AI Magazine Volume 11 Number 4 (1990) (© AAAI) BookReviews Actors: A Model of in Simula (a language owed to 0. J. tational systems are seen as models of Concurrent Computation Dahl, B. Myhrhaug, and K. Nygaard social, economical, biological, and so and developed in the Norwegian on, organizations. The goal is to in Distributed Systems Computing Center in 1970) can be understand and build such computa- VarolAkman conceived of as ancestors of actors tional ecologies. Three issues of con- because they represent an individual cern in this regard are (1) the general Gul A. Agha’s Actors: A Model of Con- data item and all the operations and issues underlying open systems current Computation in Distributed Sys- procedures on it. However, actors are (Hewitt 1985), (2) the actual imple- tems (The MIT Press, Cambridge, Mass., more powerful than sequential pro- mentations of distributed computa- 1987, 144 pages, $25.00, ISBN O-262 cesses and data flow systems. One tion in computational ecologies, and 010925) is part of the MIT Press can represent a purely functional (3) the design of appropriate lan- Series in Artificial Intelligence. This system with an actor system. Similar- guages for open systems. Agha’s book volume is edited by Patrick Winston, ly, it is possible to specify arbitrary is a must for gaining an appreciation Michael Brady, and Daniel Bobrow. sequential processes with a suitable of the problems to be solved to In the actor formalism, pioneered actor system. However, the converse obtain these actual implementations by Carl Hewitt (1977), one perceives is not true. Although actor creation is and also deals with problems of abstract computational agents, called an integral part of the computational designing languages for open sys- actors, that are distributed in space. model of Hewitt and Agha, sequen- tems. If you want to understand the Each actor has a mailbox (and a mail tial processes such as communicating more practical ideas of ecological address) and associated with each sequential processes (CSP), owed to computation, theory of games, actor is a behavior. One actor can Tony Hoare, do not create other market economies and underlying influence the actions of another actor sequential processes (but can activate mechanisms, evolution and biologi- only by sending it a communication. other sequential processes). The cre- cal systems, and so on, there is a lot An actor can send another actor a ation of actors permits one to to gain in terms of conceptual clarity communication only when it knows increase the distributivity of compu- and computational fundamentals by the address of the second actor. An tation as it evolves. starting with this elegant book. Agha actor can (should be able to) receive Is programming a collection of also commented on this sociological more than one message simultane- actors easy? It is commonly accepted aspect of the actor model elsewhere: that parallel programming is, in gen- ously (in some sense). In case of syn- The term actor brings with it the eral, hard, and the need for yet chronous communication in nonactor image of an active entity, acting another programming language such paradigms, this simultaneous com- out its role in concert with as actors might seem difficult to munication requires the sender to others. Each actor can act only argue. One crucial observation here is wait until the recipient of the com- according to its script, which rep- that because message passing is fun- munication is ready to accept it. Once resents its view of the world. At damental to computation in actors, the recipient starts processing a given this level, the actor paradigm the time complexity of communica- communication, it is blocked from stands in sharp contrast to the tion might become the dominant accepting any other messages until logicist approach, which not factor in program execution. Thus, after it is done with the message-all only postulates the existence of a architectural considerations might for the sake of maintaining the integri- unique reality, but commits us to play a crucial role in the realistic ty of communications, no doubt. representing our knowledge in implementations of actor languages. The actor model achieves the same terms of a consistent collection Note, however, that the most attrac- effect through buffering. A recipient’s of information. mailbox is always ready to receive tive feature of actor languages is that messages. Furthermore, in the actor programmers are not required to The book has several additional framework, there are no restrictions code details such as when and where strengths that make it a useful source on freely communicating mail to use parallelism. Also, although for people working on distributed addresses; thus, the interconnections some functional programming lan- computation. Problems of distributed of the actors can be dynamically guages might have problems with computing such as divergence, dead- modified, and resource management history-sensitive shared objects, lock, and mutual