Timely discovery of services in ultra-large scale (ULS) systems plays a vital role in critical areas, such as national power grids, homeland security, and health care. This paper develops a taxonomy for classifying discovery services and presents an overview of existing discovery service technologies. It then classifies these discovery services using the taxonomy and performs a gap analysis for discovery services with respect to emerging ULS systems. Our results show that while discovery

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... s are fairly mature and broadly applicable to today's systems much R&D remains to support emerging systems of ultra-large scale effectively.