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Actor Model of Computation
Published in ArXiv http://arxiv.org/abs/1008.1459 Actor Model of Computation Carl Hewitt http://carlhewitt.info This paper is dedicated to Alonzo Church and Dana Scott. The Actor model is a mathematical theory that treats “Actors” as the universal primitives of concurrent digital computation. The model has been used both as a framework for a theoretical understanding of concurrency, and as the theoretical basis for several practical implementations of concurrent systems. Unlike previous models of computation, the Actor model was inspired by physical laws. It was also influenced by the programming languages Lisp, Simula 67 and Smalltalk-72, as well as ideas for Petri Nets, capability-based systems and packet switching. The advent of massive concurrency through client- cloud computing and many-core computer architectures has galvanized interest in the Actor model. An Actor is a computational entity that, in response to a message it receives, can concurrently: send a finite number of messages to other Actors; create a finite number of new Actors; designate the behavior to be used for the next message it receives. There is no assumed order to the above actions and they could be carried out concurrently. In addition two messages sent concurrently can arrive in either order. Decoupling the sender from communications sent was a fundamental advance of the Actor model enabling asynchronous communication and control structures as patterns of passing messages. November 7, 2010 Page 1 of 25 Contents Introduction ............................................................ 3 Fundamental concepts ............................................ 3 Illustrations ............................................................ 3 Modularity thru Direct communication and asynchrony ............................................................. 3 Indeterminacy and Quasi-commutativity ............... 4 Locality and Security ............................................ -
Oral History of Sir Antony Hoare
Oral History of Sir Antony Hoare Interviewed by: Jonathan P. Bowen Recorded: September 8, 2006 Cambridge, United Kingdom CHM Reference number: X3698.2007 © 2006 Computer History Museum Oral History of Sir Antony Hoare Jonathan Bowen: Hello, Tony. Would you like to introduce yourself briefly? Sir Antony Hoare: I’m Tony Hoare, principal researcher at Microsoft Research Limited in Cambridge. Thank you for coming here to talk to me. Bowen: Thank you, Tony. I’m looking forward to our talk together. It would be interesting to know, first of all, how you grew up, and what your mother and father did. Hoare: My father was a colonial civil servant, and my mother was the daughter of a tea planter in Ceylon. She was called out to Ceylon to act as social secretary for my grandfather, and they met in Ceylon, married there, and I was born there. Bowen: And do you have any memories of Ceylon? Hoare: Oh, yes, I have quite vivid memories of going to school there. In those days it was still quite a wild place, and we used to go out to the country -- indeed into the forest -- to see animals and elephants and tigers. Had quite exciting adventures there in the school party. Bowen: And you had brothers and sisters? Hoare: I have two younger brothers and two younger sisters. My second brother was also born in Ceylon. Bowen: And you all got on well together? You were a happy family? Hoare: Oh, yes, in the end anyway. Bowen: Yes, like all families. Yes. Hoare: We still have the opportunity to meet quite frequently. -
Communicating Parallel Processes
Communicating Parallel Processes Stephen Brookes Abstract Tony Hoare's 1978 paper introducing the programming language Communicating Sequential Processes is now a classic. CSP treated input and output as fundamental programming primitives, and in- cluded a simple form of parallel composition based on synchronized communication. This paper provides an excellent example of Tony's clarity of vision and intuition. The notion of processes is easy to grasp intuitively, provides a natural abstraction of the way many par- allel systems behave, and has an abundance of applications. Ideas from CSP have influenced the design of more recent programming languages such as occam and Ada. Investigations of the semantic foundations of CSP and its successors and derivatives have flourished, bringing forth a variety of mathematical models, each tailored to focus on a particular kind of program behavior. In this paper we re-examine the rationale for some of the original language design decisions, equipped with the benefit of hindsight and the accumulation of two decades of research into programming language semantics. We propose an \ide- alized" version of CSP whose syntax generalizes the original language along familiar lines but whose semantics is based on asynchronous communication and fair parallel execution, in direct contrast with the original language and its main successors. This language permits nested and recursive uses of parallelism, so it is more appropriate for us to refer to Communicating Parallel Processes. We outline a sim- ple semantics for this language and compare its structure with the most prominent models of the synchronous language. Our semantic framework is equally well suited to modelling asynchronous processes, shared-variable programs, and Kahn-style dataflow networks, so that we achieve a unification of paradigms. -
