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# Advanced Feature Flag System

**Date:** August 2025
**Technologies:** TypeScript, MongoDB, Redis, Next.js

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## Project Overview

The Feature Flag System provides runtime control over website functionality without requiring code changes or redeployment. It enables the GDGoC UAM team to roll out features gradually, run A/B tests, and disable problematic functionality instantly.

Unlike simple boolean toggles, the system supports user-segmented rollouts based on roles, permissions, and custom criteria. A feature can be enabled for 10% of general users, 100% of organizers, and 0% of anonymous visitors simultaneously. This granularity allows the team to test features with trusted users before broad release.

## Technical Stack

- **Backend**: Elysia.js with MongoDB for flag definitions, Redis for cached evaluations
- **Frontend**: React hook (`useFeatureFlag`) with server-side hydration support
- **Evaluation Engine**: Custom rule evaluator supporting percentage rollouts, user attributes, and time windows
- **Admin Panel**: Integrated into the existing admin interface with real-time toggle controls

## Architecture Highlights

### Percentage Rollout with Consistent Hashing

To ensure a user always sees the same variant across sessions, percentage rollouts use consistent hashing of the user ID combined with the flag key. This eliminates the "flickering" experience that occurs with random assignment.

The hash function maps users deterministically to a 0-100 range. A user with hash value 23 will be included in a 30% rollout but excluded from a 20% rollout, and this assignment persists across devices and sessions.

### Redis-Backed Evaluation Cache

Flag evaluations are cached in Redis with a TTL tied to the user's session. When an admin updates a flag, the cache is invalidated via Redis pub/sub, ensuring all server instances reflect the change within seconds.

The evaluation cache stores the full flag state per user, not just boolean results. This allows the frontend to display variant information without additional API calls.

### Server-Side Hydration

The `useFeatureFlag` hook works on both server and client. During server-side rendering, flags are evaluated from the request context and embedded in the HTML as data attributes. The client hydrates from these attributes, preventing layout shifts or content flashes when flags affect visible UI.