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In this paper, we present a language framework for describing dependable systems. Our framework emphasizes modularity and composition. The dependability and functionality aspects of an application may be described independently, providing separation of design concerns. Furthermore, the dependability protocols of an application may be constructed bottom-up as simple protocols that are composed into more complex protocols. Composition makes it easier to reason about the behavior of complex protocols and supports the construction of generic reusable dependability schemes. A significant aspect of our language framework is that dependability protocols may be loaded into a running application and installed dynamically. Dynamic installation makes it possible to impose additional dependability protocols on a server as clients with new dependability demands are integrated into a system. Similarly, if a given dependability protocol is only necessary during some particular phase of execution it may be installed during that period only.

The publish/subscribe (or pub/sub) paradigm is an increasingly popular model for interconnecting applications in a distributed environment. Many existing pub/sub systems are based on pre-defined subjects, and hence are able to exploit multicast technologies to provide scalability and availability. An emerging alternative to subject-based systems, known as content-based systems, allow information consumers to request events based on the content of published events. This model is considerably more flexible than subject-based pub/sub. However, it was previously not known how to efficiently multicast published events to interested content-based subscribers within a large and geographically distributed network of broker (or router) machines. We develop and evaluate a novel and efficient distributed algorithm for this purpose, called -link matching". Link matching performs just enough computation at each node to determine the subset of links to which an event should be forwarded. We show via simulations that: link matching yields higher throughput than flooding when subscriptions are selective; and the overall CPU utilization of link matching is comparable to that of centralized matching.

Event distribution middleware supports the integration of distributed applications by accepting events from information producers and disseminating applicable events to interested consumers. We present a flexible new model, the Information Flow Graph (IFG), for specifying the flow of information in such a system. We illustrate the use of the IFG for: content-based publish/subscribe; stateless event transformations that consolidate events from diverse sources; and stateful event interpretation functions for deriving trends, summaries, and alarms from published events and for defining equivalent event sequences. We introduce two techniques for efficient implementation of such systems: a flow graph rewriting optimization which allows stateless IFGs to be converted to a form which can exploit efficient multicast technology developed for content-based publish/subscribe system; and an algorithm for converting a sequence of events to the shortest equivalent sequence of events with respect to an event interpretation function.

To optimize performance in a fault-tolerant distributed system, it is often necessary to enforce different failure semantics for different components. By choosing a custom set of failure semantics for each component and then by enforcing the semantics with a minimal set of protocols for a particular architecture, performance may be maximized while ensuring the desired system behavior. We have developed DIL, a language for specifying, on a per-component basis, protocols that transparently enforce failure semantics. These protocols may be reused with arbitrary components, allowing the development of a library of protocols. >

We present a language framework for describing dependable systems which emphasizes modularity and composition. Dependability and functionality aspects of an application may be described separately providing a separation of design concerns. Futhermore, the dependability protocols of an application may be constructed bottom-up as simple protocols that are composed into more complex protocols. Composition makes it easier to reason about dependability and supports the construction of general reusable dependability schemes. A significant aspect of our language framework is that dependability protocols may be loaded into a running application and installed dynamically. Dynamic installation makes it possible to impose additional dependability protocols on a server as clients with new dependability demands are integrated into a system. Similarly, if a given dependability protocol is only necessary during some critical phase of execution, it may be installed during that period only.