This paper describes a system for designing and classifying actor languages. This system, named Actalk, which stands for actors in Smalltalk-80, is based on some minimal kernel introducing actors into Smalltalk-80. The Actalk kernel extends passive and sequential objects activated by synchronous message passing into active and concurrent actors communicating by asynchronous message passing. This defines a sub-world of actors embedded into the Smalltalk80 programming language and environment, which is left unchanged. The Actalk kernel is composed of only two Smalltalk-80 classes. Simulating current actor languages or designing new ones is achieved by defining subclasses of these two kernel classes. Consequently all such extensions are implicitly classified by the inheritance mechanism and unified into the Actalk environment. We are currently extending the standard Smalltalk-80 programming environment to design a specific one dedicated to Actalk concurrent actors. In this paper, the moti...

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This paper aims at classifying and discussing the various ways along which the object paradigm is used in concurrent and distributed contexts. We distinguish the library approach, the integrative approach, and the reflective approach. The library approach applies object-oriented concepts, as they are, to structure concurrent and distributed systems through class libraries. The integrative approach consists in merging concepts such as: object and activity, message passing and transaction, etc. The reflective approach integrates class libraries intimately within an object-based programming language. We discuss and illustrate each of these approaches and we point out their complementary levels and goals. Technical Report - May 1998 Laboratoire de Syst`emes d'Exploitation D'epartement d'Informatique Ecole Polytechnique F'ed'erale de Lausanne CH-1015 Lausanne, Switzerland To appear in ACM Computing Surveys, September 1998. 1 Introduction It is now well accepted that the object paradi...

Abstract. We present the design and implementation of the type system for Ptolemy II, which is a tool for component-based heterogeneous modeling and design. This type system combines static typing with run-time type checking. It supports polymorphic typing of components, and allows automatic lossless type conversion at run-time. To achieve this, we use a lattice to model the lossless type conversion relation among types, and use inequalities defined over the type lattice to specify type constraints in components and across components. The system of inequalities can be solved efficiently, with existence and uniqueness of a solution guaranteed by fixed-point theorems. This type system increases the safety and flexibility of the design environment, promotes component reuse, and helps simplify component development and optimization. The infrastructure we have built is generic in that it is not bound to one particular type lattice. The type system can be extended in two ways: by adding more types to the lattice, or by using different lattices to model different system properties. Higher-order function types and extended types can be accommodated in this way. 1

Current middleware solutions such as CORBA and Java's RMI emphasize compositional design by separating functional aspects of a system (e.g. objects) i~om the mechanisms used for interaction (e.g. remote procedure call through stubs and skeletons). While this is an effective solution for handling distributed interactions, higher-level requirements such as heterogeneity, availability, and adaptability require policies for resource management as well as interaction. We describe the Distributed Connection Language (DCL): an architecture description language based on the Actor model of distributed objects. System components and the policies which govern an architecture are specified as encapsulated groups of actors. Composition operators are used to build connections between components as well as customize their behavior. This customization is realized using a metaarchitecture. We describe the syntax and semantics of DCL, and illustrate the language by way of several examples. 1

The trend towards processors with more and more parallel cores is increasing the need for software that can take advantage of parallelism. The most widespread method for writing parallel software is to use explicit threads. Writing correct multithreaded programs, however, has proven to be quite challenging in practice. The key difficulty is non-determinism. The threads of a parallel application may be interleaved non-deterministically during execution. In a buggy program, non-deterministic scheduling will lead to nondeterministic results—some interleavings will produce the correct result while others will not. We propose an assertion framework for specifying that regions of a parallel program behave deterministically despite nondeterministic thread interleaving. Our framework allows programmers to write assertions involving pairs of program states arising from different parallel schedules. We describe an implementation of our deterministic assertions as a library for Java, and evaluate the utility of our specifications on a number of parallel Java benchmarks. We found specifying deterministic behavior to be quite simple using our assertions. Further, in experiments with our assertions, we were able to identify two races as true parallelism errors that lead to incorrect non-deterministic behavior. These races were distinguished from a number of benign races in the benchmarks.

