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. is maximizing expressiveness in the selection mechanism without sacrificing scalability in the delivery mechanism [Carzaniga et al. 2000a]. Expressiveness refers to the ability of the event notification service to provide a powerful data model with which to capture information about events, to express filters and patterns on notifications of interest, and to use that data model as the basis for optimizing notification delivery. In terms of scalability, we are referring not simply to the number of event generators, the number of event notifications, and the number of notification recipients, but also to the need to discard many of the assumptions made for local-area networks, such as low latency, abundant bandwidth, homogeneous platforms, continuous and reliable connectivity, and centralized control. We recognize that there are other important attributes of an event notification service besides expressiveness and scalability, including security, reliability, and fault tolerance, but we do not address them in this paper.Intuitively, a simple event notification service that provides no selection mechanism can be reduced to a multicast routing and transport mechanism for which there are numerous scalable implementations. However, once the service provides a selection mechanism, then the overall efficiency of the service and its routing of no...

No 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 • 333 to optimize performance. We also present results of simulation studies that examine the scalability and performance of the service.

This paper describes the design of SIENA, an Internet-scale event notification middleware service for distributed eventbased applications deployed over wide-area networks. SIENA is responsible for selecting the notifications that are of interest to clients (as expressed in client subscriptions) and then delivering those notifications to the clients via access points. The key design challenge for SIENA is maximizing expressiveness in the selection mechanism without sacrificing scalability of the delivery mechanism. This paper focuses on those aspects of the design of SIENA that fundamentally impact scalability and expressiveness. In particular, we describe SIENA'S data model for notifications, the covering relations that formally define the semantics of the data model, the distributed architectures we have studied for SIENA'S implementation, and the processing strategies we developed to exploit the covering relations for optimizing the routing of notifications.

The Unified Modeling Language (UML) is a family of design notations that is rapidly becoming a de facto standard software design language. UML provides a variety of useful capabilities to the software designer, including multiple, interrelated design views, a semiformal semantics expressed as a UML meta model, and an associated language for expressing formal logic constraints on design elements. The primary goal of this work is an assessment of UML's expressive power for modeling software architectures in the manner in which a number of existing software architecture description languages (ADLs) model architectures. This paper presents two strategies for supporting architectural concerns within UML. One strategy involves using UML "as is," while the other incorporates useful features of existing ADLs as UML extensions. We discuss the applicability, strengths, and weaknesses of the two strategies. The strategies are applied on three ADLs that, as a whole, represent a broad cross-section of present-day ADL capabilities. One conclusion of our work is that UML currently lacks support for capturing and exploiting certain architectural concerns whose importance has been demonstrated through the research and practice of software architectures. In particular, UML lacks direct support for modeling and exploiting architectural styles, explicit software connectors, and local and global architectural constraints.