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<p><b>Abstract</b>—In this paper we introduce the concept of continual queries, describe the design of a distributed event-driven continual query system—OpenCQ, and outline the initial implementation of OpenCQ on top of the distributed interoperable information mediation system DIOM [<ref type="bib" rid="bibK061022">22</ref>], [<ref type="bib" rid="bibK061020">20</ref>]. Continual queries are standing queries that monitor update of interest and return results whenever the update reaches specified thresholds. In OpenCQ, users may specify to the system the information they would like to monitor (such as the events or the update thresholds they are interested in). Whenever the information of interest becomes available, the system immediately delivers it to the relevant users; otherwise, the system continually monitors the arrival of the desired information and pushes it to the relevant users as it meets the specified update thresholds. In contrast to conventional pull-based data management systems such as DBMSs and Web search engines, OpenCQ exhibits two important features: 1. it provides push-enabled, event-driven, content-sensitive information delivery capabilities, and 2. it combines pull and push services in a unified framework. By event-driven we mean that the update events of interest to be monitored are specified by users or applications. By content-sensitive, we mean the evaluation of the trigger condition happens only when a potentially interesting change occurs. And, by push-enabled, we mean the active delivery of query results or triggering of actions without user intervention.</p>
Distributed information systems, Internet scale information delivery, World Wide Web technology, distributed triggers, event monitoring.

W. Tang, L. Liu and C. Pu, "Continual Queries for Internet Scale Event-Driven Information Delivery," in IEEE Transactions on Knowledge & Data Engineering, vol. 11, no. , pp. 610-628, 1999.
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