16. Consistency and CAP
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The picture to keep in your head2
16.1 What does "consistent" actually mean?3
16.2 What is strong consistency?4
16.3 What is eventual consistency?5
16.4 What are read-your-writes and monotonic reads?6
16.5 What is a network partition?7
16.6 What does the CAP theorem actually say?8
16.7 What do CP and AP systems look like in practice?9
16.8 What is PACELC?10
16.9 How do quorums give tunable consistency?11
16.10 How do you choose consistency for each feature?12
Memory card and where the analogy breaks13
Practice: five levels14
Interview questions15
Solutions16
Sources17
Code lab16. Consistency and CAP
16.10 How do you choose consistency for each feature?
3 min read+10 XP
At the bank: The bank doesn't use one rule for everything. Withdrawals: phone first, always. Updating the newsletter address: whenever. Showing the customer their own last deposit: make sure they see it.
In the system: Choose consistency per feature, not per company. For each one, ask: what does a stale answer cost, and what does no answer cost?
Figure: one app, different consistency for each feature.
For the bookstore:
- Gift-card balance, payment, seat or stock at checkout: strong. A stale balance means spending money twice (the Level 4 bug: two ₹400 purchases from a ₹500 card during a partition). Route these to a single leader or a consensus-backed store, and accept occasional errors during failures.
- Unique username or email: strong (or a single place that owns the check; Chapter 15, section 15.7).
- "My cart", "my orders", my profile: read-your-writes. The user must see their own changes; others can lag.
- Comments and replies: causal. Replies never before the comment they answer.
- Likes, view counts, trending lists, recommendations: eventual. Fast and always available; a few seconds behind is fine.
A useful pattern: eventual for display, strong for the decision. Show an eventually consistent "only 3 left" on the product page, and do the strongly consistent stock check at checkout (Chapter 12, section 12.9).
Memory code: "Price the stale answer and the missing answer; pick per feature."