Relaxed Currency and Consistency: How to Say "Good Enough" in SQL

Despite the widespread and growing use of asynchronous copies to improve scalability, performance and availability, this practice still lacks a firm semantic foundation. Applications are written with some understanding of which queries can use data that is not entirely current and which copies are "good enough"; however, there are neither explicit requirements nor guarantees. We propose to make this knowledge available to the DBMS through explicit currency and consistency (C&C) constraints in queries and develop techniques so the DBMS can guarantee that the constraints are satisfied. In this paper we describe our model for expressing C&C constraints, define their semantics, and propose SQL syntax. We explain how C&C constraints are enforced in MTCache, our prototype mid-tier database cache, including how constraints and replica update policies are elegantly integrated into the cost-based query optimizer. Consistency constraints are enforced at compile time while ...

• Type: Conference
• Year: 2004
• Where: SIGMOD
• Authors: Hongfei Guo, Jonathan Goldstein, Per-Åke Larson, Raghu Ramakrishnan
• Keywords: Cache Dbms, Consistency Constraints, Currency Constraints, Database, SIGMOD 2004