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Algorithms are described for maintaining clock synchrony in a distributed multiprocess system where each process has its own clock. These algorithms work in the presence of arbitrary clock or process failures, including “two-faced clocks” that present different values to different processes. Two of the algorithms require that fewer than one-third of the processes be faulty. A third algorithm works if fewer than half the processes are faulty, but requires digital signatures.

Fault-tolerant distributed systems are becoming more important, but in existing systems, maintaining the consistency of replicated data is quite expensive. The Totem single-ring protocol supports consistent concurrent operations by placing a total order on broadcast messages. This total order is derived from the sequence number in a token that circulates around a logical ring imposed on a set of processors in a broadcast domain. The protocol handles reconfiguration of the system when processors fail and restart or when the network partitions and remerges. Extended virtual synchrony ensures that processors deliver messages and configuration changes to the application in a consistent, systemwide total order. An effective flow control mechanism enables the Totem single-ring protocol to achieve message-ordering rates significantly higher than the best prior total-ordering protocols.

Many applications can benefit from distributed systems based on multiple computers interconnected by a communication network. Distributed systems use inexpensive high-performance computers and can be configured closely to the application. Information can be replicated on several processors to improve performance and to provide fault tolerance. However, programming distributed applications is difficult, particularly when replicated information must remain consistent as it is updated in the presence of faults. Since many messages may be required, recovery from faults may introduce delays, making real-time performance objectives difficult to achieve.Ordered multicast group communication systems are a useful infrastructure on which complex distributed applications can be built.

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We present an innovative approach to the design of faultprocessors agree on exactly the same sequence of broadcast tolerant distributed systems that avoids the several rounds of message exchange required by current protocols for consensus agreement. The messages. approach is based on broadcast communication over a local area It is easy to demonstrate that placing a total order on network, such as an Ethernet or a token ring, and on two novel protocols, broadcast messages, so that every working processor procthe Tram protocol, which provides efficient reliable broadcast communi-esses the same messages in the same order, provides an cation, and the Total protocol, which with high probability promptly immediate solution to the agreement problem. Once this total places a total order on messages and achieves distributed agreement even in the presence of fail-stoo. omission. timing, and communication faults. order is determined, distributed actions can be carried out Reliable distributed operations such as locking, update and commitment, using simple sequential fault-tolerant algorithms. The strategy typically require only a single broadcast message rather than the several is very efficient: for example, locking records in a distributed tens of messages required by current algorithms. database typically requires only a single broadcast message to claim a lock and a single broadcast message to release it.