As many users create, upload, and share multimedia data on the Web, information explosion has created a large crowd of multimedia data which are dynamic and ever-changing. Information monitoring services appear very beneficial to timely utilize those data because they respond automatically to change or arrival of data. In this paper, we present an efficient and scalable active database system that provides a cross-database monitoring/searching mechanism over multimedia data on the Web. A unique feature of this system is that it offers a global rule management mechanism that enables users to share and execute their ECA rules in collaborative and evolutional manners, by building a rule-sharing network. We evaluate our approach using both measurements on real systems and simulations.