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CAP theorem
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<p>In <a href="/facts/Database_theory/unyQa3cK">database theory</a>, the CAP theorem, also named Brewer's theorem after computer scientist <a href="/facts/Eric_Brewer_(scientist)/MrWDQGJv">Eric Brewer</a>, states that any <a href="/facts/Distributed_data_store/Raz2puRl">distributed data store</a> can provide only <a href="/facts/Trilemma/SQ82oo6z">two of the following three</a> guarantees: </p> <a href="/facts/Consistency_model/a8Xphrs7">Consistency</a> Every read receives the most recent write or an error. Note that consistency as defined in the CAP theorem is quite different from the consistency guaranteed in <a href="/facts/ACID/XdHkYzHN">ACID</a> <a href="/facts/Database_transaction/q1Etrsl8">database transactions</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> <a href="/facts/Availability/l8SGSyyf">Availability</a> Every request received by a non-failing node in the system must result in a response. This is the definition of availability in CAP theorem as defined by Gilbert and Lynch.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> Note that availability as defined in CAP theorem is different from <a href="/facts/High_availability/y3J8GnEo">high availability</a> in software architecture.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> <a href="/facts/Network_partitioning/SehGw7dD">Partition tolerance</a> The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes. <p>When a <a href="/facts/Network_partition/SehGw7dD">network partition</a> failure happens, it must be decided whether to do one of the following: </p> <ul><li>cancel the operation and thus decrease the availability but ensure consistency</li> <li>proceed with the operation and thus provide availability but risk inconsistency. Note this doesn't necessarily mean that system is <a href="/facts/High_availability/y3J8GnEo">highly available</a> to its users.</li></ul> <p>Thus, if there is a network partition, one has to choose between consistency or availability. </p>