Distributed computer systems are now widely available but, despite a number of recent advances, the design of software for these systems remains a challenging task, involving two main difficulties the absence of a shared clock and the absence of a shared memory. The absence of a shared clock means that the concept of time is not useful in distributed systems. The absence of shared memory implies that the concept of a state of a distributed system also needs to be redefined. These two important concepts occupy a major portion of this book. br emPrinciples of Distributed Systemsem describes tools and techniques that have been successfully applied to tackle the problem of global time and state in distributed systems. The author demonstrates that the concept of time can be replaced by that of causality, and clocks can be constructed to provide causality information. The problem of not having a global state is alleviated by developing efficient algorithms for detecting properties and comput