A distributed system is a network of independent computers that communicate by message passing. This structure for distributed systems leads to three important features: concurrency of processes on the nodes of a distributed system, lack of global time, and independence of component failures from one another. Many examples can be cited for distributed systems, of which the best-known is certainly the Internet. The Internet grew out of the old ARPAnet, a network that was created by the Advanced Research Projects Agency (ARPA--now known as the Defense Advanced Research Projects Agency, DARPA). The military was concerned at the time (late 1960s) with ways to prevent and win nuclear wars, and thus decided to try to create a network that would have the characteristics of being resistant to the failures of individual nodes, and of being uniform in structure (having no specialized nodes). The distributed system called ARPAnet...