Feature Flag System

Feature Flag System

Implements feature flags for progressive rollouts, A/B testing, and kill switches. Supports user targeting and percentage rollouts.

Decision Framework

When to Apply

Use when: Progressive rollouts, A/B testing, kill switches, environment-specific features, beta programs

When NOT to Apply

Don't use when: Feature is permanent and always-on, simple config values suffice

Anti-Patterns

1. Flags That Never Get Removed

Technical debt accumulates. Schedule flag cleanup after full rollout.

2. Complex Flag Dependencies

// BAD: Hard to reason about
if (flags.newCheckout && flags.newPayment && !flags.legacyCart) { }

// GOOD: Single flag with clear meaning
if (flags.modernCheckoutFlow) { }

Trigger Phrases

• "Feature flags"
• "A/B testing"
• "Progressive rollout"
• "Kill switch"
• "Feature toggle"

Patterns

Flag Evaluator

class FeatureFlags {
  constructor(flags) { this.flags = flags; }

  isEnabled(flagName, context = {}) {
    const flag = this.flags[flagName];
    if (!flag || !flag.enabled) return false;

    // Percentage rollout
    if (flag.percentage < 100) {
      const hash = this.hashUserId(context.userId || 'anonymous');
      if (hash % 100 >= flag.percentage) return false;
    }

    // User targeting
    if (flag.targetUsers?.includes(context.userId)) return true;

    // Environment
    if (flag.environments && !flag.environments.includes(context.env)) return false;

    return true;
  }

  hashUserId(userId) {
    let hash = 0;
    for (let i = 0; i < userId.length; i++) {
      hash = ((hash << 5) - hash) + userId.charCodeAt(i);
      hash |= 0;
    }
    return Math.abs(hash);
  }
}

Integration

• Works with: deployment-pipeline-architect, multi-tenant-architecture, notification-system-builder