ACTORS - a model of concurrent computation in distributed systems

ly, we may picture an actor with a mail queue on which all . communications are placed in the order in which they arrive and an actor machine8 which points to a particular cell in the mail queue. The end of a communication on the mail queue can be indicated by some special symbol reserved for the purpose.' We represent this pictorially as in Fig. 3.1. When an actor machine Xn accepts the nth communication in a mail queue, it will create a new actor machine, Xn 1, which will carry out the replacement behavior of the actor. This new actor machine will point to drance for a theoretical investigation assuming a reliable mail system. See the discussion in Section 2.4. "The mail queue will be considered large enough to hold all communications sent to a given actor. This implies that a mail queue is, in principle, unbounded, while only a finite fragment of it is used at any given point in time. This is quite similar to a readonly tape of a Turing Machine. However, the writing is done, indirectly, using the mail .1 system. 'No assumption should be made about an actor machine being sequential, indeed, an actor machine, much like machines in the real world, may have components that function " in parallel. 7Thus the variable length of a commumication is not a problem.