What is the CAP theorem and PACELC theorem?

Question

Accepted Answer

**CAP** — Consistency, Availability, Partition tolerance. **PACELC** — extends CAP with latency.

**CAP Theorem:**
- Can only guarantee 2 of 3.
- Partition tolerance is essential.
- Choose between C and A.

```
CAP:
If Partition: choose Availability or Consistency
  AP: Cassandra (available, eventual consistency)
  CP: MongoDB (consistent, may reject)
```

**PACELC Theorem:**
- Extension of CAP.
- Even without partition, choose latency or consistency.
- More complete model.

```
PACELC:
If Partition: choose A or C
Else: choose Latency or Consistency
  EL: DynamoDB (low latency)
  EC: Spanner (strong consistency)
```

**Key differences:**
- **CAP** — partition scenario only.
- **PACELC** — partition + normal scenario.
- **PACELC** — more complete model.

Systems Analyst: What is the CAP theorem and PACELC theorem?