Design a feature flag platform for safe rollouts
Reported in Wise European engineering loops. System design exercise emphasizing control plane and low-latency reads.
Interview scenario
Context for Wise candidates:
Teams should enable features by user segment, percentage rollout, and environment.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Wise: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Separate control plane from data plane. Control plane handles flag creation, targeting rules, approvals, and audit logs; data plane serves evaluated flag state to applications with strict low latency.
Cache flag configs at SDK and edge layers with versioned polling or streaming updates. Include deterministic bucketing using user id hash for stable percentage rollouts.
Add kill switch semantics, fallback defaults, and stale config expiry so applications remain safe during control-plane outages.
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