Event broker networks -scalable versions of the publish-subscribe paradigmact as peer-to-peer overlays on broker nodes. Various frameworks support different overlay topologies and routing schemes for event dissemination, but attention is now turning to the nonfunctional attributes (such as quality of service) of such systems. Although many research efforts are starting to address the need for adaptation and QoS, no taxonomies or comprehensive surveys of adaptive middleware, which provide support for service guarantees, exist yet. To tackle this knowledge gap, the authors examine existing event-based middleware efforts, focusing on quality of service and adaptation. iddleware for event broker networks (EBNs) both alleviates the issues related to underlying platform heterogeneity and provides a uniform application interface. A common service interface that such middleware provides is publish-subscribe, 1 a paradigm in which producers publish information and consumers subscribe to it. In EBNs, information of interest is encapsulated in events; on such networks, middleware stores and manages subscriptions as well as routes events. This model's fully decoupled nature in terms of time, space, and synchronization 1 makes it highly suitable for large-scale distributed applications. Middleware for event broker networks is also known as eventdissemination middleware (EDM). To disseminate events, an EBN overlay network 2 uses a subset of the underlying existing physical network (an overlay link itself is virtual but could consist of several physical links to the underlying network). Therefore, middleware must execute on each overlay node that has to work cooperatively to accomplish its functions. Such event-based architectures appear in various domains, many of which themselves require quality-of-service (QoS) guarantees from the underlying infrastructure. However, our survey of existing efforts in research-and-industrybased event dissemination middleware indicates significant gaps in efforts to address these needs; in fact, we found that few middleware options 3-5 provide support for nonfunctional service guarantees. Because a comprehensive survey of current options doesn't yet exist, this article offers a review of existing event-based systems, classified with a taxonomy of adaptive, event-based middleware that provides QoS guarantees. As Figure 1 shows, event-based middleware has a layered architecture. The core comprises mandatory functional features, such as the event model, the subscription scheme used to disseminate events, and the overlay routing substrate. The layer on top of the core forms a set of optional services -that is, the nonfunctional QoS guarantees the middleware provides, such as reliability, delivery semantics, message ordering, security, and fault tolerance. QoS guarantees are orthogonal to middleware-supported core functions; the middleware's ability to reconfigure itself to ensure and maintain a QoS agreement is its adaptability, which can result in changes to the core's subscription scheme or overlay routing substrate. The taxonomy we developed as part of our survey efforts provides a hierarchy of the identifying features that an event-based system requires for service guarantees. Specifically, it helps us identify groups of systems and lays out the functional characteristics that come into play when providing QoS guarantees and support. As Figure 2 shows, our taxonomy begins with a differentiation between mandatory functional features and QoS properties; as mentioned in the previous section, we can break the core down further into an event model, overlay routing substrate, and subscription scheme. In any EDM, subscribers transmit subscriptions in the overlay network (filters sift out any nonmatching data elements from the event notifica-36 www.computer.org/internet/ IEEE INTERNET COMPUTING Dynamic Information Dissemination I n building our taxonomy, we first researched all the existing work we could find on event dissemination. The taxonomy presented by Rene Meier and colleagues 1 came closest to what we were looking for: it deals with the classification of existing event-based systems as a programming paradigm and identifies fundamental properties of event-based middleware. Their taxonomy then classifies middleware broadly based on event model and event service criteria, with a further classification on the basis of the middleware's functional and nonfunctional features and organization and interaction model. Although we reviewed core functionality, our focus is on quality-of-service guarantees and adaptation in middleware rather than with programming models for functional aspects of event-dissemination middleware (EDM). We also extend Meier and colleagues' definition of an event model to a communication model by taking into account event characteristics such as attributes and hierarchical and compositional relationships between event types. Patrick Eugster and colleagues 2 also present a detailed survey of event-based systems based on a common denominator of the various modes of decoupling in asynchronous systems. Although they discuss decoupling in three dimensions (time, space, and synchronization), their main focus is on implementing the issues underlying publish-subscribe schemes. Their survey forms the basis of our work. Bruce Martin and colleagues 3 present a taxonomy as a hierarchy of questions and answers about the features of distributed computing systems (DCS). Although their taxonomy isn't specific to the publish-subscribe paradigm, its comparison of existing general-purpose DCSs provides a means of classifying systems and also serves as a basis for designing a DCS that offers novel feature combinations.
Mahambre et al present a taxonomy of pub-sub middleware with QoS features, acknowledging significant gaps in the provision of QoS features to the extent that some environments need REF .
A Taxonomy of QoS-Aware, Adaptive Event-Dissemination Middleware
IEEE Internet Computing
IEEE Internet Computing
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