Mark Astley

Bio: Mark Astley is an academic researcher from University of Illinois at Urbana–Champaign. The author has contributed to research in topics: Architecture description language & Modular design. The author has an hindex of 7, co-authored 7 publications receiving 920 citations.

Matching events in a content-based subscription system

Abstract: Content-based subscription systems are an emerging alternative to traditional publish-subscribe systems, because they permit more flexible subscriptions along multiple dimensions. In these systems, each subscription is a predicate which may test arbitrary attributes within an event. However, the matching problem for content-based systems — determining for each event the subset of all subscriptions whose predicates match the event — is still an open problem. We present an efficient, scalable solution to the matching problem. Our solution has an expected time complexity that is sub-linear in the number of subscriptions, and it has a space complexity that is linear. Specifically, we prove that for predicates reducible to conjunctions of elementary tests, the expected time to match a random event is no greater than O(N 1 ) where N is the number of subscriptions, and is a closed-form expression that depends on the number and type of attributes (in some cases, 1=2). We present some optimizations to our algorithms that improve the search time. We also present the results of simulations that validate the theoretical bounds and that show acceptable performance levels for tens of thousands of subscriptions. Department of Computer Science, Cornell University, Ithaca, N.Y. 14853-7501, aguilera@cs.cornell.edu IBM T.J. Watson Research Center, Yorktown Heights, N.Y. 10598, fstrom, sturman, tusharg@watson.ibm.com Department of Computer Science, University of Illinois at Urbana-Champaign, 1304 W. Springfield Ave, Urbana, I.L. 61801, astley@cs.uiuc.edu

Customizable middleware for modular distributed software

Abstract: autonomous computing elements that interact through a shared network. The asynchronous nature of distributed systems significantly complicates application development. In particular, software executing on distributed systems represents a unique synthesis of application code and code for managing requirements such as heterogeneity, scalability, security, and availability. As an example, consider a distributed video-ondemand service: such an application requires policies for opening new connections to the service, ensuring that service providers are compensated (that is, charging for service), and managing the delivery of media to clients. While such policies express basic requirements for the service, the protocols that implement these policies are determined by orthogonal factors such as the presence of faulty hardware or insecure networks. These factors are orthogonal in the sense that they only affect the implementation of the protocol and do not alter the basic implementation of the service. As a result, changing requirements and the rapid pace at which new hardware is introduced lead to protocols that are constantly evolving. Application code and protocol code are often intermixed and this lack of modularity significantly reduces the flexibility, maintainability, and portability of distributed code. We describe a system architecture and programming techniques that enable the clean separation of protocols from application code. In particular, we provide modularity via two mechanisms (see Figure 1):

Customization and composition of distributed objects: middleware abstractions for policy management

Abstract: Current middleware solutions such as CORBA and Java's RMI emphasize compositional design by separating functional aspects of a system (e.g. objects) from 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 meta-architecture. We describe the syntax and semantics of dcl, and illustrate the language by way of several examples.

Customization and composition of distributed objects: policy management in distributed software architectures

Abstract: 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 semantics which provides a foundation for reasoning about composition in the model. As an instantiation of the model, we describe an architecture description language called the Distributed Connection Language (DCL). DCL allows the specification of distributed architectures which incorporate traditional elements (i.e. components and connectors) together with new elements, called policies, which specify resource constraints. We provide a Java-based implementation of DCL to demonstrate that the increased modularity of the approach does not entail prohibitive performance tradeoffs.

Modular heterogeneous system development: a critical analysis of Java

Abstract: Java supports heterogeneous applications by transforming a heterogeneous network of machines into a homogeneous network of Java virtual machines. This approach abstracts over many of the complications that arise from heterogeneity, providing a uniform API to all components of an application. However, for many applications heterogeneity is an intentional feature where components and resources are co-located for optimal performance. The authors argue that Java's API does not provide an effective means for building applications in such an environment. Specifically, they suggest improvements to Java's existing mechanisms for maintaining consistency (e.g. synchronized), and controlling resources (e.g. thread scheduling). They also consider the recent addition of a CORBA API in JDK 1.2. They argue that while such an approach provides greater flexibility for heterogeneous applications, many key problems still exist from an architectural standpoint. Finally, they consider the future of Java as a foundation for component-based software in heterogeneous environments and suggest architectural abstractions which will prove key to the successful development of such systems. They drive the discussion with examples and suggestions from their work on the Actor model of computation.

