High-performance computing increasingly occurs on “computational grids” composed of heterogeneous and geographically distributed systems of computers, networks, and storage devices that collectively act as a single “virtual” computer. One of the great challenges for this environment is to provide efficient access to data that is distributed across remote data servers in a grid. In this paper, we describe our solution, a framework we call Armada. Armada allows applications to flexibly compose modules to access their data, and to place those modules at appropriate hosts within the grid to reduce network traffic.