ing Interaction Patterns: A Programming Paradigm for Open Distributed Systems Gul A. Agha Open Systems Laboratory Department of Computer Science, 1304 W. Springfield Avenue, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA, Email: agha@cs.uiuc.edu, Web: http://www-osl.cs.uiuc.edu To Appear: Formal Methods for Open Object-based Distributed Systems IFIP Transactions, E. Najm and J.-B. Stefani, Editors Chapman & Hall, 1997 Abstract This paper discusses mechanisms addressing the complexity of building and maintaining Open Distributed Systems. It is argued that a new programming paradigm based on modular specification of interaction patterns is required to address the complexity of such systems. Our research is based on developing abstraction mechanisms to simplify the task of developing and maintaining open systems. We define actors as a model of concurrency for open systems. We then review a number of programming abstractions that are useful in modular specification an...

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

With the evolution of the global information infrastructure, service providers will need to provide effective and adaptive resource management mechanisms that can serve more concurrent clients and deal with applications that exhibit Quality of Service (QoS) requirements. Flexible, scalable and customizable middleware can be used as enabling technology for next generation systems that adhere to the QoS requirements of applications that execute in highly dynamic distributed environments. To enable application aware resource management, we are developing a customizable and composable middleware framework called CompOSE|Q based on a reflective metamodel. In this paper, we describe the architecture and runtime environment for CompOSE|Q and briefly assess the performance overhead of the additional flexibility. We also illustrate how flexible communication mechanisms can be supported efficiently in the CompOSE|Q framework. 1.

Consider a distributed internet-based market place with sellers and buyers represented by autonomous mobile agents. Individual users of the system may create agents to purchase or offer goods or services; these agents may travel over the network searching for bargains or for potential markets. What framework would best support such a system? An opportunistic user would always selfishly seek the best deal, which may translate into having access to as much information as possible (in a managable form, of course). In a distributed environment, this would mean spawning a very large number of agents, possibly organized as a tree, to disperse over the network. In a more volatile market needing local decisions, seller and buyer agents would want to be omnipresent. In both cases, agents serving the same interests will often need some type of coordination; an ability to coordinate the behavior of agents in agent ensembles is a key challenge for Distributed AI. From the perspective of a node hosting such activity, there has to be some incentive to allow it. Malicious or erroneous agents may threaten the node

Actors are a model of concurrent objects which unify synchronization and data abstraction boundaries. Because they hide details of parallel execution and present an abstract view of the computation, actors provide a promising building block for easy-to-use parallel programming systems. However, the practical success of the concurrent object model requires two conditions be satisfied. Flexible communication abstractions and their efficient implementations are the necessary conditions for the success of actors. This thesis studies how to support communication between actors efficiently. First, we discuss communication patterns commonly arising in many parallel applications in the context of an experimental actor-based language, THAL. The language provides as communication abstractions concurrent call/return communication, delegation, broadcast, and local synchronization constraints. The thesis shows how the abstractions are efficiently implemented on stock-hardware distributed memory mul...

Advances in networking, communication, storage, computing, and multimedia technologies coupled with many emerging application areas is fueling the merger of computing and communication systems. This will result in a global information infrastructure of the size and magnitude erstwhile unimaginable. Such an infrastructure will have numerous services and hundreds of thousands of subscribers. A key issue in developing a global information infrastructure is that of effective management and utilization of resources. Increasingly, applications require delivery of multifaceted digital information services with stringent requirements on the delivery of information. For instance, multimedia applications have QoS (Quality of Service) parameters that define the extent to which performance specifications such as responsiveness, reliability, availability, security and cost-effectiveness may be violated. Varying requirements posed by applications, customers, and service providers makes the task of resource management in the evolving global information infrastructure a challenging research problem- one with significant commercial impact as well. In this thesis, we present a new paradigm for developing safe, customizable middleware

We present a two-level model of distributed computation based on the actor model. This two-level model is the basis for developing a semantic framework that supports dynamic customizability and separation of concerns in designing and reasoning about components of open distributed systems (ODS). ODS evolve dynamically and components of ODS interact with an environment that is not under their control. In particular, we would like to be able to consider separately issues such as: functional behavior of a service; failure semantics and fault tolerance protocols; and resource management issues such as memory management, migration, load balancing, and scheduling. In this report we consider remote creation, migration, and reachability snapshot services: their specification at different levels of abstraction, and their composition.

Advances in networking, communication, storage and computing technologies coupled with emerging novel application areas is enabling the widespread use of large scale distributed computing systems. These systems exhibit constant evolution as new applications place specialized requirements from the computing and communication infrastructure. Many applications provide QoS (Quality of Service) parameters that define the extent to which performance specifications such as responsiveness, reliability, availability, security and cost-effectiveness may be violated. These requirements are often implemented via resource management mechanisms in the middleware. In this paper, we develop a Qos-enabled customizable middleware framework called CompOSEjQ that can safely and effectively manage change in large scale distributed systems. We illustrate how to achieve flexible, safe and efficient composability of resource management services in the middleware layer while ensuring QoS to the application. I...

Communication between distributed objects may have to be protected against random failures and malicious attacks; also, communication timeliness may be essential or highly desired. Therefore, a distributed application often has to be extended with communication services providing some kind of fault-tolerance, secrecy, or quality-of-service guarantees. Ideally, such services should be defined in a highly modular and dynamically composable way, so that the combined assurance of several services can be achieved by composition in certain cases, and so that services can be added or removed from applications at runtime in response to changes in the environment. To reason about the formal properties of such composable communication services one first needs to give them a precise semantics. This paper proposes a rewriting logic semantics for the so-called "onion skin" model of distributed object reection, in which different meta-objects, providing different communication services, can be stacked on top of a basic application object. Since the correct behavior of a service depends on the type of hostile environment against which the service must protect the application, rewriting logic should also be used to specify such hostile environments. The service guarantees are then guarantees about the behavior specified by the union of the rewrite theories specifying the basic application, the services, and the hostile environment.

Research in software architecture has emphasized compositional development, where the computational aspects of a system are modularly separated from communication and coordination aspects. Typically, software architectures are factored into a set of components, which encapsulate computation, and connectors, which encapsulate interactions. In terms of design, development and debugging, this separation has several important advantages. In particular, by separating application code from the protocols used for interaction, software components may be independently developed and tested. Moreover, as requirements change, existing architectural elements may be modularly replaced by new elements with appropriate properties. A fundamental problem with these abstractions is their interaction with "cross-cutting" architectural features such as heterogeneity, availability, and adaptability. Availability, for example, requires protocols that manipulate both communication and resources. Controlling architectural resources, however, requires access to the internal resource usage patterns of components and connectors. Unfortunately, current architectural abstractions have inflexible interfaces which obscure these patterns. This loss of information forces the implementation of such features to be hard-coded within architectural elements, eliminating many advantages of the modular approach. In this thesis, we propose a model for distributed software architectures that exposes resource access in a modular fashion. Our model extends current architectural abstractions by providing a meta-architecture for customization. This meta-architecture augments the functional interface of architectural elements with an operational interface for controlling resources. We also develop a formal semantic...