"Although distributed computer systems are highly desirable, putting together a properly functioning system is notoriously difficult. Some of the difficulties are pragmatic, for instance, the presence of heterogeneous hardware and software and the lack of adherence to standards. More fundamental difficulties are introduced by three factors: asynchrony, llimited local knowledge, and failures. The term asynchrony means that the absolute and relative times at which events take place cannot always be known precisely. Because each computing entity can only be aware of information that it acquires, it has only a local view of the global situation. Computing entities can fail independently, leaving some components operational while others are not."