We describe an efficient decentralized monitoring algorithm that monitors a distributed program's execution to check for violations of safety properties. The monitoring is based on formulae written in PT-DTL, a variant of past time linear temporal logic that we define. PT-DTL is suitable for expressing temporal properties of distributed systems. Specifically, the formulae of PT-DTL are relative to a particular process and are interpreted over a projection of the trace of global states that represents what that process is aware of. A formula relative to one process may refer to other processes' local states through remote expressions and remote formulae. In order to correctly evaluate remote expressions, we introduce the notion of KNOWLEDGEVECTOR and provide an algorithm which keeps a process aware of other processes' local states that can affect the validity of a monitored PT-DTL formula. Both the logic and the monitoring algorithm are illustrated through a number of examples. Finally, we describe our implementation of the algorithm in a tool called DIANA.

Abstract. We present an algorithm for automatic testing of distributed programs, such as Unix processes with inter-process communication, Web services, etc. Specifically, we assume that a program consists of a number of asynchronously executing concurrent processes or actors which may take data inputs and communicate using asynchronous messages. Because of the large numbers of possible data inputs as well as the asynchrony in the execution and communication, distributed programs exhibit very large numbers of potential behaviors. Our goal is two fold: to execute all reachable statements of a program, and to detect deadlock states. Specifically, our algorithm uses simultaneous concrete and symbolic execution, or concolic execution, to explore all distinct behaviors that may result from a program’s execution given different data inputs and schedules. The key idea is as follows. We use the symbolic execution to generate data inputs that may lead to alternate behaviors. At the same time, we use the concrete execution to determine, at runtime, the partial order of events in the program’s execution. This enables us to improve the efficiency of our algorithm by avoiding many tests which would result in equivalent behaviors. We describe our experience with a prototype tool that we have developed as a part of our Java program testing tool jCUTE. 1

We introduce a rewrite-based specification language for modelling probabilistic concurrent and distributed systems. The language, based on PMaude, has both a rigorous formal basis and the characteristics of a high-level rule-based programming language. Furthermore, we provide tool support for performing discrete-event simulations of models written in PMaude, and for statistically analyzing various quantitative aspects of such models based on the samples that are generated through discrete-event simulation. Because distributed and concurrent communication protocols can be modelled using actors (concurrent objects with asynchronous message passing), we provide an actor PMaude module. The module aids writing specifications in a probabilistic actor formalism. This allows us to easily write specifications that are purely probabilistic – and not just non-deterministic. The absence of such (un-quantified) non-determinism in a probabilistic system is necessary for a form of statistical analysis that we also discuss. Specifically, we introduce a query language called Quantitative Temporal Expressions (or QuaTEx in short), to query various quantitative aspects of a probabilistic model. We also describe a statistical technique to evaluate QuaTEx expressions for a probabilistic model. 1

Systems with both continuous and discrete behaviors can be modeled using a mixed-signal style or a hybrid systems style. This paper presents a component-based modeling and simulation framework that supports both modeling styles. The component framework, based on an actor meta-model, takes a hierarchical approach to manage heterogeneity in modeling complex systems. We describe how ordinary differential equations, discrete-event systems, and finite state machines can be built under this meta-model. A mixed-signal system is a hierarchical composition of continuous-time and discrete-event models, and a hybrid system is a hierarchical composition of continuous-time and finite-state-machine models. Hierarchical composition and information hiding help building clean models and efficient execution engines. Simulation technologies, in particular, the interaction between a continuous-time ODE solving engine and various discrete simulation engines are discussed. A signal type system is introduced to schedule hybrid components inside a continuous-time environment. Breakpoints are used to control the numerical integration step sizes so that discrete events are handled properly. A "refiring" mechanism and a "rollback" mechanism are designed to manage continuous components inside a discrete-event environment. The technologies are implemented in the Ptolemy II software environment. Examples are given to show the applications of this framework in mixed-signal and hybrid systems.