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  • 单元化与分布式架构的切分问题


    • 由于每个单元数据规模可控,相关维度内的所有资料可放在一个数据库中(如上例中的用户资料),不需要复杂的sharding分库分表逻辑,存储及缓存访问得到极大的简化。同时开发也变得简单,工程师不需要有丰富的“大规模大并发系统”开发经验。
    • 同时由于计算离存储更近,也可以让数据离用户更近,比如用户数据存储在地理上靠近用户的位置,数据有了更好的局部性(locality),因此也会获得更好的访问性能。部署上相关单元的前端、缓存、数据库、数据挖掘等节点可在同一个机柜,架构上让大数据的访问变得低廉,也在部分程度上让大数据更为快速及敏捷。
    • 可以自然支持不同用户分片支持不同的功能特性,天然的A/B testing试验场。

    分布式是将一个系统的数据分布到多个单元,以便使系统能够scale out,具有更好的可扩展性。当今大型网站基本上是分布式设计的。分布式系统除了机房内的,考虑到系统扩展性、用户访问的便捷性、机房规模的物理限制、异地容灾(比如2013年4月及8月的微信机房故障)等需要,大型系统也会考虑地理分布在多个机房。



    1. 局部性,单元内的访问,大部分的数据可以在单元内命中。
    2. 封装性,单元内的应用程序使用统一的方法访问数据,不需要关注数据的具体位置。

    那么这个数据总线应该如何设计?放在哪个层级比较合适?比如service layer, cache or storage(e.g. Google Spanner)?


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