In active database systems, events are used in ECA rules to specify the time to check the conditions of the rules. Composite events can be constructed in an intuitive way by applying event operators, such as and, or, sequence, etc., to primitive events. Where timing is important, these event operators may introduce ambiguity if there is no formal semantics defining the occurrence of the composite events. In this paper, we propose a formalism to specify a wider range of composite events with formal semantics in the logic RTL which is especially amenable for specifying timing constraints in real-time systems. The use of RTL to define the formal semantics also allows us to exploit compilation methods which can be used to translate the enabling conditions of ECA rules into timing constraints. Thus the detection of composite events can be handled by monitoring the corresponding timing constraints, a subject which has been explored in our previous work [18, 19]. A prototype implementation of...

Real-Time, Active Database Systems (RTADBSs), have attracted considerable amount of research attention in the very recent past and a number of important applications have been identified for such systems, such as telecommunications network management, automated air traffic control, automated financial trading, process control and military command and control systems. In spite of the recognized importance of this area, very little research has been devoted to exploring the dynamics of transaction processing in RTADBSs. Concurrency Control (CC) constitutes an integral part of any transaction processing strategy and thus deserves special attention. In this paper we study CC strategies in RTADBSs and postulate a number of CC algorithms. These algorithms exploit the special needs and features of RTADBSs and are shown to deliver substantially superior performance to conventional real-time CC algorithms. Keywords: Real-Time Database Systems, Active database Systems, Concurrency Control, Perfo...

In the DeeDS prototype, active database functionality and critical timing constraints are combined with integrated monitoring techniques. In the scope of DeeDS, this paper presents a mathematical model which is used to derive two important design constraints; worst-case minimum delay and maximum frequency of events. This model is based on a dual-processor hybrid-monitoring solution. Furthermore, different interaction styles between the scheduler and the event monitor are evaluated. I. Introduction Distributed processing and sharing of large amounts of data under critical timing constraints are needed by many complex real-time systems. Real-time database systems are thus an important component in such complex systems, since they provide efficient storage and access to shared data as well as predictable transaction execution, thereby eliminating the need for ad hoc methods in these respects. Real-time systems have two fundamental characteristics. First, the time at which a result is pre...

Real-Time, active database systems (RTADBs) have attracted the attention of researchers in recent times. Such systems are envisioned as control systems for environments as diverse as process control, network management and automated financial trading. Sensors distributed throughout the system report the state of the system to the database. Unacceptable state reports typically results in corrective actions being triggered with deadlines. Thus RTADBs incorporate both real-time as well as active characteristics. In this paper we study buffer management in RTADBs. Buffer management is recognized as not being a well studied area in real-time systems. As a result of our work, we postulate PAPER, a new buffer management scheme that relies on two strategies: prefetching and priority based buffer replacement. We report the result of studies of the performance of PAPER, as compared to that of existing buffer management algorithms. The insights derived from this paper impact both real-t...

The DeeDS prototype integrates active database functionality and critical timing constraints from active and real-time databases. This is combined with unique distribution and integrated monitoring techniques. The aims are to avoid important sources of unpredictability, to guarantee schedulability, and to make scheduling and event monitoring predictable. Our solution to the first item is based on the use of bounded as-soon-as-possible replication and main memory residency. The second item is handled by overload filtering, which is based on the concepts of criticality and contingency plans. The last item, based on off-loading, is covered in-depth by introducing necessary assumptions for predictability. We present a mathematical model for refining important application design constraints, which is useful for prototyping. 1 INTRODUCTION Distributed processing and sharing of large amounts of data under critical timing constraints are needed by many complex real-time systems. Real-time da...

. The DeeDS architecture is a distributed real-time systems architecture developed for complex real-time systems by the DeeDS project. A major goal is to evaluate the effectiveness of certain approaches to ensuring predictability and efficiency in a dynamic and complex environment. DeeDS features time-cognizant reactive mechanisms complemented by multi-purpose predictable event monitoring and dynamic deadline scheduling with overload management, as well as support for delayed replication with eventual consistency. These functions are considered orthogonal and only a minimum of functionality included, to be able to control internal complexity. Predictability and efficiency are ensured by allowing all transactions to execute locally without delays due to disk accesses, network communication, or distributed commit processing. Further, a mix of criticality from hard to soft allows hard critical time constraints to be met at the expense of softer constraints, and contingency a...

Introduction There is a great need for building research prototypes of real-time database systems, because existing techniques are often ad hoc or concealed in internal solutions, and few, if any, commercial offerings exist. As real-time applications inherently use reactive mechanisms, and a wealth of results now exists on active database management systems [5], we decided to build DeeDS, a [D]istributed Activ[e], Real-Tim[e] [D]atabase [S]ystem prototype. This research effort is challenging due to the unique combination of reactive mechanisms, distributed technology, dynamic scheduling, and integrated monitoring, under a mixture of soft and hard real-time constraints. To make all this feasible with a limited amount of man-power, we have taken a deliberate approach of building with existing components where possible, and including only those features absolutely necessary to obtain the benefit of each of the architectural dimensions. At the same time, features a

In the DeeDS prototype, active database functionality and critical timing constraints are combined with integrated monitoring techniques. In the scope of DeeDS, this paper presents a mathematical model which is used to derive two important design constraints; worst-case minimum delay and maximum frequency of events. This model is based on a dual-processor hybrid-monitoring solution. Furthermore, different interaction styles between the scheduler and the event monitor are evaluated. I. Introduction Distributed processing and sharing of large amounts of data under critical timing constraints are needed by many complex real-time systems. Real-time database systems are thus an important component in such complex systems, since they provide efficient storage and access to shared data as well as predictable transaction execution, thereby eliminating the need for ad hoc methods in these respects. Real-time systems have two fundamental characteristics. First, the time at which a result is pr...