exclusion are treat- is more flexibly done. actors can easily model such objects. ed. There is also an excellent chapter Should the reader begin pondering Recently, there has been interest- on abstraction and compositionality. the difference between actors and ing research into what is somewhat An excellent glossary of actor terms is objects (as found in object-oriented exotically termed the ecology of included as well as a list of references programming), I would like to make a computation (Hubermann 1988). In and a useful index. Although the book few clarifying observations. Objects this philosophy, distributed compu- is a revised version of the author’s 92 AI MAGAZINE BookReviews Readings from AI Magazine The First Five Years: 1980-1985 Edited with a Preface by Robert Engelmore AAAI is pleased to announce publication of Readings from AI Magazine, the complete collection of all the articles that appeared during AI Magazine’s first five years. Within this 650-page indexed volume, you will find articles on AI written by the foremost practitioners in the field-articles that earned AI Magazine the title “journal of record for the artificial intelligence community.” This collection of classics from the pre- mier publication devoted to the entire field of artificial intelligence is available in one large, paperbound desktop reference. SubjectsInclude: q Automatic Programming n Distributed Artificial Intelligence n Games n Infrastructure n Learning n Natural Language Understanding n Problem Solving q Robotics n Education q General Artificial Intelligence n Knowledge Acquisition n Legal Issues n Object Oriented Programming n Programming Language n Simulation n Technology Transfer n Discovery w Expert Systems n Historical Perspectives n Knowledge Representation n Logic n Partial Evaluation n Reasoning with Uncertainty n Computer Architectures $74 95 plus $2 postage and handling. 650 pages, illus., appendix, index ISBN O-929280-01-6. Send prepaid orders to The MIT Press, 55 Hayward Street, Cambridge, MA 02142. doctoral dissertation, it doesn’t carry physics, knowledge representation, and MIT Press, 1989, 266 pages, $25.00, the usual blemish associated with mathematical logic. ISBN O-262-12140-9), Dan Lloyd theses in general; that is, it is not seeks to provide answers that will written in the stream-of-conscious- References bridge the gap between computation- ness mode. al and connectionist models of the There is a well-known saying: Any Agha, G Foundational Issues in Concur- mind. He notes that naturalism theory that can be put in a nutshell rent Computing, Dept of Computer Sci- assumes that the human brain and ence, Yale Univ Unpublished. Also in the brains of some animals are belongs there. Well, running this Foundational Issues in Concurrent Com- risk, I’ll say that in a nutshell, the puting. In Proceedings of the ACM/- organs of thought or representation, actor approach is about the future of SIGPLAN Workshop on Object-Based but what is it about these brains that computing. Concurrent Programming, SIGPLAN makes them thoughtful? Additional- Postscript: A new (and probably Notices, eds. G. Agha, P. Wagner, and A. ly, just what sorts of physical systems the definitive) source has joined the Yonezawa. New York: Association of Com- are sufficient to embody the repre- ranks of actor literature: Concurrent puting Machinery. sentations typical of thinking? Final- Systems for Knowledge Processing: An Hewitt, C 1977. Viewing Control Struc- ly, could a nonbiological system-a Actor Perspective, edited by C. Hewitt tures as Patterns of Passing Messages Arti- computer perhaps-think? With and G. Agha (Cambridge, Mass., The ficial Intelligence 8(3): 323-364 these fundamental questions, Lloyd MIT Press, 1989). This book brings Hewitt, C 1985 The Challenge of Open introduces the central question to be together over 20 important contribu- Systems Byte lO(4): 223-242. answered by Simple Minds: How does tions on the actor concept and its Hubermann, B., ed. 1988. The Ecologyof the brain comprise the mind? applications. Although I haven’t seen Computation: Studies in Computer Science Lloyd seeks to outline a theory of this book, given my positive views and Artificial Intelligence, volume 2. The representing systems and lay the about the book under review, I believe Hague, The Netherlands: Elsevier. foundations of a natural philosophy that this new text will also be a plea- of mind. Such a reductionist theory, sure to read. he observes, must explain how repre- sentations are made out of nonrepre- Varol Akman is an assistant professor in Simple Minds sentational parts. Only by this means the Department of Computer Engineering can we solve the problem of mind and Information Science, Bilkent Univer- Lee A. Gladwin and brain and the open question of sity, Ankara, Turkey His current research concentrates on various theoretical aspects Of what are minds made? Internal the convergence of psychology and of AI, especially from the angle of intelli- mental representations? Matter? In neuroscience.