From Numbers to Architecture — Decision Gates
Estimation is only useful if it changes a decision
The point of the math is to trip a gate — a threshold where the design must change. Memorize these rough triggers; quoting them turns an estimate into an architectural argument.
| If your estimate is… | …the design changes to |
|---|---|
| Write QPS > ~50K (or one primary saturates) | Shard / multiple primaries; partition by key |
| Read QPS ≫ write QPS (e.g. 100:1) | Cache + read replicas; precompute/fan-out |
| Cache working set > ~200–300 GB | Shard the cache (consistent hashing) |
| Storage in PB of immutable blobs | Object storage + CDN, not a database |
| Egress > a single NIC / tens of Gbps | CDN / edge serving |
| Strong consistency across regions needed | Consensus (Raft/Paxos) or accept higher latency (PACELC) |
| Spiky 10×+ peaks (flash sale) | Queue + autoscale + backpressure; don’t size for peak synchronously |
The Hello-Interview principle
Don’t estimate for its own sake. Calculate the number that changes a decision — then state the gate it trips. “6K writes/s is fine on one primary; but reads at 600K/s force a cache layer” is the whole point.
Now apply all of this in the drills below — estimate first, then reveal the worked solution and check your bounds.
Formulas are standard/public-domain engineering math. Approach and reference-table format adapted from the System Design Primer (CC BY 4.0), Jeff Dean’s latency numbers, the DesignGurus capacity-estimation guide, and Little’s Law.
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