Providing scalable distributed Web-based eventing services has been an important research topic. It is desirable to have an effective mechanism for the servers to summarize their filters for in-network preprocessing in order to optimize system performance. In this paper, we propose a summary-based routing mechanism and introduce the notion of imprecise summaries to provide a trade-off between routing overhead and event traffic. Our system uses similarity-based filter clustering to reduce overall event traffic and performs self-tuning summary precision selection to optimize throughput. We have implemented summary-based routing on top of an XML-based infrastructure that closely follows the proposed Web services standards. Measurements from the actual implementation validate our analytical and simulation results, and demonstrate the practical benefits of the proposed techniques.