Eric Brewer about CAP Twelve Years Later: How the “Rules” Have Changed:

The CAP theorem asserts that any net­worked shared-data system can have only two of three desirable properties. How­ever, by explicitly handling partitions, designers can optimize consistency and availability, thereby achieving some trade-off of all three. In the decade since its introduction, designers and researchers have used (and sometimes abused) the CAP theorem as a reason to explore a wide variety of novel distributed systems. The NoSQL movement also has applied it as an argument against traditional databases. [...]

The “2 of 3″ formulation was always misleading because it tended to oversimplify the tensions among properties. Now such nuances matter. CAP prohibits only a tiny part of the design space: perfect availability and consistency in the presence of partitions, which are rare. Although designers still need to choose between consistency and availability when partitions are present, there is an incredible range of flexibility for handling partitions and recovering from them. The modern CAP goal should be to maximize combinations of consistency and availability that make sense for the specific application. Such an approach incorporates plans for operation during a partition and for recovery afterward, thus helping designers think about CAP beyond its historically perceived limitations.

And Todd Hoff recently wrote about a later presentation Brewer gave, and that motivated me to finally blog about above article… Myth: Eric Brewer on Why Banks are BASE Not ACID – Availability Is Revenue:

In NoSQL: Past, Present, FutureEric Brewer has a particularly fine section on explaining the often hard to understand ideas of BASE (Basically Available, Soft State, Eventually Consistent), ACID (Atomicity, Consistency, Isolation, Durability), CAP (Consistency Availability, Partition Tolerance), in terms of a pernicious long standing myth about the sanctity of consistency in banking.

Some good examples about banking and ACID requirements… or the lack thereof, and how that risk is contained.

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