The PACELC theorem is an extension of the CAP theorem that is used to analyze the behavior of distributed computer systems. The PACELC theorem can help developers and architects choose the appropriate consistency model for a given system by considering the trade-offs between consistency, availability, and partition tolerance.
PACELC stands for the following six consistency models:
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Partition tolerance (P): This means that the system continues to function even when network partitions occur, i.e., when a network communication failure causes a subset of nodes to be unable to communicate with the rest of the network.
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Availability (A): This means that every request made to the system receives a response, either success or failure, without any guarantee of consistency.
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Consistency (C): This means that every read from the system returns the most recent write or an error, and every write to the system will be visible to every subsequent read.
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Eventual consistency (E): This means that after all updates to the system have ceased, all nodes will eventually return the same data.
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Latency (L): This refers to the amount of time it takes to complete a request.
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Cost (C): This refers to the monetary cost of executing an operation.
PACELC acknowledges that partition tolerance is essential in any distributed system. PACELC states that a system can provide high availability and strong consistency when there is no network partition, but when a network partition occurs, the system must choose between availability and consistency.