Regular users and enterprises are now increasingly dependent on web services. This growing dependence on one hand has simplified routine tasks, but on the other hand it has resulted in loss of direct control over the data. Nevertheless, both users and enterprises require simplified and generic solutions to access their data. The classical mediation approach from the data integration field provides a uniform query interface to diverse data sources hiding the underlying heterogeneity. But using this approach over multiple heterogeneous and autonomous web services has several open challenges. In this article, we will take a look at some of these challenges that need to be addressed for achieving a fully automated solution.