Question

If a system is not currently experiencing a network partition, which two properties does the PACELC theorem force the system to choose between?

Accepted Answer

When a distributed system is operating normally without a network partition, the PACELC theorem forces the system to choose between Latency and Consistency. The PACELC theorem extends the CAP theorem, which states that during a network partition, a system must choose between Consistency and Availability. The ELC portion of the theorem describes system behavior when there is no partition. Consistency refers to the requirement that all nodes in a distributed system see the same data at the same time, meaning a read request will always return the most recent write. Latency refers to the time it takes for a system to process a request and return a response to the user. When a system prioritizes Consistency, it must ensure that updates are synchronized across nodes before confirming success, which increases the time a user must wait, thereby increasing Latency. When a system prioritizes Latency, it allows for faster responses by potentially serving data that is not yet perfectly synchronized across all nodes, which reduces the level of Consistency. Therefore, outside of a partition, the system must decide whether to provide a high-performance, low-latency experience by sacrificing strict data synchronization, or to guarantee strict consistency at the expense of slower response times.

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If a system is not currently experiencing a network partition, which two properties does the PACELC theorem force the system to choose between?