CAP and Consistency
What the CAP theorem really says, how PACELC adds latency, and the consistency levels from strong to eventual.
Why consistency is a question
Once data lives on several machines (replication), two copies can disagree for a moment. Consistency describes what a reader is allowed to see when that happens.
Stronger promises are simpler for programmers but cost speed and availability. Weaker promises are fast and resilient but push complexity into your application. Every distributed database picks a point on that line.
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