document: Monitoring the dynamic web to respond to continuous queries Continuous queries are queries for which responses given to users must be continuously updated , as the sources of interest get updated . Such queries occur , for instance , during on-line decision making , e.g. , traffic flow control , weather monitoring , etc. . The problem of keeping the responses current reduces to the problem of deciding how often to visit a source to determine if and how it has been modified , in order to update earlier responses accordingly . On the surface , this seems to be similar to the crawling problem since crawlers attempt to keep indexes up-to-date as pages change and users pose search queries . We show that this is not the case , both due to the inherent differences between the nature of the two problems as well as the performance metric . We propose , develop and evaluate a novel multi-phase ( Continuous Adaptive Monitoring ) ( CAM ) solution to the problem of maintaining the currency of query results . Some of the important phases are : The tracking phase , in which changes , to an initially identified set of relevant pages , are tracked . From the observed change characteristics of these pages , a probabilistic model of their change behavior is formulated and weights are assigned to pages to denote their importance for the current queries . During the next phase , the resource allocation phase , based on these statistics , resources , needed to continuously monitor these pages for changes , are allocated . Given these resource allocations , the scheduling phase produces an optimal achievable schedule for the monitoring tasks . An experimental evaluation of our approach compared to prior approaches for crawling dynamic web pages shows the effectiveness of CAM for monitoring dynamic changes . For example , by monitoring just 5 % of the page changes , CAM is able to return 90 % of the changed information to the users . The experiments also produce some interesting observations pertaining to the differences between the two problems of crawling -- to build an index -- and the problem of change tracking -- to respond to continuous queries .
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