Pretty well-known prompt at this point, so I'd seen it before going in.
Start by clarifying requirements: who uses the system, what types of flags, and expected scale. Then design the core components: flag definition, evaluation service, and SDKs, focusing on low-latency evaluation and consistency. Discuss trade-offs around data modeling, caching, and rollout strategies, and how to ensure reliability and auditability.
Pro tip: Emphasize the importance of a kill switch and gradual rollout capabilities, and how you would handle flag evaluation at scale with minimal latency. Mention that flag changes should be auditable and reversible, which is critical for a payments company like Stripe.
Ask about scale (number of flags, evaluations per second), latency requirements, consistency needs, and user types (internal, external). Understand the use cases: release toggles, experiments, ops flags, etc.
Outline the main components: a flag management UI/API, a flag store (database), an evaluation service, and client SDKs. Describe how flags are created, updated, and evaluated.
Design the schema for flags, including targeting rules, segments, and rollout percentages. Consider versioning and audit logs for changes.
Explain how to evaluate flags quickly, using caching, local evaluation in SDKs, and streaming updates. Discuss consistency vs. availability trade-offs.
Discuss trade-offs: push vs. pull, centralized vs. decentralized evaluation, and how to handle failures. Cover edge cases like flag dependencies and stale caches.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.