IntermediateSystem-Design
30 min
Design a Feature Flag Service
Release EngineeringCachingSecurity
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Interview Question
Design a low-latency feature flag platform with targeting rules, audit logs, and mobile/edge delivery.
Key Points to Cover
- Flag store with versioned rules and segments
- SDKs with local cache and streaming updates
- Eval at client/edge vs server; consistency trade-offs
- Audit logs, approvals, and change rollbacks
- Multi-tenant isolation and rate limits
Evaluation Rubric
Clear data model & rule evaluation25% weight
Low-latency delivery & caching25% weight
Audit, approvals, and rollbacks25% weight
Multi-tenant safety and limits25% weight
Hints
- 💡Consider streaming (SSE) for live updates.
Common Pitfalls to Avoid
- ⚠️**Over-reliance on client-side evaluation without robust consistency mechanisms:** Leading to fragmented user experiences and potential bugs.
- ⚠️**Inefficient targeting rule evaluation:** Causing high latency during flag lookups, negating the benefits of client-side caching.
- ⚠️**Insufficient versioning and rollback capabilities:** Making it difficult to recover from faulty flag deployments.
- ⚠️**Lack of comprehensive audit trails:** Hindering debugging, security analysis, and compliance efforts.
- ⚠️**Ignoring security implications of the flag store and SDKs:** Leaving the platform vulnerable to unauthorized access or manipulation of flags.
Potential Follow-up Questions
- ❓How do you prevent flag drift between envs?
- ❓How to support experiment variants?
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