笔记 MIT6.824 Lecture 17: COPS, Causal Consistency

前言

主要讲了有一定consistency保证的情况下有很不错性能的DB方案Causal Consistency, 其中notion of dependency一定程度上解决了写在不同servers上异步的特点

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一、geo-replication

1.1 Spanner

– 通过Paxos保证强consistency
– Two phase commit
– no one site can --write on its own
– but has read transactions, consistent, fairly fast

1.2 Memchche

– 写操作只能通过primary site完成
– read非常快

1.3 新的需求

– 写操作可以再任何server上进行
– 先满足performance再满足consistency

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二、预备方案One

每个server都可以有写操作，然后再同步给其它server
这个属于"eventually consistent" design

特点

1. clients may see updates in different orders
2. if no writes for long enough, all clients see same data
   a pretty loose spec, many ways to implement, easy to get good performance
   used in deployed systems, e.g. Dynamo and Cassandra