# Consistency Trade-offs: CAP, PACELC and Quorums — Scalability, Availability & Reliability

Source: https://www.skillbyai.com/en/scalability/d-consistency

> Reason about consistency and availability under partitions and in normal operation.

## You cannot have everything during a partition

The **CAP theorem** says that when a **network partition** separates nodes, a distributed data store must choose between **consistency** (every read sees the latest write, or gets an error) and **availability** (every request gets a non-error response, possibly stale). Partitions are not optional in real networks, so the choice is really C or A *during* a partition. **PACELC** extends this: if there is a Partition, choose Availability or Consistency; **E**lse, in normal operation, choose lower **L**atency or stronger **C**onsistency. Many systems let you tune per request with **quorums**: with N replicas, a write waits for W acknowledgements and a read queries R replicas; if **R + W > N**, every read overlaps at least one replica with the latest write. Choose by business need: account balances and inventory counts usually need strong consistency; like counts, feeds and recommendations tolerate eventual consistency.

## Quorum settings with three replicas

Tuning R and W trades latency and availability against consistency.

```text
N = 3 replicas

W=3, R=1   strong reads, fast reads; any one replica down blocks writes
W=2, R=2   R+W=4 > 3: reads see latest write; tolerates one replica down
W=1, R=1   fastest, most available; reads may be stale (eventual consistency)

rule of thumb: R + W > N  -> read and write sets overlap -> no stale reads
               (assuming no concurrent failures or sloppy quorums)
```

## Consistency is a product decision

Ask the business what happens if two users see different values for a few seconds. "Two people buy the last ticket" and "the like count is off by one" deserve very different designs.

**Quiz:** With N = 3 replicas, which setting guarantees reads overlap the latest write?

- [ ] W = 1, R = 1
- [ ] W = 1, R = 2
- [x] W = 2, R = 2
- [ ] W = 0, R = 3

*Answer:* W = 2, R = 2. R + W = 4 > 3, so the read set always includes a replica with the latest write.