Continual queries for Internet scale event-driven information delivery

In this paper we introduce the concept of continual queries, describe the d esign of a distributed event-driven continual query system-OpenCQ, and outl ine the initial implementation of OpenCQ on top of the distributed interope rable information mediation system DIOM [22], [20]. Continual queries are s tanding queries that monitor update of interest and return results whenever the update reaches specified thresholds. In OpenCQ, users may specify to t he 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 rele vant users; otherwise, the system continually monitors the arrival of the d esired information and pushes it to the relevant users as it meets the spec ified update thresholds. In contrast to conventional pull-based data manage ment systems such as DBMSs and Web search engines, OpenCQ exhibits two impo rtant features: 1, it provides push-enabled, event-driven, content-sensitiv e information delivery capabilities, and 2. it combines pull and push servi ces 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 cont ent-sensitive, we mean the evaluation of the trigger condition happens only when a potentially interesting change occurs. And, by push-enabled, we mea n the active delivery of query results or triggering of actions without use r intervention.