Clinger, W.D. Foundations of Actor Semitics. AI-TR- 633, MIT Artificial Intelligence Laboratory, .May, 1981.  Home/Search   Document Not in Database   Summary   Related Articles   Check

.... for communication in such protocols, and as an illustration for the Mobile UNITY proof logic, which was not exercised to the same degree in [11] While formal models capable of expressing reconfiguration have been explored from the algebraic perspective [12] and from a denotational perspective [5, 2], very few state based models can naturally express reconfiguration of components. Also, while algebraic models such as the 7r calculus may be adequate for expressing reconfiguration, it is not so clear how to handle the issue of disconnection. Recent work has recognized the importance of ....

William D. Clinger. Foundations of actor semantics. Technical Report AI-TR-633, MIT Artificial Intelligence Laboratory, 1981.

....in terms of power and clarity of expression of the module, type and exception content of the example. Three of the languages are computer algebra environments (Maple [ea85] Mathematica [Wol91] and Axiom [JS92] two are typically used in compiler implementation and the study of algorithms (Scheme [CRea89] and Setl2 [Sny90a, Sny90b] and one is in a family of languages explicitly intended as metalanguage for the implementation of proof checkers for axiomatic logics (SML [AM90] the example could equally have been done in CAML [WAL 87] which, however, has a less developed module facility) ....

....of languages explicitly intended as metalanguage for the implementation of proof checkers for axiomatic logics (SML [AM90] the example could equally have been done in CAML [WAL 87] which, however, has a less developed module facility) 3.2. 2 Stack in Scheme, an algorithm language Scheme [CRea89] doesn t have user defined dataypes or exceptions, so the expression is purely functional: define push (lambda (x S) cons x S) define pop (lambda (S) if (null S) cantPop (cdr S) define top (lambda (S) if (null S) cantTop (car S) 27 (define isEmpty (lambda (S) null S) ....

William Clinger, Jonathan Rees, and et al. Revised 3.99 report on the algorithmic language Scheme. MIT Artificial Intelligence Laboratory, 31 August 1989.

....framework. The mapping of the programming model to the execution model described here is largely conceptual; further information about our approach and actual implementation of COOP can be found in [9, 30] 2. 1 Programming Model The programming model we assume is the synergistic union of Actors [1, 12, 21] and the objectoriented model [17] Each object can act concurrently to update its own state, create new objects or invoke methods on other objects. An object provides a set of abstract operations, of which only one may be active at a time. This allows objects to control updates to their internal ....

William D. Clinger. Foundations of actor semantics. Technical Report AI-TR-633, MIT Artificial Intelligence Laboratory, 1981.

....Thus the topology can change dynamically. See [14, 2, 3] for more discussion of the actor model, and for many examples of programming with actors. Axioms expressing the essential features of actor computation such as the acquaintance relation and ordering of events are given in [7] Will Clinger [12] developed a powerdomain semantics of actor systems, showing the consistency of these axioms. An interleaving transition system semantics for an actor language is given in [6, 5] This work builds on the formulation in [2] and develops methods for reasoning about equivalence of actor programs. ....

W. D. Clinger. Foundations of actor semantics. AI-TR633, MIT Artificial Intelligence Laboratory, May 1981.

....objects is flexible because it does not require any language extensions and allows different thread and synchronization objects, and scheduling strategies to be implemented according to the programmer s needs. The actor approach offers a well studied theoretical concurrent object oriented model [71, 2], but is not without its problems (e.g. inheritance anomaly and efficient implementation) PSather follows the model with passive objects and threads as independent loci of control, and has thread creation constructs (Section 2.3) It also supports data parallel computation with a dist statement ....

W. D. Clinger. Foundations of Actor Semantics. Technical Report AI-TR-633, MIT Artificial Intelligence Laboratory, May 1981.

....between autonomous interacting components. The local computation carried out by the components may be specified in any sequential language. Axioms expressing the essential features of actor computation such as the acquaintance relation and ordering of events are given in [19] Will Clinger [51] developed a powerdomain semantics of actor systems, showing the consistency of these axioms. An interleaving transition system semantics for an actor language is given in [9, 10] This work builds on the formulation in [4] and develops methods for reasoning about equivalence of actor programs ....

W. D. Clinger. Foundations of actor semantics. AI-TR- 633, MIT Artificial Intelligence Laboratory, May 1981.

....investigated both in theory and in practice [17, 58, 18, 57, 1] programmers still write their parallel applications in a low level message passing paradigm [112, 42] or a shared memory paradigm. By almost any measure, massively parallel MIMD machines remain difficult to program. Because actors [53, 55, 30, 1] (or, concurrent active objects) hide details of parallel execution and present a transparent view of the computation, they provide a promising building block for efficient easy to use parallel programming systems. In particular, actors extend sequential object models by abstracting over threads ....

