Mariano Cilia

Event composition in time-dependent distributed systems

Many interesting application systems, ranging from workflow management and CSCW to air traffic control, are event-driven and time-dependent and must interact with heterogeneous components in the real world. Event services are used to glue together distributed components. They assume a virtual global time base to trigger actions and to order events. The notion of a global time that is provided by synchronized local clocks in distributed systems has a fundamental impact on the semantics of event-driven systems, especially the composition of events. The well studied 2g-precedence model, which assumes that the granularity of global time-base g can be derived from a priori known and bounded precision of local clocks may not be suitable for the Internet where the accuracy and external synchronization of local clocks is best effort and cannot be guaranteed because of large transmission delay variations and phases of disconnection. We introduce a mechanism based on NTP synchronized local clocks with global reference time injected by GPS time servers. We agree that timestamps of events can be related to global reference time with bounded accuracy and propose that event timestamps are modeled using accuracy intervals. We present algorithms for event composition and event consumption which make use of accuracy interval based timestamping and illustrate the problems that arise due to inaccuracy and message transmission delays.

Towards multi-purpose wireless sensor networks

Current wireless sensor network (WSN) architectures are based on the assumption that all sensor nodes are participating in a single global task. In many scenarios, however it will be desirable to use a single sensor network for multiple concurrent applications. In order to enable such multipurpose WSNs efficiently, delimiting each application to its specific set of relevant nodes is one of the key issues that needs to be solved. We present scoping as a general concept for the creation and maintenance of network-wide node subsets and describe a flexible and modular architecture that meets the requirements of multi-purpose WSNs.

Looking into the past: enhancing mobile publish/subscribe middleware

Publish/subscribe (pub/sub) middleware facilitates loosely coupled cooperation and fits well the needs of spontaneous, ad-hoc interaction. However, newly started mobile applications have to be bootstrapped to interpret the current flow of notifications correctly and commence normal operation. This problem is aggravated in mobile environments where disconnections and context changes occur frequently.In this paper, we propose two forms of subscriptions that allow consumers to subscribe to past events to improve the bootstrapping process. The first form uses logical mobility to harness possible client movements and subscribe in future locations to bootstrap virtual counterparts before the application needs the data. The second form is based on buffers and offers a way to integrate data repositories distributed in the network.

DREAM: Distributed Reliable Event-Based Application Management

New applications and the convergence of technologies, ranging from sensor networks to ubiquitous computing and from autonomic systems to event-driven supply chain management, require new middleware platforms that support proactive event notification. We present a system overview and discuss the principles of Dream, a reactive middleware platform that integrates event detection and composition mechanisms in a highly distributed environment; fault-tolerant and scalable event notification that exploits a variety of filter placement strategies; content-based notification to formulate powerful filters and concept-based notification to extend content-based filtering to heterogeneous environments; middleware-mediated transactions that integrate notifications and transactions; and scopes, which are administration primitives for both deployment- and runtime configurability, as well as for the management of policies. We discuss four prototypes that were implemented as proof-of-concept systems and present lessons learned from them.

'Publish-subscribe grows up: support for management, visibility control, and heterogeneity

Message-oriented middleware is used to decouple the operation of cooperating applications. Existing approaches have concentrated mainly on scalability issues, but dynamic business processes and the integration of a wide range of data sources and applications, require a middleware that is customizable. The Rebeca publish-subscribe service uses scoping to structure both middleware and applications. It thus offers advanced routing mechanisms to subsystems that need high scalability and it allows for heterogeneous message models that are transparently mapped onto each other.

CREAM: An Infrastructure for Distributed, Heterogeneous Event-Based Applications

Applications ranging from event-based supply chain management to enterprise application integration and pervasive computing depend on the timely detection and notification of events. We present Cream the event-based reactive component of the Dream middleware platform. Here we address four key issues in distributed and heterogeneous environments: event detection and notification, event composition, an active functionality service, and ontology support. We show the need for ontology support at all levels in heterogeneous environments and present a distributed active functionality service that addresses the difficult issues of event composition in widely distributed environments. We illustrate the practicality of the proposed approach through two prototypes that are based on this infrastructure: a meta-auction service and a personalized service offering in Internet-enabled vehicles.

An active functionality service for e-business applications

Service based architectures are a powerful approach to meet the fast evolution of business rules and the corresponding software. An active functionality service that detects events and involves the appropriate business rules is a critical component of such a service-based middleware architecture. In this paper we present an active functionality service that is capable of detecting events in heterogeneous environments, it uses an integral ontology-based approach for the semantic interpretation of heterogeneous events and data, and provides notifications through a publish/subscribe notification mechanism. The power of this approach is illustrated with the help of an auction application and through the personalization of car and driver portals in Internet-enabled vehicles.

Moving Active Functionality from Centralized to Open Distributed Heterogeneous Environments

Active functionality is especially useful for enforcing business rules in applications, such as Enterprise Application Integration (EAI) and e-commerce. It can be used as glue among existing applications, and for data transformations between heterogeneous applications. However, traditional active mechanisms have been designed for centralized systems and are monolithic, thus making it difficult to extend and adapt them to the requirements imposed by distributed, heterogeneous environments. To correct this we present a flexible, extensible, service-based architecture built on ontologies, services and events/notifications. The main contributions of this work are: i) the homogeneous use of ontologies for a semantically meaningful exchange and combination of events in open heterogeneous environments, and for the infrastructure itself; ii) a flexible architecture for the composition of autonomous, elementary services to provide Event-Condition-Action (ECA) functionality in different configurations; iii) the interaction of these services via notifications using a publish/subscribe mechanism (concept-based addressing).

The convergence of AOP and active databases: towards reactive middleware

Reactive behavior is rapidly becoming a key feature of modern software systems in such diverse areas as ubiquitous computing, autonomic systems, and event-based supply chain management. In this paper we analyze the convergence of techniques from aspect oriented programming, active databases and asynchronous notification systems to form reactive middleware. We identify the common core of abstractions and explain both commonalities and differences to start a dialogue across community boundaries. We present existing options for implementation of reactive software and analyze their run-time semantics. We do not advocate a particular approach but concentrate on identifying how the various disciplines can benefit from each other. We believe that AOP can solve the impedance mismatch found in reactive systems that are implemented through inherently static languages, while AOP can benefit from the active database communitys experience with event detection/composition and fault tolerance in large scale systems. The result could be a solid foundation for the realization of reactive middleware services.

Scoping in wireless sensor networks: a position paper

One of the trends of wireless sensor networks (WSN) is to allow multiple applications to run on top of the same sensor network. This will have an enormous impact on the way WSN applications are developed, deployed and maintained. Many applications for WSN are still developed on very low level functions provided by simple operating systems or bare hardware. Alternatively, generic WSN middleware focuses on very high-level system abstractions, such as declarative query languages, and acts as black box that tries to automatically map applications to the underlying resources. In this paper, we propose scopes as a generic abstraction for the definition of groups of nodes. They bridge the gap between high- and low-level interfaces and enable the partitioning of WSN functionality. As middleware building block they facilitate the construction of tailored services in multipurpose WSNs.