An Efficient Optimal Algorithm for Minimizing the Overall Communication Cost in Replicated Data Management

Quorum consensus methods have been widely applied to managing replicated data. In this paper, we study the problem of vote and quorum assignments for minimizing the overall communication cost of processing the typical demands of transactions. This problem is left open, even restricted to a uniform network, and we shall show that it can be solved by an efficient polynomial time algorithm. 1 I n t r o d u c t i o n The problem of managing replicated copies of data in a distributed database has received a great deal of attention [2, 3, 4, 5, 6, 9] throughout the last decade. The main issue is focussed on providing high availability of data through replicating data. Meanwhile, the replicated copies of data must be kept mutually consistent by synchronizing transactions at different sites so that a global serialization order can be ensured. A number of standard methods for replicated data concurrency control are based on the formation of quorums [1, 2]. They are called quorum consensus methods. In this paper, we follow the model where replicated data is represented by multiple copies and the transactions are simple read/write. A quorum consensus (QC) method has several common features as follows. A vote vi (integer) is assigned to each site si. Two threshold values (integers) are assigned: one is referred to as read threshold Qr (called read quorum size), and the other is referred to as write threshold Q,~ (called write quorum size) . Each read (write) transaction must assemble a read (write) quorum of sites such that the votes of all of the sites in the quorum add up tt, a value not less than Qr (Qw)Two quorum intersection invariants are assigned: Qr + Q~ > ~ i n x vi, and 2 , where n is the number of sites. If a 2-phase locking mechanism [1] is applied, these two quorum intersection invariants ensure that a read and a write cannot take place simultaneously on different copies of the same data, and neither can two writes.