....in actor languages because actors are active by definition. Every actor is associated with a thread; however, the thread makes its presence manifest only when a message is scheduled. 2. 3 Actor Based Languages The Actor model was first introduced by Carl Hewitt [53] refined by many others [55, 54, 30, 56] and defined in its current standard form by Agha [1] Particularly, since its introduction many actor based languages [121, 13, 88, 16, 11, 85, 135, 89, 70, 29] have been proposed for programming concurrent computation. Act1 [88] was an early actor language which was implemented in Lisp. It ....

W. Clinger. Foundations of Actor Semantics. Technical Report AI-TR-633, MIT Artificial Intelligence Laboratory, May 1981.

....Programming We outline salient features of our programming model that facilitate irregular applications and briefly describe its implementation in the Concert System. 2.1. Programming Model Concert supports a fine grained concurrent objectoriented programming model based on Actors [1, 7, 18]. Computation is expressed as method invocations on objects and collections of objects. Method invocations on objects conceptually operate within dynamically created fine grained threads that are inherently concurrent, with execution order constrained only by data or programmer imposed control ....

William D. Clinger. Foundations of actor semantics. Technical Report AI-TR-633, MIT Artificial Intelligence Laboratory, 1981.

.... for communication in such protocols, and as an illustration for the Mobile UNITY proof logic, which was not exercised to the same degree in [11] While formal models capable of expressing reconfiguration have been explored from the algebraic perspective [12] and from a denotational perspective [5, 2], very few state based models can naturally express reconfiguration of components. Also, while algebraic models such as the calculus may be adequate for expressing reconfiguration, it is not so clear how to handle the issue of disconnection. Recent work has recognized the importance of ....

William D. Clinger. Foundations of actor semantics. Technical Report AI-TR-633, MIT Artificial Intelligence Laboratory, 1981.

....the other hand there are models which express mobility directly but which still require, in our view, a mathematical analysis of their basic concepts such as we provide in this paper. A well known model of this kind, which has had considerable success in applications, is the Actors model of Hewitt [5]. In such models, mobility is often achieved by allowing processes to be passed as values in communication; we shall instead achieve it by allowing references to processes, i.e. links, to be communicated. This presents an interesting contrast with recent attempts to combine the ideas of calculus ....

....There have been a number of formalisms which allow mobility, but have not developed its algebraic theory. The first was Hewitt s Actor formalism. Hewitt s ideas on the changing configuration among actors were developed in the early 1970s; a semantic treatment is given by Clinger in his PhD thesis [5]. More recently, Kennaway and Sleep invented their LNET and DyNe formalisms specifically to describe parallel graph reduction processes, such as we present in Section 4, in the context of a project to design a parallel processor [15] Also Astesiano and Zucca [2] have extended CCS to include ....

Clinger, W.D., Foundations of Actor Semantics, AI-TR-633, MIT Artificial Intelligence Laboratory, 1981.

....implement an ideal algorithm. For our purposes, a PDS is fixed or dynamic with respect to some set of events in computation. Events in a distributed system are partially ordered but may be mapped to a linear global time which represents the events as they may be observed by a hypothetical observer [17]. The global time is not unique. A PDS for an actor group is fixed with respect to a set of events E if the placement of actors in the group is fixed for all events bounded by E. We define an event as bounded by a set of events if it must occur between some two events in the set regardless of ....

W. Clinger. Foundations of actor semantics. AI-TR- 633, MIT Artificial Intelligence Laboratory, May 1981.

....computation and messaging dominate respectively. 5 Discussion and Related Work We have described a hybrid execution model for fine grained concurrent programs. While the mechanisms described in this paper were developed for concurrent object oriented languages [20, 33] based on the Actor model [18, 9, 1], we believe they are applicable to other programming models that support implicit synchronization and communication. In particular, this model is useful as a compiler target. It has a portable implementation and provides a hierarchy of mechanisms of varying power and cost supporting a wide range ....

William D. Clinger. Foundations of actor semantics. Technical Report AI-TR-633, MIT Artificial Intelligence Laboratory, 1981.

.... in process calculi related formalisms, which plays a crucial role in our semantic construction, was concurrently discovered by us (early 1990) by Meseguer [16] and by Nierstrasz [25] 13 We can even trace back the idea to the representation of asynchronous messages in the actor event diagram [7] in the context of the actor model by Hewitt and others [9, 2] But semantic significance of asynchronous communication in the general theoretical setting in contrast to synchronous communication, seems not to have been studied so much, in spite of the early work such as [3] Our work develops the ....

Clinger, W. Foundations of Actor Semantics. AI-TR-633, MIT Artificial Intelligence Laboratory.

