Indatabase theory, theCAP theorem, also namedBrewer's theorem after computer scientistEric Brewer, states that anydistributed data store can provide onlytwo of the following three guarantees:[1][2][3]
When anetwork partition failure happens, it must be decided whether to do one of the following:
Thus, if there is a network partition, one has to choose between consistency or availability.
No distributed system is safe from network failures, thus network partitioning generally has to be tolerated.[6][7] In the presence of a partition, one is then left with two options: consistency oravailability. When choosing consistency over availability, the system will return an error or a time out if particular information cannot be guaranteed to be up to date due to network partitioning. When choosing availability over consistency, the system will always process the query and try to return the most recent available version of the information, even if it cannot guarantee it is up to date due to network partitioning.
In the absence of a network partition, both availability and consistency can be satisfied.[8]
Database systems designed with traditionalACID guarantees in mind such asRDBMS chooseconsistency over availability, whereas systems designed around theBASE philosophy, common in theNoSQL movement for example, choose availability over consistency.[9]
Some cloud services choose strong consistency but use worldwide private fiber networks and GPS clock synchronization to minimize the frequency of network partitions[citation needed]. Finally, consistent shared-nothing architectures may use techniques such as geographic sharding to maintain availability of data owned by the queried node, but without being available for arbitrary requests during a network partition[citation needed].
According to computer scientistEric Brewer of theUniversity of California, Berkeley, the theorem first appeared in autumn 1998.[9] It was published as the CAP principle in 1999[10] and presented as aconjecture by Brewer at the 2000Symposium on Principles of Distributed Computing (PODC).[11] In 2002,Seth Gilbert andNancy Lynch ofMIT published a formal proof of Brewer's conjecture, rendering it atheorem.[1]
In 2012, Brewer clarified some of his positions, including why the often-used "two out of three" concept can be somewhat misleading because system designers only need to sacrifice consistency or availability in the presence of partitions; partition management and recovery techniques exist. Brewer also noted the different definition of consistency used in the CAP theorem relative to the definition used inACID.[9][12]
A similar theorem stating the trade-off between consistency and availability in distributed systems was published by Birman and Friedman in 1996.[13] Birman and Friedman's result restricted this lower bound to non-commuting operations.
ThePACELC theorem, introduced in 2010,[8] builds on CAP by stating that even in the absence of partitioning, there is another trade-off between latency and consistency. PACELC means, if partition (P) happens, the trade-off is between availability (A) and consistency (C); Else (E), the trade-off is between latency (L) and consistency (C). Some experts like Marc Brooker argue that the CAP theorem is particularly relevant in intermittently connected environments, such as those related to theInternet of Things (IoT) andmobile applications. In these contexts, devices may become partitioned due to challenging physical conditions, such aspower outages or when entering confined spaces like elevators. Fordistributed systems, such ascloud applications, it is more appropriate to use thePACELC theorem, which is more comprehensive and considers trade-offs such aslatency andconsistency even in the absence of network partitions.[14]
In February 2012, Eric Brewer provided an updated perspective on his CAP theorem ... Brewer now describes the "2 out of 3" axiom as somewhat misleading. He notes that designers only need sacrifice consistency or availability in the presence of partitions, and that advances in partition recovery techniques have made it possible for designers to achieve high levels of both consistency and availability.
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