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This entry has been published in the Encyclopedia of Database Systems by Springer. The Encyclopedia, under the editorial guidance of Ling Liu and M. Tamer Özsu, is a multiple volume, comprehensive, and authoritative reference on databases, data management, and database systems. Since it is available in both print and online formats, researchers, students, and practitioners benefit from advanced search functionality and convenient interlinking possibilities with related online content. The Encyclopedia’s online version is accessible on the platform SpringerLink. Visit http://www.springer.com/computer/database+management+%26+information+retrieval/book/978-0-387-49616-0 for more information about the Encyclopedia of Database Systems.