Art of Codeartofcode
Lesson 4 of 17

16. Consistency and CAP

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The picture to keep in your head
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16.1 What does "consistent" actually mean?
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16.2 What is strong consistency?
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16.3 What is eventual consistency?
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16.4 What are read-your-writes and monotonic reads?
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16.5 What is a network partition?
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16.6 What does the CAP theorem actually say?
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16.7 What do CP and AP systems look like in practice?
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16.8 What is PACELC?
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16.9 How do quorums give tunable consistency?
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16.10 How do you choose consistency for each feature?
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Memory card and where the analogy breaks
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Practice: five levels
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Interview questions
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Solutions
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Sources
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Code lab
16. Consistency and CAP

16.3 What is eventual consistency?

3 min read+10 XP

At the bank: Each counter keeps its own copy of the ledger and posts changes to the other by courier. For a while, the two might show different balances. But if no new transactions happen, once the couriers arrive, both agree.

In the system: Eventual consistency promises only this: if no new writes happen, all replicas will eventually return the same value. In between, a read may return an older value.

You've already met it:

  • DNS (Chapter 4): changes spread as caches expire.
  • Caches (Chapter 12): a copy can be stale until its TTL or a delete.
  • Asynchronous replicas (Chapter 14): followers lag behind the leader.

Why choose it: replicas can answer locally, immediately, without waiting for each other, and keep answering even when they can't reach each other. That gives low latency and high availability.

What to watch:

  • "Eventually" has no deadline in the definition. In practice it's usually milliseconds to seconds, but measure it (replication lag, Chapter 14).
  • Conflicts: if two replicas accept different writes to the same data, something must decide the final value: last-write-wins (which loses one write), merging, or data types designed to merge, like CRDTs. In the chapter's code, last-write-wins keeps only one of two profile edits, while a counter CRDT adds 3 likes from Mumbai and 5 from Frankfurt to get the correct 8.

Use it where a slightly old answer does no harm: likes, view counts, product descriptions, recommendations, feeds.

Memory code: "Couriers arrive eventually; until then, branches may disagree."