Event Orderings Versus Global State
A fundamental challenge in defining the Actor model is that it did not provide for global states so that a computational step could not be defined as going from one global state to the next global state as had been done in all previous models of computation.
In 1963 in the field of Artificial Intelligence, John McCarthy introduced situation variables in logic in the Situational Calculus. In McCarthy and Hayes 1969, a situation is defined as "the complete state of the universe at an instant of time." In this respect, the situations of McCarthy are not suitable for use in the Actor model since it has no global states.
From the definition of an Actor, it can be seen that numerous events take place: local decisions, creating Actors, sending messages, receiving messages, and designating how to respond to the next message received. Partial orderings on such events have been axiomatized in the Actor model and their relationship to physics explored (see Actor model theory).
Read more about this topic: History Of The Actor Model
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