feature-flags

# Feature Flags > Ship safely. Separate deployment from release. --- ## 1. Types of Feature Flags | Type | Purpose | Lifetime | |------|---------|---------| | **Release toggle** | Gradual rollout, hide incomplete | Days–weeks | | **Experiment toggle** | A/B testing | Duration of test | | **Ops toggle** | Kill switch, circuit breaker | Indefinite | | **Permission toggle** | Feature per plan/role | Indefinite | | **Config flag** | Dynamic configuration | Indefinite | --- ## 2. Implementation Approaches | Approach | When | |----------|------| | Environment variables | Simple, per-deployment | | Database flags | Dynamic, no redeploy | | Redis flags | Fast reads, dynamic | | Managed service (LaunchDarkly, Unleash) | Complex targeting | --- ## 3. Simple Implementation (DB + Redis) ```ts // Schema model FeatureFlag { key String @id enabled Boolean @default(false) rolloutPct Int @default(0) // 0-100 allowList String[] @default([]) // User IDs updatedAt DateTime @updatedAt } // Flag evaluation async function isEnabled(flagKey: string, userId?: string): Promise<boolean> { const cached = await redis.get(`flag:${flagKey}`); const flag: FeatureFlag = cached ? JSON.parse(cached) : await loadFromDB(flagKey); if (!flag || !flag.enabled) return false; // Allow list if (userId && flag.allowList.includes(userId)) return true; // Percentage rollout (consistent per user) if (flag.rolloutPct > 0 && userId) { const hash = murmurhash(`${flagKey}:${userId}`) % 100; return hash < flag.rolloutPct; } return flag.rolloutPct === 100; } ``` --- ## 4. LaunchDarkly Integration ```ts import { init } from "launchdarkly-node-server-sdk"; const client = init(process.env.LAUNCHDARKLY_SDK_KEY!); await client.waitForInitialization(); // Evaluate flag with user context const enabled = await client.variation("new-checkout", { key: user.id, email: user.email, country: user.country, plan: user.plan, }, false); // default value if (enabled) { return <NewCheckout />; } else { return <OldCheckout />; } ``` --- ## 5. React Hook ```tsx function useFeatureFlag(key: string): boolean { const { data } = useQuery({ queryKey: ["flags", key], queryFn: () => fetchFlag(key), staleTime: 60_000, // 1 min }); return data?.enabled ?? false; } // Usage function Checkout() { const newCheckout = useFeatureFlag("new-checkout"); return newCheckout ? <NewCheckout /> : <OldCheckout />; } ``` --- ## 6. Gradual Rollout Strategy ``` Rollout stages: 0% → Deployed but hidden (dark launch) 1% → Canary (catch crashes) 10% → Limited beta 25% → Wider test 50% → Half users 100% → Full release Flag deleted → Code cleaned up Monitor per stage: - Error rate - Latency - Business metrics - User feedback ``` --- ## 7. Flag Management Rules ``` Lifecycle rules: ├── Every flag has an owner + expiry date ├── Release flags: delete after 2 weeks of 100% rollout ├── Experiment flags: delete after analysis ├── Name: {domain}.{feature} (e.g. checkout.new-flow) └── Document purpose in flag description Tech debt prevention: ├── Track flags in JIRA/Linear ticket ├── PR to remove flag after release └── Alert when flag older than 90 days ``` --- ## 8. Kill Switch Pattern ```ts // Ops toggle for circuit breaker const USE_STRIPE = isEnabled("payments.stripe"); const USE_PAYPAL_FALLBACK = isEnabled("payments.paypal-fallback"); async function processPayment(order: Order) { if (USE_STRIPE) { return await stripeCharge(order); } if (USE_PAYPAL_FALLBACK) { return await paypalCharge(order); } throw new Error("No payment provider available"); } ``` --- ## 9. Anti-Patterns | ❌ Don\'t | ✅ Do | |----------|-------| | Nested flag conditions | One flag, one decision | | Flags forever | Set expiry, clean up | | No monitoring | Track flag evaluation metrics | | Flag in wrong layer | Evaluate server-side for security | | Hardcoded user lists | Use targeting rules |