The World Wide Web is witnessing an increase in the amount of structured content --
vast heterogeneous collections of structured data are on the rise due to the Deep
Web, annotation schemes like Flickr, and sites like Google Base. While this
phenomenon is creating an opportunity for structured data management, dealing with
heterogeneity on the web-scale presents many new challenges. In this paper we
articulate challenges based on our experience with addressing them at Google, and
offer some principles for addressing them in a general fashion.