System Design - 6.CAP Theorem & PACELC, CAP Theorem & PACELC: The Most Important Trade-off in Distributed Systems

The Theorem That Changed How We Think About Databases

In 2000, Eric Brewer stood at a conference and proposed a conjecture that would reshape distributed systems forever:

"You can only guarantee two of these three properties at the same time: Consistency, Availability, and Partition Tolerance."

Two years later, Seth Gilbert and Nancy Lynch proved it mathematically. It became known as the CAP Theorem — and every distributed system architect since has had to wrestle with it.

It sounds abstract. But once you understand it, you'll never look at a database choice the same way again. You'll understand why Amazon DynamoDB and Google Spanner make opposite architectural choices. You'll know why your bank uses PostgreSQL while Twitter uses Cassandra.