....every other node via some sequence of links. We further assume that communication links behave as unbounded buffers with finite message transfer delay. Although there is no global clock in a truly distributed system, we can assume that a meaningful notion of global state of the system exists (cf. [5]) We call such a global state a multicomputer system configuration. It is a snapshot of the entire state of the system, relative to some external point of view, that encompasses both the processing and communication activities in the system. It accounts for both the abstract level of processing ....

W. D. Clinger. Foundations of actor semantics. AI-TR- 633, MIT Artificial Intelligence Laboratory, May 1981.

....capability to generate and pass communication ports turns out to be essential not only for object orientation (which is obvious) but also for control of causality chains in the face of pure asynchrony. This reminds us of the studies on the actor model of computation by Hewitt and his colleagues [8, 7, 5, 1]. Also readers may refer to the authors work on conceptual framework for open distributed computing environments [22, 23] to understand their general orientation in a different context. This paper only provides basic concepts and definitions for the formal system along with several illustrative ....

Clinger, W. : Foundations of Actor Semantics. AI-TR-633, MIT Artificial Intelligence Laboratory.

....the super prescript u X to indicate that X belongs to the user language and k X to indicate that X belongs to the kernel language. So for example u K is an user language configuration. However to prevent a notational quagmire we use this convention sparingly. 3 behaviors used by Clinger [Cli81]. They are a simplification and generalization of the notion of abstract actor structures presented in [Tal96b, Tal96a] 2.1 Actor Theories and Their Semantics an Introduction An actor theory describes individual actor behaviors and their local interactions in a representation independent ....

....rules are labelled. These labels are used in deriving a labelled transition system semantics. In this way the labels provide information concerning the basic observations that can be made as an actor system evolves. An actor theory must obey the fundamental acquaintance (locality) laws of actors [BH77, Cli81] in addition to renaming laws that express the fact that computation is uniformly parameterized in the choice of actor names renaming commutes with everything. To state these laws an actor theory also provides a primitive operation to determine the acquaintances of (actor names occurring in) the ....

W. D. Clinger. Foundations of actor semantics. AI-TR- 633, MIT Artificial Intelligence Laboratory, May 1981.

.... one approach to handling fairness is through a second transition relation (for example, Costa and Sterling [CS84] define such a relation for CCS) The concept of actors was originally proposed by Hewitt [Hew77] The actor model has been formally characterized by means of power domain semantics [Cli81], by a transition system [Agh86] and by Colored Petri Nets [SVN91] Complexity measures for actor programs have been defined [BVN91] The model has been proposed as a basis for multiparadigm programming [Agh89] and has been used as a programming model for multicomputers [AS88] Recently, Milner ....

W. Clinger. Foundations of actor semantics. AI-TR- 633, MIT Artificial Intelligence Laboratory, May 1981.

....actions implied by the withdrawal request are still being carried out. This allows concurrent execution of actions specified within the body of the actor. The concept of actors was originally proposed by Hewitt [9] The actor model has been formally characterized by means of power domain semantics [6], by a transition system [1] and by Colored Petri Nets [12] Complexity measures for actor programs have been defined [5] The model has been proposed as a basis for multiparadigm programming [2] and has been used as a programming model for multicomputers [4] and [7] Rosette Our work uses ....

W. Clinger. Foundations of Actor Semantics. AI-TR- 633, MIT Artificial Intelligence Laboratory, May 1981.

....bounded by E. We define an event as bounded by a set of events if it must occur between some two events in the set regardless of the observer. Note that events in a distributed system are partially ordered but may be mapped to a linear global time as they may be observed by a hypothetical observer [12]. In intuitive terms, a fixed placement policy for a computation performed by a group of actors places the actors before the computation starts and doesn t change their location during the computation. A placement policy which is not fixed is said to be dynamic. Fixed placement policies are often ....

W. Clinger, Foundations of Actor Semantics, AI-TR- 633, MIT Artificial Intelligence Laboratory, May 1981.

....with time floating from the top of the line to the bottom. Events that occurred earlier lie above those that occur later. Event diagrams bring out the concept of local time and local state in the actor model. In fact, it is believed that the notion of global time is itself quite constraining [Cli81] Causality connections form the fundamental synchronization mechanism in the Actor model. Arrival ordering imposes a notion of local time while activation ordering indicates communication patterns and causality among events. The combined ordering includes all possible dependencies. target ....

W. D. Clinger. Foundations of actor semantics. AI-TR- 633, MIT Artificial Intelligence Laboratory, May 1981.

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Clinger, W.D. Foundations of Actor Semitics. AI-TR- 633, MIT Artificial Intelligence Laboratory, .May, 1981.

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