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Abstract. The bandwidth limitations of multimedia systems force trade-offs between presentation-data fidelity and real-time performance. For example, digital video is commonly encoded with lossy compression to reduce bandwidth, and frames may be skipped during playback to maintain synchronization. These trade-offs depend on device performance and physical data representations that are hidden by a database system. If a multimedia database is to support digital video and other continuous media data types, we argue that the database should provide a quality-of-service (QOS) interface to allow application control of presentation timing and information-loss trade-offs. This paper proposes a data model for continuous media that preserves device and physical data independence. We show how to define formal QOS constraints from a specification of ideal presentation outputs. Our definition enables meaningful requests for endto-end service guarantees, while leaving the database system free to optimize resource management. We propose one set of QOS parameters that constitute a complete model for presentation error, and we show how this error model extends the opportunities for resource optimization.

Abstract. Recent works on self-adaptivity use a middleware-based approach where the adaptation mechanisms and meta-level information are separated and externalized from the application code. Current solutions generally target individual life-cycle phases of an application in isolation, preventing easy integration of design-time and run-time adaptability. Integration is needed in order to support the introduction of new adaptive behavior during run-time. Self-adapting systems therefore need to support both planning, instantiation and maintenance of applications throughout their life-time.In this paper we propose middleware managed adaptation, in which services are specified by their behavior, and planned, instantiated and maintained by middleware services in such a way that the behavioral requirements are satisfied throughout the service life-time. Central to this approach is mirror-based reflection, which supports introspection and intercession on an application, or any service, through all the phases of its life-cycle, including pre-runtime. The mirror of a service may contain information about its implementation, including the developer's knowledge about how this implementation will perform in different contexts. By making this knowledge available to the middleware, we facilitate the implementation of a wide range of self-adaptive behaviors.

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