Federated system is a popular paradigm for multiple organizations to collaborate and share resources for common benefits. In practice, however, these organizations are often within different administrative domains and thus demand autonomous resource management, as opposed to blindly exporting their resources for efficient search. To address these challenges, we present a new resource discovery middleware, called ROADS, that can facilitate voluntary sharing. In ROADS, the participants can associate with each other at their own will and dictate the extent of sharing by properly exporting summaries, a condensed representation of their resource records. To enable efficient search, these summaries are aggregated and replicated along an overlayassisted server hierarchy, and the queries are routed to those servers that are likely to hold the desired resources. Our experimental results show that ROADS provides not only flexible resource sharing for federated systems but also efficient resource discovery, with performance comparable to a centrally managed system.