The Theory and Practice of Concurrency A.W. Roscoe
Contents i The Theory and Practice of Concurrency A.W. Roscoe Published 1997, revised to 2000 and lightly revised to 2005. The original version is in print in April 2005 with Prentice-Hall (Pearson). This version is made available for personal reference only. This version is copyright (c ) Pearson and Bill Roscoe. Contents Preface ix 0 Introduction 1 0.1 Background 1 0.2 Perspective 4 0.3 Tools 7 0.4 What is a communication? 8 I A FOUNDATION COURSE IN CSP 11 1 Fundamental concepts 13 1.1 Fundamental operators 14 1.2 Algebra 29 1.3 The traces model and traces refinement 35 1.4 Tools 48 2 Parallel operators 51 2.1 Synchronous parallel 51 2.2 Alphabetized parallel 55 2.3 Interleaving 65 2.4 Generalized parallel 68 iv Contents 2.5 Parallel composition as conjunction 70 2.6 Tools 74 2.7 Postscript: On alphabets 76 3 Hiding and renaming 77 3.1 Hiding 77 3.2 Renaming and alphabet transformations 86 3.3 A basic guide to failures and divergences 93 3.4 Tools 99 4 Piping and enslavement 101 4.1 Piping 101 4.2 Enslavement 107 4.3 Tools 112 5 Buffers and communication 115 5.1 Pipes and buffers 115 5.2 Buffer tolerance 127 5.3 The alternating bit protocol 130 5.4 Tools 136 5.5 Notes (2005) 137 6 Termination and sequential composition 139 6.1 What is termination? 139 6.2 Distributed termination 143 6.3 Laws 144 6.4 Effects on the traces model 147 6.5 Effects on the failures/divergences model 148 II THEORY 151 7 Operational semantics 153 7.1 A survey of semantic approaches to CSP 153 7.2 Transition systems and state machines 155 Contents v 7.3 Firing rules -
DEPARTMENT of COMPUTER SCIENCE Carneg,E-,Mellon Un
A MODEL FOR COMMUNICATING SEQUENTIAL PF_OCESSES Stephen D. Brookes DEPARTMENT of COMPUTER SCIENCE Carneg,e-,Mellon Un,vers,ty (7__[J--_ q - _ 2-- _ 4 :_ A MOI)EL FOR COMMUNICATING SEQUENTIAL PROCESSES Stephen D. Brookes Thesis submitted for the degree of Doctor of Philosophy. University College Oxford University January 1983 ABSTRACT This thesis describes tile construction and mathematical properties of a model for communicating sequential processes. We define a semantic model for processes based on failures, which encapsulate certain finite aspects of process behaviour and allow an elegant treatment of nondeterminism. We define a set of process operations, including nondeter- ministic choice, conditional composition, and various forms of' parallel composition. These process operations enjoy many interesting mathematical properties, which allow us to prove many process identities. The failures model is well suited to reasoning about deadlock properties of processes, and some examples are given to illustrate this. The failures model does not treat in a reasonable way the phenomenon of divergence, which occurs when a process performs a potentially infinite sequence of internal actions and never interacts with its environment. We offer an extension of" the model in Chapter 5, which treats divergence more satisfactorily. We also give a complete proof system for proving semantic equivalence of terms. Tile thesis also contains some results on the relationship of these models to other models of processes in the literature, specifically relating to the work of Milner, Kennaway, Hennessy and de Nicola. Chapter 3 gives an alternative formula t;ion of the failures model, a]iowing comparison with Kennaway's work. -
Translating Between Models of Concurrency
Translating between models of concurrency David Mestel∗ A.W. Roscoe† April 23, 2019 Abstract Hoare’s Communicating Sequential Processes (CSP) [8] admits a rich universe of semantic models closely related to the van Glabbeek spectrum. In this paper we study finite observational models, of which at least six have been identified for CSP, namely traces, stable failures, revivals, ac- ceptances, refusal testing and finite linear observations [20]. We show how to use the recently-introduced priority operator ([21], ch.20) to transform refinement questions in these models into trace refinement (language in- clusion) tests. Furthermore, we are able to generalise this to any (rational) finite observational model. As well as being of theoretical interest, this is of practical significance since the state-of-the-art refinement checking tool FDR4 [5] currently only supports two such models. In particular we study how it is possible to check refinement in a discrete version of the Timed Failures model that supports Timed CSP. 1 Introduction In this paper we re-examine part of the Linear-Time spectrum that forms part of the field of study of van Glabbeek in [31, 30], specifically the part characterised by finite linear observations. A number of different forms of process calculus have been developed for the modeling of concurrent programs, including Hoare’s Communicating Sequential Processes (CSP) [8], Milner’s Calculus of Communicating Systems (CCS) [12], arXiv:1904.09875v1 [cs.FL] 22 Apr 2019 and the π-calculus [13]. Unlike the latter two, CSP’s semantics are traditionally given in behavioural semantic models coarser than bisimulation, normally ones that depend on linear observations only. -
Security and Trust for Ubiquitous Communication
