Abstract: Modern search engines are expected to make documents searchable shortly after they appear on the ever changing Web. To satisfy this requirement, the Web is frequently crawled. Due to the sheer size of their indexes, search engines distribute the crawled documents among thousands of servers in a scheme called local index-partitioning, such that each server indexes only several million pages. To ensure documents from the same host (e.g., www.nytimes.com) are distributed uniformly over the servers, for load balancing purposes, random routing of documents to servers is common. To expedite the time documents become searchable after being crawled, documents may be simply appended to the existing index partitions. However, indexing by merely appending documents, results in larger index sizes since document reordering for index compactness is no longer performed. This, in turn, degrades search query processing performance which depends heavily on index sizes. A possible way to balance quick document indexing with efficient query processing, is to deploy online document routing strategies that are designed to reduce index sizes. This work considers the effects of several online document routing strategies on the aggregated partitioned index size. We show that there exists a tradeoff between the compression of a partitioned index and the distribution of documents from the same host across the index partitions (i.e., host distribution). We suggest and evaluate several online routing strategies with regard to their compression, host distribution, and complexity. In particular, we present a term based routing algorithm which is shown analytically to provide better compression results than the industry standard random routing scheme. In addition, our algorithm demonstrates comparable compression performance and host distribution while having much better running time complexity than other document routing heuristics. Our findings are validated by experimental evaluation performed on a large benchmark collection of Web pages.