ACTORS

Motivation

Develop language for concurrency “Parallel Execution of actions”.

Brief History

A number of individuals have contributed to the development of the actor model.

Actor model was first described by Carl Hewitt (70’s)– The model was object-oriented: every object was a

computationally active entity capable of receiving and reacting to messages. The Objects were called Actors.

Gul Agha, later develop the actor model and develop a mathematical theory of actors.

What are actors ?

Actors are independent concurrent objects that interact by sending asynchronous messages; each actor has its mail address and a behavior.

An actor can do the following: Send communication to other actors. Create new actors. Define a new behavior for itself, which may be the same or

different to its previous behavior.

Behavior change

Higher level than state change (assignment)

The current behavior of an actor will specify its replacement behavior

– How it will perform the next incoming message

A behavior accepts only one message

A causally connected chain of behaviors isomorphic to the queue of messages

Mail System

Two important facts about mail system in the actor system:

Mail arrives in random, non deterministic order (asynchronous).

Ensures concurrent execution.

The mail delivery is guaranteed. The system guarantees to execute all tasks eventually.

Communication Mechanisms

Communication is asynchronous, without any guaranteed arrival order.

– Message order: if actor A sends a sequence of communications to actor B, then B may not receive them in the same order that A sent them.

– It is possible for A to tag each message with a sequence number, so that B may rearrange messages into the correct

order.

Extending the Actalk framework towards ACTORS

ActorBehavior subclass: #AghaActorBehavior

setProcess

[self acceptNextMessage] fork

replace: replacementBehavior

aself initializeBehavior: replacementBehavior

Example

AghaActorBehavior subclass: #AghaCounter

instanceVariables: ‘contents’

consultAndReplyTo: replyDestination

self replace: (self class contents: contents).

replyDestination reply: contents

incr

self replace: (self class contents: contents + 1)

reset

self replace: (self class contents: 0)

References

Agha, Gul, Actors: A Model of Concurrent Computation in Distributed Systems. MIT Press, 1986, 144 p.

Open Systems Laboratory– http://osl.cs.uiuc.edu/