OBJECT-ORIENTED ALERTER SYSTEM DESIGN.

摘要

Database systems are usually 'passive.' Database alerting techniques provide a database system with the capability to take actions by itself. Hence, the database system with alerting techniques can play a more intelligent role. The primary objective of this research effort is the development of a distributed intelligent database system. This design problem encompasses the issues of the development of methodology to decompose the complex alerters into simple alerters, the investigation of algorithm for allocating the simple alerters and design of protocol for ensuring that alerters correctly monitor database. The proposed system is useful in office information systems, decision-support systems and user-friendly specification of 'chip' expert systems.; We approach the problem of monitoring database updates by using the object-oriented approach. A methodology is proposed to decompose a complex alerter into several objects. These objects form a tree-structure. Each object can be considered as monitoring a virtual database view. If the database view is updated, the updated message is sent up the tree for further monitoring. The top object can check whether alert condition is met and then invoke the alert action.; An alerter can usually be expressed in terms of an associated query on database view. The conventional approach for implementing the complex alerters is that the database view is constructed whenever related elementary relations are updated. The response time of view-construction approach is longer than that of object-oriented approach. However, object-oriented approach needs much more space to store customized alerters. Another approach we proposed is the combination of both approaches. The performance of three approaches is compared. The combination approach can achieve better time-space tradeoff.; If the object-oriented approach is applied in the centralized computer systems, the complex alerter can be correctly invoked. But in the distributed systems, the actions which should be invoked may be missing or the extraneous actions are invoked. Some concurrency-control methods are proposed to make sure that the complex alerters monitor the database updates correctly in the distributed systems.; Some allocation algorithms are also proposed to allocate the alerting objects so that the response time is minimal under object-number constraints.