Current web search engines are retrospective in that they limit users to searches against already existing pages. Prospective search engines, on the other hand, allow users to upload queries that will be applied to newly discovered pages in the future. We study and compare algorithms for efficiently matching large numbers of simple keyword queries against a stream of newly discovered pages.

Given two versions of a file, a current version located on one machine and an outdated version known only to another machine, the remote file synchronization problem is how to update the outdated version over a network with a minimal amount of communication. In particular, when the versions are very similar, the total data transmitted should be significantly smaller than the file size. File synchronization problems arise in many application scenarios such as Web site mirroring, file system backup and replication, and Web access over slow links. An open source tool for this problem, called rsync and included in many Linux distributions, is widely used in such scenarios, rsync uses a single round of messages between the two machines. While recent research has shown that significant additional savings in bandwidth consumption are possible through the use of optimized multi-round protocols, there are many scenarios where multiple rounds are undesirable. In this paper, we study single-round protocols for file synchronization that offer significant improvements over rsync. Our main contribution is a new approach to file synchronization based on the use of erasure codes. Using this approach, we design a single-round protocol that is provably efficient with respect to common measures of file distance, and another optimized practical protocol that shows promising improvements over rsync on our data sets. In addition, we show how to obtain moderate improvements by engineering the rsync approach.