Relaxed Index Consistency for a Client-Server Database

Client-Server systems cache data in client buffers to deliver good performance. Several efficient protocols have been proposed to maintain the coherence of the cached data. However, none of the protocols distinguish between index pages and data pages. We propose a new coherence protocol, called Relaxed Index Consistency, that exploits the inherent differences in the coherence and concurrency-control (C&CC) requirements for index and data pages. The key idea is to incur a small increase in computation time at the clients to gain a significant reduction in the number of messages exchanged between the clients and the servers. The protocol uses the concurrency control on data pages to maintain coherence of index pages. C&CC for the data pages is enforced using a traditional protocol such as Callback Locking. A performance-conscious implementation of the protocol that makes judicious use of version numbers is proposed. We show, through both qualitative and quantitative analysis, the performance benefits of making the distinction between index pages and data pages for the purposes of C&CC. Our simulation studies show that the Relaxed Index Consistency protocol improves system throughput by as much as 15% to 88%, based on the workload.