History of The Actor Model - Abstracting Away Implementation Details

Abstracting Away Implementation Details

An important challenge in defining the Actor model was to abstract away implementation details.

For example, consider the following question: "Does each Actor have a queue in which its communications are stored until received by the Actor to be processed?" Carl Hewitt argued against including such queues as an integral part of the Actor model. One consideration was that such queues could themselves be modeled as Actors that received messages to enqueue and dequeue the communications. Another consideration was that some Actors would not use such queues in their actual implementation. E.g., an Actor might have a network of arbiters instead. Of course, there is a mathematical abstraction which is the sequence of communications that have been received by an Actor. But this sequence emerged only as the Actor operated. In fact the ordering of this sequence can be indeterminate (see Indeterminacy in concurrent computation).

Another example of abstracting away implementation detail was the question of interpretation: "Should interpretation be an integral part of the Actor model?" The idea of interpretation is that an Actor would be defined by how its program script processed eval messages. (In this way Actors would be defined in a manner analogous to Lisp which was "defined" by a meta-circular interpreter procedure named eval written in Lisp.) Hewitt argued against making interpretation integral to the Actor model. One consideration was that to process the eval messages, the program script of an Actor would itself have a program script (which in turn would have ...)! Another consideration was that some Actors would not use interpretation in their actual interpretation. E.g., an Actor might be implemented in hardware instead. Of course there is nothing wrong with interpretation per se. Also implementing interpreters using eval messages is more modular and extensible than the monolithic interpreter approach of Lisp.