---
title: "Consistency Models and the CAP Trade-off"
description: "A replicated store can behave as though there were one copy, or it can keep serving both sides of a network split and reconcile later. It cannot do both during a partition, and the choice belongs to t"
canonical: https://lightmysky.com/learn/computing/consistency-models-and-the-cap-trade-off-mt_j6qtpYPHsr
source: https://lightmysky.com/learn/computing/consistency-models-and-the-cap-trade-off-mt_j6qtpYPHsr.md
retrieved: 2026-09-12
---

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# Consistency Models and the CAP Trade-off

A replicated store can behave as though there were one copy, or it can keep serving both sides of a network split and reconcile later. It cannot do both during a partition, and the choice belongs to the application rather than to the database.

Subject: Computing · Area: Networks & Security · Ages 20 to 22
Page: https://lightmysky.com/learn/computing/consistency-models-and-the-cap-trade-off-mt_j6qtpYPHsr

## Ready when they can

- Say what a client can observe under strong consistency and under eventual consistency
- Work out what each side of a partitioned system may do under each choice
- Choose a consistency model for a stated application and defend the choice

## Lesson: One copy or always open

You already know majorities agree on one value and isolation levels permit chosen anomalies. Consistency is the same idea for replicas. Strong consistency behaves as though there were one copy: every reader sees the latest write. Eventual consistency lets copies disagree for a while and reconciles them later.

**Example.** A network split divides the store in two. Under strong consistency one side stops answering writes so no split brain grows. Under eventual consistency both sides keep serving and merge their changes when the link heals.

During a partition you cannot have both, and the choice belongs to the application. A bank balance needs strong consistency because spent money must not reappear. A shopping basket can be eventual because merging two baskets later annoys nobody much.

**Tip.** Decide per feature, not per database. Ask what a wrong-then-fixed answer costs your users, and give each feature the weakest model it survives.

**Recap.** Strong behaves like one copy, eventual keeps serving and merges later, and each feature deserves its own choice.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Consensus and Why Agreement Is Hard](https://lightmysky.com/learn/computing/consensus-and-why-agreement-is-hard-mt_cF-JiM_xRz)
- [Isolation Levels and the Anomalies They Allow](https://lightmysky.com/learn/computing/isolation-levels-and-the-anomalies-they-allow-mt_utxPNgxveB)

## Opens up

- [Linearizability and Checking a History](https://lightmysky.com/learn/computing/linearizability-and-checking-a-history-mt_lTaHmlGtuo)