented framework over native operating system calls as its basic GUI architecture. This allowed an Actor application to look and feel exactly like a Windows application written in C, but with all the advantages of an interactive Smalltalk-like development environment. Both a downside and upside to this architecture was a tight coupling to the Windows OS architecture, with a thin abstraction layer into objects. This allowed direct use of the rich Windows OS API, but also made it nearly impossible to support any other OS without a significant rewrite of the application framework.
What is it???
actor language which is an extension of a simple functional language, and provide an operational semantics for this extension. Actor configurations represent open distributed systems, by which we mean that the specification of an actor system explicitly takes into account the interface with external components. We study the composability of such systems. We define and study various notions of testing equivalence on actor expressions and configurations. The model we develop provides fairness. An important result is that the three forms of equivalence, namely, convex, must, and may equivalences, collapse to two in the presence of fairness.
how does it work???
The actor model operates on message passing. Individual processes (actors) are allowed to send messages asynchronously to each other. What distinguishes this from what we normally think of as the threading model, is that there is (in theory at least) no shared state. And if one believes (justifiably, I think) that shared state is the root of all evil, then the actor model becomes very attractive.
We shouldn't get over excited, however. The actor model does not (contrary to some allegations) make it impossible to have deadlocks. The actor model also does not prevent you from having contention for resources between different processes -- message queues, for instance. The model is only "lock-free" above a certain level. At a lower level, for coordinating message queues, locking is still required.