Grid computing environments typically federate heterogeneous resource "clusters" belonging to several organizations. To fully realize the promise of a grid environment, it is necessary to support analysis tools that help obtain insights into the behavior of individual clusters. The Grid Research Centre at the University of Calgary (http://grid.ucalgary.ca/) is developing a cluster service called "replay" that simplifies the development andmaintenance of such tools. The service provides a single, consistent, model-based interface for obtaining current and historical information about a cluster.