非关系型数据库近年来又热门起来了，虽然它没有关系型数据库那么多的功能，但是也很适合逻辑简单的业务，而且更容易进行规模上的扩张，构建分布式系统更方便，一般使用BASE (basically available, soft state, eventually consistent)。事务等功能放到业务逻辑层处理。 分布式系统中的CAP原理 Consistency. The client perceives that a set of operations has occurred all at once. Availability. Every operation must terminate in an intended response. Partition tolerance. Operations will complete, even if individual components are unavailable. Twitter计划迁移数据库到Cassandra，Digg已经在转换数据到Cassandra
