分布式数据库中一致性与可用性的关系

随着各类应用在数据量和业务量上的扩展，单机数据库系统越发难以应对现实需求。分布式数据库能够根据业务的需求动态地扩容，因此逐步开始受到应用的青睐。近年来，分布式数据库产品层出不穷，并在互联网应用中被大量投入使用。然而，分布式数据库的系统复杂度前所未有。为了让系统可用，设计者需要在多种属性中作合理选择和折中。这造成现有的数据库产品形态各异、优缺点对比分明。至今为止，尚未有人对分布式数据库的设计空间和折中方案进行过深入分析和整理。本文作者在对多个分布式数据库产品进行深入理解之后认识到：分布式数据库系统的设计方案可以通过三个属性进行基本刻画–操作一致性、事务一致性和系统可用性。虽然这三个属性并不新颖，但它们在数据库语境下的含义在文献中尚未得到充分澄清。本文对这三个属性进行澄清，并通过它们对典型数据库产品的格局进行概括、对现有的分布式数据库技术进行综述。此外，本文还对这三个属性之间的相互关系进行深入分析，以期帮助未来的开发者在分布式数据库的设计过程中作出合理选择。

Consistency and Availability in Distributed Database Systems

The rapid growth of data and workload makes centralized database systems less and less favorable to today''s applications. A distributed database system can scale out dynamically to satisfy the business development. As a result, it has gained much more attention from applications. Since the needs for distributed DB became apparent, an increasing number of products have emerged and been adopted by the Web. However, due to the complexity of distributed DB systems, their designers have to trade off among several desired properties, resulting dramatical difference in their designs and advantages.To the best of our knowledge, no one has performed a comprehensive analysis on the design space and the tradeoff choices of modern distributed DB systems. After reviewing and understanding a significant number of real world DB products, we believe that a distributed DB system can be generally described using three dimensions-operational consistency, transactional consistency and availability. While these dimensions are not new, their concepts are somehow blurred in the literature. In this paper, we clarify the three concepts in the context of database, based on which we manage to draw a sensible landscape of the existing products and technologies. We alsoprovide an analysis of the relationship among the three dimensions, which we wish can help developers make right choice when designing new distributed DB systems.