Scalable Trigger Processing and Change Noti

Over the past years, the World Wide Web (the Web) has grown into a huge global information source, at an astounding pace. While we are enjoying the rich and fresh information on the Web, the rapid and often unpredictable updates to the widely distributed information sources have made it harder to detect and capture the changes. Update monitoring is a promising area of research where the system brings the right information to the right user at the right time. Update monitoring in open systems, such as the Web, presents many new challenges. Two most critical ones are the extraction and monitoring of information ows from heterogeneous and less structured data sources, and the distributed control of thousands or millions of triggers ring at multiple sources. This paper presents the design and implementation methodology of the JCQ system, a Java-based Continual Query system for update monitoring over Web information sources. A continual query is a standing query that monitors updates of interest using distributed triggers and notiies users whenever the updates reach speciied thresholds. In this paper we focus on the strategies and techniques developed in JCQ for scalable and eecient trigger ring and the execution model for exible and robust change notiication. We evaluate our approach through a performance study of the most recent release of the JCQ system and a comparison with related work.