CONQUER: a continual query system for update monitoring in the WWW
The World Wide Web (the Web) has made an enormous amount of data freely accessible over the Internet. However, finding the right information in the midst of this mountain of data has been likened to finding the proverbial needle in a haystack. Commonly used search engines (e.g. AltaVista) and directory services (e.g. Yahoo) have practical but limited success. The exponential growth of the Web is increasing the haystack rapidly. Instead of pull-based browsing, update monitoring is a promising area of research where the system brings the right information to the right user at the right time. In this paper we present the design and implementation of the CONQUER continual query system, designed for update monitoring over the Web information sources. A Continual Query (CQ) is a standing query that monitors update of interest using distributed triggers and notifies the user of changes whenever an update of interest reaches specified thresholds or some time limit is reached. In contrast to normal queries whose scope is limited to past and present data, the scope of a continual query also includes future data. The first contribution of the CONQUER system is the specification language and the inherent semantics of continual queries. The second contribution consists of the mechanisms for efficient and scalable processing of large numbers of continual queries. The third contribution is the three-tier architecture that provides active capabilities at both mediator tier and wrapper tier and interoperability among multiple web information sources.
continual queries update monitoring world wide web distributed triggers distributed event monitoring