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Inside Cloud Spanner and the CAP Theorem

By Eric Brewer, Vice President of Infrastructure, Google Cloud Building systems that manage globally distributed data, provide data consistency and are also highly available is really hard. The beauty of the cloud is that someone else can build that for you.The CAP theorem says that a database can only have two of the three following desirable properties:C: consistency, which implies a single value for shared data
A: 100% availability, for both reads and updates
P: tolerance to network partitions
This leads to three kinds of systems: CA, CP and AP, based on what letter you leave out. Designers are not entitled to two of the three, and many systems have zero or one of the properties.For distributed systems over a “wide area,” it’s generally viewed that partitions are inevitable, although not necessarily common. If you believe that partitions are inevitable, any distributed system must be prepared to forfeit either consistency (AP) or availability


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