Stay correct · 3 minutes
Which copy is the truth
With several copies, you choose between always-latest and always-fast.
Open this lesson
Two copies, two answers
With a primary and a replica, two people can read different values for a moment. Strong consistency means every read sees the latest write, and costs a little speed. Eventual consistency means copies catch up soon, and is faster and cheaper.
Choose per screen
Read a replica before delivery, then route to the primary. Choose the guarantee for each screen, including the authority needed to serve a latest-value read.
Try the idea
Users must see their own edits at once. Make writes safe and read the latest from the primary.
Key numbers
- Async write
- ≈ 5 ms. approx.
- Sync write to a second copy
- ≈ 8 to 15 ms. adds a round trip, approx.
- Replica lag in trouble
- ≈ 30 s. rare, approx.
Check your reasoning
Think through these questions before opening the lesson. In the game you can test the idea and compare your reasoning.
- Strong consistency means:
- Which screen can be eventually consistent?
- What does sync replication protect against?
The key point
Pick the strongest guarantee only where being wrong would hurt.
Practise this idea
- Hard
Group chat and presence
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- Hard
Bank ledger
You run the system of record for a regional bank: every deposit, withdrawal and card settlement is a posting that must be exact and permanent. Customers in two continents read balances all day, and regulators can ask for proof at any time. A bad day for you is a loss of even one posting.
- Hard
Stock order matching
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