What is the CAP theorem and its implications?

Question

Accepted Answer

**CAP Theorem** — distributed system can guarantee only 2 of 3: Consistency, Availability, Partition Tolerance.

**Three Properties:**
- **Consistency** — all nodes see same data.
- **Availability** — every request gets response.
- **Partition Tolerance** — system works despite network failures.

```
CAP Triangle:
         Consistency
            /\
           /  \
          /    \
         /  CA  \
        /________\
  Availability   Partition
                 Tolerance
```

**Possible Combinations:**
- **CA** — No partition tolerance (rare).
- **CP** — Consistent but may reject requests.
- **AP** — Available but may return stale data.

**Examples:**
- **CP** — HBase, MongoDB (strong consistency).
- **AP** — Cassandra, DynamoDB (high availability).

**Implications:** Choose based on business requirements. Most systems choose AP (partition tolerance is essential).

Systems Analyst: What is the CAP theorem and its implications?