Synchronization of replicated data in distributed systems

摘要

Replication is the key factor in improving the availability of data in distributed systems. Replicated data is stored redundantly at multiple sites so that it can be used by the user even when some of the copies are not available due to site failures. A major restriction in using replication is that replicated copies must behave like a single copy, i.e. mutual consistency as well as internal consistency must be preserved.Synchronization techniques based on the maintenance of multiple versions of data objects have been studied by many researchers in order to increase the degree of concurrency and to reduce the possibility of transaction rollback. Improved availability and increased degree of concurrency would result if multiversion concurrency control techniques can be used with replication control methods in distributed systems. This paper surveys synchronization methods for multiversion distributed systems with replicated data objects that have appeared in the literature. Different synchronization methods are grouped by the underlying mechanisms they use in ordering the operations, and their advantages and disadvantages are discussed. A theory that is used in analyzing the correctness of synchronization methods is reviewed, and a proving method based on the serializability theory is demonstrated using a simple synchronization scheme.