In a distributed database system, data replicas are placed at different locations to achieve high data availability in the presence of link failures. With a majority voting protocol, a location survives for read/write operations if and only if it is accessible to more than half of the replicas. The problem is to find out the optimal placements for a given number of data replicas in a ring network. When the number of replicas is odd, it was conjectured by Hu et al. [X.-D. Hu, X.-H. Jia, D.-Z. Du, D.-Y. Li, H.-J. Huang, Placement of data replicas for optimal data availability in ring networks, J. Parallel Distrib. Comput., 61 (2001) 1412-1424] that every uniform placement is optimal, which is proved by Shekhar and Wu later. However, when the number of replicas is even, it was pointed out by Hu et al. that uniform placements are not optimal and the optimal placement problem may be very complicated. In this paper, we study the optimal placement problem in a ring network with majority voting protocol and an even number of replicas, and give a complete characterization of optimal placements when the number of replicas is not too large compared with the number of locations.
Bibliographical noteFunding Information:
The first author’s research is supported by NSFC (60603003), the Key Project of Chinese Ministry of Education (208161), and Program for New Century Excellent Talents in University. The work was completed when the first author was visiting Department of Computing Science, the University of Texas at Dallas. The second author’s work is supported in part by the National Science Foundation under grant CCF-0514796 and CNS-0524429.
- Data replica
- Distributed database
- Majority voting protocol
- Optimal placement
- Ring network