The CAP theorem states that a distributed system cannot guarantee consistency, availability, and network partition tolerance at the same time. Consistency means every request reads the newest write; availability means the system continues serving requests; partition tolerance means it handles disconnected nodes. section 1.2.1
Because distributed systems must tolerate partitions, they choose between consistency and availability during a network split. A consistent system may stop serving requests until replicas receive the required data, while an available system may serve temporarily inconsistent results. section 1.2.1 section 1.4
Consistency levels make the trade-off operational:
- QUORUM requires a majority of replicas, balancing correctness and availability. section 1.4
- ONE requires a single replica, preserving service through more failures but allowing weaker consistency. section 1.2.1
ScyllaDB is typically classified as AP: during a partition, it prioritizes availability and may converge through eventual consistency. MongoDB is described as CP because writes wait for a primary during a partition. section 1.2.1 section 1.2.5