Security and trust for ubiquitous communication S. Creese Systems Assurance Group, QinetiQ, Malvern Technology Centre, UK G. M. Reed International Institute for Software Technology United Nations University Macau A. W. Roscoe and J. W. Sanders Programming Research Group University of Oxford Oxford, UK Prepared for the ITU WSIS Thematic Meeting on Cybersecurity 28 June – 1 July, 2005, Geneva Switzerland June 15, 2005 1 Abstract In this paper we report on an ethical approach to security and trust in contemporary computing and communication systems and we discuss some of its specific detailed consequences. The approach is that of the principle of distribution, in which control resides as much as possible with the individual rather than in centralised agents. The consequences on which we elaborate here include: the importance of establishing entirely distributed protocols, in particular for the secure and trusted dynamic networking of mobile ICT devices; that in view of their pervasive popularity, cellphones be subject to the same stringent criteria of security and trust as networked computers; the promotion of open-source software to bridge the digital divide and empower developing nations in configuring accepted software in their own languages and to their own needs. We report on the United Kingdom’s Department of Trade and Industry’s FOR- WARD program concerning security and trust in human-centric systems and on the United Nations University’s International Institute for Software Technology’s recent Open Computing Initiative, and end by raising some pertinent questions. 2 Introduction 1. We live in an age in which information and communications technologies span the globe, providing users with mobile and real-time access to information, services and each other. -
Communicating Parallel Processes Stephen Brookes 2
Communicating Parallel Processes Stephen Brookes 2 Abstract Tony Hoare’s 1978 paper introducing the programming language Communicating Sequential Processes is now a classic. CSP treated input and output as fundamen- tal programming primitives, and included a simple form of parallel composition based on synchronized communication. This paper provides an excellent example of Tony’s clarity of vision and intuition. The notion of processes is easy to grasp intuitively, provides a natural abstraction of the way many parallel systems behave, and has an abundance of applications. Ideas from CSP have influenced the design of more recent programming languages such as occam and Ada. Investigations of the semantic foundations of CSP and its successors and derivatives have flourished, bringing forth a variety of mathematical models, each tailored to focus on a par- ticular kind of program behavior. In this paper we re-examine the rationale for some of the original language design decisions, equipped with the benefit of hind- sight and the accumulation of two decades of research into programming language semantics. We propose an “idealized” version of CSP whose syntax generalizes the original language along familiar lines but whose semantics is based on asyn- chronous communication and fair parallel execution, in direct contrast with the original language and its main successors. This language permits nested and recur- sive uses of parallelism, so it is more appropriate for us to refer to Communicating Parallel Processes. We outline a simple semantics for this language and compare its structure with the most prominent models of the synchronous language. Our semantic framework is equally well suited to modelling asynchronous processes, shared-variable programs, and Kahn-style dataflow networks, so that we achieve a unification of paradigms. -
Discrete Analysis of Continuous Behaviour in Real-Time Concurrent Systems
Discrete Analysis of Continuous Behaviour in Real-Time Concurrent Systems Jo¨el Ouaknine St Cross College and University College Thesis submitted for the degree of Doctor of Philosophy at the University of Oxford, Michaelmas 2000 Discrete Analysis of Continuous Behaviour in Real-Time Concurrent Systems Jo¨el Ouaknine Thesis submitted for the degree of Doctor of Philosophy at the University of Oxford, Michaelmas 2000 Abstract This thesis concerns the relationship between continuous and discrete mod- elling paradigms for timed concurrent systems, and the exploitation of this relationship towards applications, in particular model checking. The frame- work we have chosen is Reed and Roscoe's process algebra Timed CSP, in which semantic issues can be examined from both a denotational and an operational perspective. The continuous-time model we use is the timed fail- ures model; on the discrete-time side, we build a suitable model in a CSP-like setting by incorporating a distinguished tock event to model the passage of time. We study the connections between these two models and show that our framework can be used to verify certain specifications on continuous-time processes, by building upon and extending results of Henzinger, Manna, and Pnueli's. Moreover, this verification can in many cases be carried out directly on the model checker FDR1. Results are illustrated with a small railway level crossing case study. We also construct a second, more sophisticated discrete- time model which reflects continuous behaviour in a manner more consistent with one's intuition, and show that our results carry over this second model as well.