Feature Flag Service — Simple

Problem statement (interviewer prompt)

Design a feature-flag service (LaunchDarkly-style). Define flags with targeting rules (per user / segment / percentage rollout), serve evaluations to SDKs in <10ms via push-based config updates, audit every change, and integrate with the experimentation platform.

flowchart LR
  ADM[Admin]
  CFG[Flag Service]
  KV[(Flag rules)]
  SDK([App SDK])
  ADM --> CFG --> KV
  SDK --> CFG
  CFG -. push .-> SDK

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class SDK client;
    class ADM,CFG service;
    class KV datastore;