Feature Flags

Feature Flags

When to Use

• Gradual rollout of new features (canary, percentage)
• A/B testing
• Kill switch for risky features in production
• Trunk-based development (merge without releasing)
• Customer-specific feature enablement

Flag Types

| Type | Purpose | Lifespan | Example | |------|---------|----------|---------| | Release | Control feature rollout | Short (days-weeks) | ENABLE_NEW_BILLING_UI | | Experiment | A/B testing | Medium (weeks) | EXPERIMENT_CHECKOUT_V2 | | Ops | Kill switch | Permanent | ENABLE_EXTERNAL_PAYMENTS | | Permission | Per-customer features | Permanent | PREMIUM_ANALYTICS |

Implementation

Simple (Config-based)

// Feature flags from environment/config
const FLAGS = {
  ENABLE_NEW_BILLING: process.env.FF_NEW_BILLING === 'true',
  ENABLE_DARK_MODE: process.env.FF_DARK_MODE === 'true',
};

// Usage
if (FLAGS.ENABLE_NEW_BILLING) {
  return this.newBillingService.process(order);
} else {
  return this.legacyBillingService.process(order);
}

Advanced (Database-backed)

@Entity('feature_flags')
class FeatureFlag {
  @PrimaryColumn()
  key: string;                    // 'ENABLE_NEW_BILLING'

  @Column({ default: false })
  enabled: boolean;               // Global toggle

  @Column({ type: 'int', default: 0 })
  rollout_percentage: number;     // 0-100

  @Column({ type: 'simple-array', nullable: true })
  allowed_tenants: string[];      // Specific tenants

  @Column({ type: 'simple-array', nullable: true })
  allowed_users: string[];        // Specific users

  @Column({ type: 'timestamp', nullable: true })
  expires_at: Date;               // Auto-disable date
}

@Injectable()
export class FeatureFlagService {
  constructor(
    @InjectRepository(FeatureFlag) private repo: Repository,
    private cache: CacheManager,
  ) {}

  async isEnabled(
    key: string,
    context: { userId?: string; tenantId?: string },
  ): Promise {
    const flag = await this.getFlag(key);
    if (!flag || !flag.enabled) return false;

    // Check expiration
    if (flag.expires_at && flag.expires_at  0 && context.userId) {
      const hash = this.hashUserId(context.userId);
      return (hash % 100)  3 months old)
- ❌ Testing only with flags ON (also test OFF)
- ❌ Using flags for permanent business logic (use permissions)
- ❌ Too many flags (> 20 active = confusion)

## Quality Gates

- [ ] Every flag has an owner and expiration date
- [ ] Cleanup task created when flag reaches 100%
- [ ] Dashboard showing all active flags and their rollout %
- [ ] Alert if flag is > 90 days old without cleanup
- [ ] Both flag-on and flag-off paths tested

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [camilooscargbaptista](https://github.com/camilooscargbaptista)
- **Source:** [camilooscargbaptista/cto-toolkit](https://github.com/camilooscargbaptista/cto-toolkit)
- **License:** MIT

Install and usage instructions live in the source repository linked above.