Tractable near-optimal policies for crawling

The problem of maintaining a local cache of n constantly changing pages arises in multiple mechanisms such as web crawlers and proxy servers. In these, the resources for polling pages for possible updates are typically limited. The goal is to devise a polling and fetching policy that maximizes the utility of served pages that are up to date. Cho and Garcia-Molina [(2003) ACM Trans Database Syst 28:390–426] formulated this as an optimization problem, which can be solved numerically for small values of n, but appears intractable in general. Here, we show that the optimal randomized policy can be found exactly in O(n log n) operations. Moreover, using the optimal probabilities to define in linear time a deterministic schedule yields a tractable policy that in experiments attains 99% of the optimum.