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From Total Order to Database Replication



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From Total Order to Database Replication Yair Amir and Ciprian Tutu Department of Computer Science Johns Hopkins University Baltimore MD 21218 USA yairamir ciprian cnds jhu edu Technical Report CNDS 2001 6 http www cnds jhu edu November 5 2001 Abstract This paper presents in detail an efficient and provably correct algorithm for database replication over partitionable networks Our algorithm avoids the need for end to end acknowledgments for each action while supporting network partitions and merges and allowing dynamic instantiation of new replicas One round of end to end acknowledgments is required only upon a membership change event such as a network partition New actions may be introduced to the system at any point not only while in a primary component We show how performance can be further improved for applications that allow relaxation of consistency requirements We provide experimental results that demonstrate the superiority of this approach 1 Introduction Database replication is quickly becoming a critical tool for providing high availability survivability and high performance for database applications However to provide useful replication one has to solve the non trivial problem of maintaining data consistency between all the replicas The state machine approach 27 to database replication ensures that replicated databases that start consistent will remain consistent as long as they apply the same deterministic actions transactions in the same order Thus the database replication problem is reduced to the problem of constructing a global persistent consistent order of actions This is often mistakenly considered easy to achieve using the Total Order service e g ABCAST Agreed order etc provided by group communication systems Early models of group communication such as Virtual Synchrony did not support network partitions and merges The only failures tolerated by these models were process crashes without recovery Under these circumstances total order is sufficient



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