Overview of CAM


Overview of CAM

Consider a user who is worried about a hurricane in progress and wants to keep abreast of the hurricane-related updates. To achieve this, he poses a continuous m-keyword query q = {w1, w2,...wm}. In this section, we present an overview of the major ingredients of the CAM approach.

Identifying pages relevant to a set of queries

Each query is submitted to a Web search engine which returns a set of pages relevant to the queries. The returned URLs determine the domain of pages which we will monitor. Continuing our earlier example, we may find, say, that the National Hurricane Center, National Weather Organization, and other tropical cyclone sites as well as news sites are relevant. The relevance of a page to a query can be measured by standard IR techniques based on the vector-space model (see Appendix B for details).

Tracking and characterizing relevant page changes

By visiting each relevant page (identified via a search engine) at frequent intervals during a tracking period, changes to these pages are tracked, update statistics collected, and the relevance of the changes, vis-a-vis the queries, is assessed. This data is used to build a statistical model of the changes to the pages relevant to a set of queries. These statistics include page update instances, page change frequency, and relevance of the changes to the pages for current queries.

Let Q denote the set of all queries submitted in the system and $ \omega_{i}^{}$ denote the importance of ith query. These are input to the system. Let P denote the set of web pages relevant to the continuous queries, Qpi be the set of queries for which ith page is found to be relevant, and ri, j be the estimated relevance of ith page for jth query. It is positive for all queries q $ \in$ Qpi and zero for all q $ \in$ Q - Qpi. These relevance measures are initially calculated during the tracking period (and get updated continually, as explained later).

It is clear that not all pages will be equally important for each query in the system. So we rank the pages by assigning a weight to each page using its relevance for queries. The weight of a page, computed as $ \sum_{j \in Q}^{}$($ \omega_{j}^{}$ri, j), denotes the value of current version of the page. If the page gets updated before its current version is monitored, we assume that we incur a loss of Wi.

Change monitoring considerations

CAM does its monitoring in epochs. Each epoch has duration T. The purpose of resource allocation is to decide how to allocate a limited number of monitoring tasks for an epoch and the goal of scheduling is to decide when each allocated task should execute, given the resource allocation decisions. Monitoring is done by a monitoring task which fetches a specified page from its source, determines if it has changed and, if so, propagates the effects of these changes to responses of affected queries.

Let C denote the total number of monitoring tasks that can be employed in a single monitoring epoch. C is derived as an aggregation of the resources needed for monitoring, including CPU cycles, communication bandwidth, and memory. E.g., Heydon and Najork [9] report that with two 533MHz Alpha processors, 2GB of RAM, 118GB of local disk, a 100Mbit/sec FDDI connection to the Internet, their crawler (Mercator) downloaded 112 documents/sec and 1,682 KB/sec (on an average).

$ \lambda_{i}^{}$ is the estimated number of changes that occur in page i in T time units. Henceforth we will call it the change frequency for page i. Suppose Ui denotes the sequence of time instances ui, 1, ui, 2,..., ui, pi at which the tracking phase determines that possible updates occur to page i. We assume 0 $ \leq$ ui, 1 $ \leq$ ui, 2 ... $ \leq$ ui, pi $ \leq$ T, ui, 0 = 0, and ui, pi = T. pi is the total number of update instances for ith page during T, i.e., cardinality of sequence Ui (pi=| Ui|).

Note that a page may not be updated at these time instances and so there is a probability $ \rho_{i,j}^{}$ associated with each time instance ui, j that denotes the chances of ith page being updated at the jth instance. The overall goal of the resource allocation and scheduling phases is to monitor in such a way that the monitoring events occur just after updates are expected to take place. The number of missed updates is an indication of the amount of lost information and minimizing this is the goal of the system.

With these considerations in mind, decisions are made about the allocation of a given number of monitoring tasks among a set of relevant pages while also deciding the best instant when these allocated tasks should occur within an epoch. The basic idea is that these monitoring epochs of length T repeat every T units of time and we will make decisions pertaining to the monitoring tasks to be carried out in one monitoring epoch using both new data and the results from the previous epochs.

Resource Allocation Phase

It should be clear that if we decide to monitor at some instant, then it should be at the potential update time instant only because there is no reason to delay it beyond when a update might occur. If the number of monitoring tasks allocated for a page is equal to the number of update instants, then we can always maintain a fresh version of this page by monitoring at all possible update instants. But in practice, we will not be able to perform as many monitoring tasks as the number of update instants. So we need to pick a set of update instants at which the page is to be monitored and not at others. Hence with every update time instant, we associate a variable yi, j where yi, j = 1 if the ith page is monitored at time ui, j, and 0 otherwise. If we monitor the ith page xi times, then $ \sum_{j=0}^{p_i}$yi, j = xi. The resource allocation phase decides yi.j values, that is, the time instants at which monitoring should be done given that the tracking phase has identified the time instants when changes may occur.

Scheduling the monitoring tasks

In the scheduling phase, we take the yi, j values as inputs and prepare a feasible schedule to meet our optimization measures. In practice, we have a set of M parallel monitoring processes which continuously perform these monitoring tasks. Now our goal is to map a monitoring task to one of these M parallel monitoring processes and determine its time of invocation. While determining any schedule, our aim is to minimize the total delay occurring between the ideal time instants and the actual scheduled time instants. The scheduling step involves taking the ideal timings for the monitoring of each page and obtaining an optimal achievable schedule out of it. We map this problem to flow-shop scheduling problem [15] with the goal of minimizing the average completion time. Next we monitor these pages according to the designed schedule and, at the end of the current monitoring epoch, update the statistics of these pages on the basis of the observations made during the current epoch. In general, based on the results of monitoring tasks of an epoch, scheduling, resource allocations, change statistic computations, and page relevance can all be revisited. These, as mentioned earlier, correspond to the arcs going from the monitoring phase to the earlier phases of Figure 
1.

Sandeep Pandey 2003-03-05