Feature Flags : Déployer en prod sans stress (et rollback en 1 clic)

// Nouvelle feature en développement
if (featureFlags.isEnabled('new-checkout-flow')) {
  return <NewCheckoutFlow />;
}
return <OldCheckoutFlow />;

// Supprimer le flag après rollout complet (2-4 semaines)

// Kill switch en cas d'incident
if (featureFlags.isEnabled('enable-recommendations')) {
  return await fetchRecommendations();  // Coûteux
}
return [];  // Fallback si service recommendations down

// A/B test sur pricing
const variant = featureFlags.getVariant('pricing-test');

if (variant === 'premium') {
  return <PremiumPricing />;
} else if (variant === 'standard') {
  return <StandardPricing />;
}

// Features par tier abonnement
if (user.tier === 'premium' && featureFlags.isEnabled('advanced-analytics')) {
  return <AdvancedAnalytics />;
}

// feature-flags.ts
export class FeatureFlags {
  private flags: Record<string, boolean> = {
    'new-dashboard': false,
    'dark-mode': true,
  };

  isEnabled(flag: string): boolean {
    // Override local pour dev
    const localOverride = localStorage.getItem(`flag:${flag}`);
    if (localOverride !== null) {
      return localOverride === 'true';
    }

    return this.flags[flag] ?? false;
  }
}

export const featureFlags = new FeatureFlags();

// feature-flags.ts
import { LDClient } from 'launchdarkly-js-client-sdk';

export class FeatureFlags {
  private client: LDClient;

  async init() {
    this.client = LDClient.initialize('sdk-key', {
      key: user.id,
      email: user.email,
      custom: {
        tier: user.subscriptionTier,
        country: user.country
      }
    });

    await this.client.waitForInitialization();
  }

  isEnabled(flag: string): boolean {
    return this.client.variation(flag, false);
  }

  getVariant(flag: string): string {
    return this.client.variation(flag, 'control');
  }
}

// Unleash : Self-hosted, gratuit
import { UnleashClient } from 'unleash-proxy-client';

const unleash = new UnleashClient({
  url: 'https://unleash.mycompany.com/proxy',
  clientKey: 'proxy-secret',
  appName: 'my-app',
});

await unleash.start();

if (unleash.isEnabled('new-feature')) {
  // Feature code
}

featureFlags.configure('new-dashboard', {
  enabled: true,
  targeting: {
    userIds: ['admin1', 'admin2']
  }
});

targeting: {
  custom: {
    email: { endsWith: '@mycompany.com' }
  }
}

targeting: {
  custom: {
    betaTester: true
  }
},
rollout: 100  // 100% des beta testers

rollout: 5  // 5% de tous les users

// Supprimer le flag du code
- if (featureFlags.isEnabled('new-dashboard')) {
-   return <NewDashboard />;
- }
- return <OldDashboard />;
+ return <NewDashboard />;

// Monitoring auto-rollback
const metrics = await getMetrics('new-payment-flow');

if (metrics.errorRate > 0.5) {  // > 0.5%
  await featureFlags.disable('new-payment-flow');
  await alert.page(oncall, 'Auto-rollback triggered');
}

// Rollout automatique si métriques OK
const rolloutSchedule = [
  { day: 1, percentage: 5 },
  { day: 3, percentage: 25 },
  { day: 5, percentage: 50 },
  { day: 7, percentage: 100 },
];

for (const stage of rolloutSchedule) {
  await wait(stage.day);

  const metrics = await getMetrics();
  if (metrics.errorRate < threshold) {
    await featureFlags.setRollout('feature', stage.percentage);
  } else {
    await featureFlags.disable('feature');
    break;
  }
}

// Rollout par région
featureFlags.configure('new-feature', {
  targeting: {
    custom: {
      country: { in: ['FR', 'BE', 'CH'] }  // DACH d'abord
    }
  }
});

// 1 semaine plus tard : élargir
country: { in: ['FR', 'BE', 'CH', 'DE', 'AT', 'NL'] }

// Rings : Dev → Staging → Prod
const rings = [
  { env: 'dev', users: ['all'] },
  { env: 'staging', users: ['internal'] },
  { env: 'prod', users: ['beta'] },
  { env: 'prod', rollout: 10 },
  { env: 'prod', rollout: 100 },
];

# Prometheus alert
- alert: FeatureFlagHighErrorRate
  expr: |
    feature_flag_error_rate{flag="new-feature"} > 0.5
  for: 5m
  annotations:
    summary: "Feature flag {{ $labels.flag }} error rate high"
    action: "Consider rolling back via dashboard"

-- Mesurer impact conversion
SELECT
  CASE
    WHEN feature_flag = 'new-checkout'
    THEN 'new'
    ELSE 'old'
  END AS variant,
  COUNT(*) as sessions,
  SUM(converted) as conversions,
  SUM(converted) / COUNT(*) as conversion_rate
FROM sessions
WHERE date > NOW() - INTERVAL '7 days'
GROUP BY variant;

// ❌ Flags qui durent 6+ mois
if (featureFlags.isEnabled('old-feature-from-2024')) {
  // Debt technique
}

// ❌ Flags imbriqués = cauchemar
if (featureFlags.isEnabled('feature-a')) {
  if (featureFlags.isEnabled('feature-b')) {
    if (featureFlags.isEnabled('feature-c')) {
      // Quelle combinaison est testée ???
    }
  }
}

// ❌ Si service Feature Flags down = app crash
const isEnabled = await featureFlags.isEnabled('feature');

// ✅ Fallback si service down
const isEnabled = await featureFlags
  .isEnabled('feature')
  .catch(() => false);  // Safe default

-- ❌ Query DB pour chaque check
SELECT enabled FROM feature_flags WHERE name = 'feature';

# Installer Unleash (Docker)
docker run -d -p 4242:4242 unleashorg/unleash-server

# Ou LaunchDarkly
npm install launchdarkly-js-client-sdk

// Feature simple, low-risk
if (featureFlags.isEnabled('new-footer')) {
  return <NewFooter />;
}
return <OldFooter />;