Three Distributed Data Replication Models
Virtual Synchrony is a popular computing model, closely related to the transactional one-copy serializability model (used mostly in replicated database systems) and the state machine (consensus) model, sometimes known as "Paxos", the name given to the most widely cited state-machine implementation.
- Among these, transactional replication is probably the most widely known model -- most database textbooks discuss it. Yet overheads are very high when using true one-copy serializability, hence the approach to replication has never been a commercial success. Turing Award winner Jim Gray offers some thoughts on this issue in a paper he wrote about "The Dangers of Replication and a Solution". Indeed, few database products support true replication of the sort Gray warns about. Instead, they more often support a form of log-based fault-tolerance that performs well, but can leave inconsistencies (updates can be lost) if a failure occurs just as the log is being transmitted.
- Virtual synchrony has been adopted, and even standardized as part of the CORBA reference model. There are many real-world systems that use this model, and achieve extremely high performance. On the other hand, virtual synchrony is a technology that can be hard to use correctly -- programmers need some training, and without it, may make mistakes. For this reason, virtual synchrony is not often supported in a form that end-users or programmers would encounter directly.
- The state machine / Paxos is used in commercial products that power large scalable systems, such as Chubby, a locking service used by Google applications.
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