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BUILDSENIOR

Build a Feature Flag Service

90 minutes
0 completed
nextjssystemsfeature-flagssdk

Description

Build a Feature Flag Service

Build a feature flag management service with percentage rollouts, targeting, and a JavaScript SDK.

Requirements

  • Create flags with: key, description, default value (on/off)
  • Percentage rollout: flag is on for X% of users (consistent per user ID)
  • User targeting rules: enable for specific user IDs, emails, or attributes
  • Environment support: dev, staging, production
  • Audit log: who changed what flag and when
  • Dashboard: list all flags, toggle, edit rules, view usage
  • JavaScript SDK for easy flag evaluation

    • Technical Notes

    Use consistent hashing for percentage rollouts (hash userId + flagKey for deterministic bucket). Store flags in database with caching.

    Bonus Points

  • A/B testing mode (multiple variants, not just on/off)
  • Flag dependencies (flag B only on if flag A is on)
  • SDK with real-time updates via SSE

    • You will be guided through MCP setup before the timer starts