illustrations by DaVe bollinger

tant property of these systems, but they were struggling with what it should be traded off against. Eric Brewer, systems professor at the University of California, Berkeley, and at that time head of Inktomi, brought the different trade-offs together in a keynote address to the Principles of Distributed Computing (PODC) conference in 2000.¹ He presented the CAP theorem, which states that of three properties of shared-data systems—data consistency, system availability, and tolerance to network partition—only two can be achieved at any given time. A more formal confirmation can be found in a 2002 paper by Seth Gilbert and Nancy Lynch. ⁴

A system that is not tolerant to network partitions can achieve data consistency and availability, and often does

so by using transaction protocols. To make this work, client and storage systems must be part of the same environment; they fail as a whole under certain scenarios and as such clients cannot observe partitions. An important observation is that in larger distributed-scale systems, network partitions are a given; therefore, consistency and availability cannot be achieved at the same time. This means there are two choices on what to drop: relaxing consistency will allow the system to remain highly available under the partitionable conditions; making consistency a priority means that under certain conditions the system will not be available.

Both options require the client developer to be aware of what the system is offering. If the system emphasizes

consistency, the developer has to deal with the fact that the system may not be available to take, for example, a write. If this write fails because of system unavailability, then the developer will have to deal with what to do with the data to be written. If the system emphasizes availability, it may always accept the write, but under certain conditions a read will not reflect the result of a recently completed write. The developer then has to decide whether the client requires access to the absolute latest update all the time. There is a range of applications that can handle slightly stale data, and they are served well under this model.

In principle the consistency property of transaction systems as defined in the ACID properties (atomicity,