A new form of computation is emerging. Propelled by advances in software design and increasing connectivity, networks of enormous complexity known as distributed computational systems are spreading throughout offices and laboratories, across countries and continents.

Unlike standalone computers, these growing networks seldom offer centralized scheduling and resource allocation. Instead, computational processes (the active execution of programs) migrate from workstations to printers, servers, and other machines in a network as the need arises, without a priori knowledge of the state or availability of other network components at run time. This novel form of delocalized and asynchronous computation is called open because the closed-world assump tion of traditional computer science is no longer valid.

In a closed system, the computer executes its programs at the command of a central scheduler that has global knowledge of the system. But in an open system, processes make local decisions on how to access given resources...

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