Code Room
System designMedium
Question
Design feature-flag / A-B-test evaluation at the edge: 200 PoPs each must evaluate flags for every request with near-zero latency (no central call per request), yet flag changes from a central config service must roll out globally within ~30 seconds, and a user must get a stable, consistent variant (not flip between variants on consecutive requests routed to different PoPs). Design the config distribution, the local evaluation model, and how you guarantee per-user variant stability across PoPs.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts — data model, bottlenecks, consistency, failure modes — and name the trade-offs you are making.
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