Feature Flag Manager

Skill: Feature Flag Manager

Implement feature flags for controlled rollouts, A/B testing, and safe deployments with cleanup workflows.

Trigger

When the user asks to add a feature flag, gate a feature, set up gradual rollout, or clean up old flags.

Prerequisites

• [ ] Feature to gate identified
• [ ] Rollout strategy decided (on/off, percentage, user segment)

Steps

Step 1: Choose Flag Strategy

Step 2: Define the Flag

• [ ] Name: FEATURE_{DOMAIN}_{DESCRIPTION} (e.g., FEATURE_ORDERS_ASYNC_PROCESSING)
• [ ] Type: boolean, number (percentage), string (variant)
• [ ] Default value: false (new features default to off)
• [ ] Description: one sentence explaining what it controls
• [ ] Owner: team or person responsible
• [ ] Expiry: target date for cleanup

Step 3: Create Flag Configuration

// config/feature-flags.ts
export const FeatureFlags = {
  FEATURE_ORDERS_ASYNC_PROCESSING: {
    key: 'FEATURE_ORDERS_ASYNC_PROCESSING',
    description: 'Enable async order processing pipeline',
    defaultValue: false,
    owner: 'orders-team',
    createdAt: '2026-04-12',
    targetCleanup: '2026-06-12',
  },
} as const;

export type FeatureFlagKey = keyof typeof FeatureFlags;

Step 4: Implement Flag Check Utility

// utils/feature-flags.ts
export function isFeatureEnabled(
  flagKey: FeatureFlagKey,
  context?: { userId?: string; orgId?: string }
): boolean {
  const envValue = process.env[flagKey];
  if (envValue !== undefined) {
    return envValue === 'true' || envValue === '1';
  }
  return FeatureFlags[flagKey].defaultValue;
}

Step 5: Gate the Feature in Code

// In handler/service:
if (isFeatureEnabled('FEATURE_ORDERS_ASYNC_PROCESSING', { userId })) {
  await processOrderAsync(order);
} else {
  await processOrderSync(order);
}

Step 6: Add to Environment Config

• [ ] Add flag to .env.example with false default
• [ ] Add to staging .env with true for testing
• [ ] Keep production as false until ready for rollout

Step 7: Add Tests for Both Paths

• [ ] Test with flag enabled
• [ ] Test with flag disabled
• [ ] Test flag default behavior (env var not set)

describe('when FEATURE_ORDERS_ASYNC_PROCESSING is enabled', () => {
  beforeEach(() => { process.env.FEATURE_ORDERS_ASYNC_PROCESSING = 'true'; });
  afterEach(() => { delete process.env.FEATURE_ORDERS_ASYNC_PROCESSING; });

  it('should process order asynchronously', async () => { /* ... */ });
});

describe('when FEATURE_ORDERS_ASYNC_PROCESSING is disabled', () => {
  it('should process order synchronously', async () => { /* ... */ });
});

Step 8: Document Rollout Plan

## Rollout Plan: FEATURE_ORDERS_ASYNC_PROCESSING
1. **Dev/Staging**: Enable, run integration tests
2. **Canary (5%)**: Monitor error rate and latency for 24h
3. **Gradual (25% → 50% → 100%)**: Each stage monitored for 48h
4. **Cleanup**: Remove flag after 2 weeks at 100%

Flag Cleanup Workflow

When to Clean Up

• Flag has been at 100% for 2+ weeks
• No incidents related to the feature
• Target cleanup date reached

Cleanup Steps

1. Remove the flag check — keep the "enabled" code path 2. Remove the "disabled" code path 3. Remove flag from FeatureFlags config 4. Remove from .env files 5. Remove flag-specific tests (keep the feature tests) 6. Update documentation

Rules

• ALWAYS default new flags to false (off)
• ALWAYS set a cleanup target date when creating a flag
• ALWAYS test both code paths (enabled and disabled)
• NEVER nest feature flags (flag inside flag)
• NEVER use feature flags for permanent configuration — use config instead
• Flag names must be descriptive and follow FEATURE_DOMAIN_DESCRIPTION pattern
• Maximum 15 active flags at any time — clean up before adding more

Completion

Feature flag created with config, utility function, code gating, tests, and rollout plan. Cleanup date documented.

If a Step Fails

• Too many active flags: Audit and clean up stale flags first
• Complex branching: Consider a feature flag service (LaunchDarkly, Unleash) instead of env vars
• Tests flaky with flag: Ensure env var cleanup in afterEach
• Can't determine rollout strategy: Default to boolean toggle, upgrade later