The many faces of publish/subscribe

Abstract: Well adapted to the loosely coupled nature of distributed interaction in large-scale applications, the publish/subscribe communication paradigm has recently received increasing attention. With systems based on the publish/subscribe interaction scheme, subscribers register their interest in an event, or a pattern of events, and are subsequently asynchronously notified of events generated by publishers. Many variants of the paradigm have recently been proposed, each variant being specifically adapted to some given application or network model. This paper factors out the common denominator underlying these variants: full decoupling of the communicating entities in time, space, and synchronization. We use these three decoupling dimensions to better identify commonalities and divergences with traditional interaction paradigms. The many variations on the theme of publish/subscribe are classified and synthesized. In particular, their respective benefits and shortcomings are discussed both in terms of interfaces and implementations.

Design and evaluation of a wide-area event notification service

Abstract: The components of a loosely coupled system are typically designed to operate by generating and responding to asynchronous events. An event notification service is an application-independent infrastructure that supports the construction of event-based systems, whereby generators of events publish event notifications to the infrastructure and consumers of events subscribe with the infrastructure to receive relevant notifications. The two primary services that should be provided to components by the infrastructure are notification selection (i. e., determining which notifications match which subscriptions) and notification delivery (i.e., routing matching notifications from publishers to subscribers). Numerous event notification services have been developed for local-area networks, generally based on a centralized server to select and deliver event notifications. Therefore, they suffer from an inherent inability to scale to wide-area networks, such as the Internet, where the number and physical distribution of the service's clients can quickly overwhelm a centralized solution. The critical challenge in the setting of a wide-area network is to maximize the expressiveness in the selection mechanism without sacrificing scalability in the delivery mechanism. This paper presents SIENA, an event notification service that we have designed and implemented to exhibit both expressiveness and scalability. We describe the service's interface to applications, the algorithms used by networks of servers to select and deliver event notifications, and the strategies used to optimize performance. We also present results of simulation studies that examine the scalability and performance of the service.

Sold!: auction methods for multirobot coordination

Abstract: The key to utilizing the potential of multirobot systems is cooperation. How can we achieve cooperation in systems composed of failure-prone autonomous robots operating in noisy, dynamic environments? We present a method of dynamic task allocation for groups of such robots. We implemented and tested an auction-based task allocation system which we call MURDOCH, built upon a principled, resource centric, publish/subscribe communication model. A variant of the Contract Net Protocol, MURDOCH produces a distributed approximation to a global optimum of resource usage. We validated MURDOCH in two very different domains: a tightly coupled multirobot physical manipulation task and a loosely coupled multirobot experiment in long-term autonomy. The primary contribution of the paper is to show empirically that distributed negotiation mechanisms such as MURDOCH are viable and effective for coordinating physical multirobot systems.

Processing flows of information: From data stream to complex event processing

Abstract: A large number of distributed applications requires continuous and timely processing of information as it flows from the periphery to the center of the system. Examples include intrusion detection systems which analyze network traffic in real-time to identify possible attacks; environmental monitoring applications which process raw data coming from sensor networks to identify critical situations; or applications performing online analysis of stock prices to identify trends and forecast future values.Traditional DBMSs, which need to store and index data before processing it, can hardly fulfill the requirements of timeliness coming from such domains. Accordingly, during the last decade, different research communities developed a number of tools, which we collectively call Information flow processing (IFP) systems, to support these scenarios. They differ in their system architecture, data model, rule model, and rule language. In this article, we survey these systems to help researchers, who often come from different backgrounds, in understanding how the various approaches they adopt may complement each other.In particular, we propose a general, unifying model to capture the different aspects of an IFP system and use it to provide a complete and precise classification of the systems and mechanisms proposed so far.

High-performance complex event processing over streams

Abstract: In this paper, we present the design, implementation, and evaluation of a system that executes complex event queries over real-time streams of RFID readings encoded as events. These complex event queries filter and correlate events to match specific patterns, and transform the relevant events into new composite events for the use of external monitoring applications. Stream-based execution of these queries enables time-critical actions to be taken in environments such as supply chain management, surveillance and facility management, healthcare, etc. We first propose a complex event language that significantly extends existing event languages to meet the needs of a range of RFID-enabled monitoring applications. We then describe a query plan-based approach to efficiently implementing this language. Our approach uses native operators to efficiently handle query-defined sequences, which are a key component of complex event processing, and pipeline such sequences to subsequent operators that are built by leveraging relational techniques. We also develop a large suite of optimization techniques to address challenges such as large sliding windows and intermediate result sizes. We demonstrate the effectiveness of our approach through a detailed performance analysis of our prototype implementation under a range of data and query workloads as well as through a comparison to a state-of-the-art stream processor.