Product requires strong consistency and p99 latency under 100 ms for a globally distributed user base. Both are non-negotiable. What do you do?
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What the interviewer is testing
Whether you can identify a requirement that physics forbids and turn it into a productive conversation.
The physics
Strong consistency across regions requires coordination — a quorum acknowledgement or a leader round trip. Light takes roughly 60 ms to cross the Atlantic and back in fibre, before any processing. A globally distributed quorum write cannot complete in under 100 ms for users far from the quorum.
Both requirements cannot hold simultaneously as stated. Saying so plainly is the first contribution.
The productive move: decompose
The requirement is almost never uniform across the system. Ask which specific operations need strong consistency, and it is usually a small set:
| Operation | Consistency needed | Latency achievable |
|---|---|---|
| Read product catalogue | Eventual | Local, fast |
| Read own profile | Read-your-writes | Local, fast |
| Add to basket | Eventual | Local, fast |
| Check inventory availability | Approximate | Local, fast |
| Commit an order against inventory | Strong | Cross-region, slower |
| View order history | Eventual | Local, fast |
Almost everything is fast; the one operation genuinely needing a global invariant is slower — and users tolerate a slower confirmation step far better than a slow browsing experience.
The other resolutions
Partition by region. Each user's data has a home region where it is strongly consistent, and they are routed there. Strong consistency locally, no cross-region coordination, fast for everyone in their own region. This resolves the conflict entirely for most consumer systems and is under-used.
Reservations rather than commitments for contended resources, so the fast path takes a local hold and the slow global confirmation happens asynchronously.
Relax the latency target for the specific operation, having established that it applies to one step rather than to everything.
What a strong answer adds
Presenting it as a menu with what each option costs and forbids, rather than as a refusal. The stakeholder stated both requirements because nobody had shown them the trade; showing it lets them choose.
Common weak answers
Accepting both and hoping. Choosing eventual consistency unilaterally without surfacing the decision.