The Three Properties
CAP stands for:
- Consistency — every read returns the most recent write (or an error)
- Availability — every request receives a response (never an error), even if it's stale
- Partition Tolerance — the system continues operating even if network partitions split nodes
Brewer's theorem: you can guarantee at most two of the three simultaneously.
Why Partition Tolerance Is Not Optional
In any distributed system, network partitions will happen. Nodes go offline, switches fail, packets get dropped. You cannot opt out of P.
This means the real choice is: CP or AP when a partition occurs.
CP Systems
Choose consistency over availability. During a partition, refuse requests that might return stale data.
Examples: HBase, Zookeeper, etcd, MongoDB (with majority write concern)
Use when: financial transactions, inventory counts, any place where returning wrong data is worse than returning an error.
AP Systems
Choose availability over consistency. During a partition, serve potentially stale data rather than returning errors.
Examples: Cassandra, DynamoDB, CouchDB, DNS
Use when: social media feeds, product catalogs, caches — places where a slightly stale response is fine.
PACELC — The Full Picture
CAP only describes behaviour during a partition. PACELC extends it to the normal case:
If there is a Partition, choose between Availability and Consistency. Else (when running normally), choose between Latency and Consistency.
Most "CP" systems still trade consistency for latency in the normal path (e.g., reading from a replica).
Practical Takeaways
| Scenario | Choose |
|---|---|
| Bank balance, order stock | CP — never show wrong number |
| User profile, social feed | AP — stale is fine |
| Config values, leader election | CP — correctness critical |
| Shopping cart, recommendations | AP — availability wins |
| Real-time chat delivery | Depends on requirements |
Common Misconception
CAP doesn't say you pick two and ignore the third permanently. It says during a network partition you must sacrifice one. Healthy systems can aim for all three — the tradeoff only activates under failure.