While any object can be sent as a user-level message between Actors, the convention is that instances of ActorRequest are the only kind of user-level message exchanged.

Class ActorRequest

An ActorRequest (a “request”) is a triple of a Message destined for a remote behavior (the “message”), a Process interested in the reply (the “sender”), and a Promise by which the reply is to be delivered to the Process (the “promise”).

The sender is not always an ActorProcess. An ordinary Process can send ActorRequests and can wait for eventual replies or exceptions.

A request is not intrinsically targeted at any actor in particular; it does not store any information about the identity of its target.

Asynchronous requests

If the promise is nil, the request is asynchronous and no-one cares about the reply to the eventual evaluation of the message. However, even in this case, the sender is almost always non-nil: an asynchronous request can still usefully have a notion of “sender” associated with it.

Sending requests

Requests can be sent to an actor with ActorRequest >> #sendTo: or the #redirectTo: family of methods. There are also convenience methods Actor class >> #sendRequest:to: and sendAsyncRequest:to:, and of course any instance of ActorProxy builds and sends ActorRequest instances.

Once a request has been sent, its reply may be retrieved via the Promise it holds.

Sending replies or notifying of failures

Ultimately, when a response is ready for a request, it is transmitted by invoking ActorRequest >> #resolveWith: or #rejectWith:, as appropriate. Compare these to similar and similarly-named methods on Promise.

In order to ensure that every request receives an answer, a request sometimes stores the identity of an Actor (in its worker instance variable) that has taken responsibility for the request, so that it can send a default answer if no-one else supplies anything first. The request object takes care to signal the worker whenever someone supplies a reply to it, so that the worker knows it no longer needs to bother with the request.