2nd/10_Wiki/Topics/DevOps_and_Security/Feature-Flags.md

---
id: wiki-2026-0508-feature-flags
title: Feature Flags
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Feature Toggles, Feature Switches, Feature Gates]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [devops, deployment, experimentation, release-engineering]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: TypeScript/Go
  framework: LaunchDarkly/Unleash/OpenFeature
---

# Feature Flags

## 매 한 줄
> **"매 deploy 와 release 를 매 분리하는 runtime conditional"**. Pete Hodgson 의 매 4-axis taxonomy (release/experiment/ops/permission) 가 매 baseline. 매 modern stack — OpenFeature spec + provider (LaunchDarkly, Unleash, ConfigCat, Statsig) — 매 progressive delivery, A/B test, kill switch 의 매 통합 layer.

## 매 핵심

### 매 4 종류 (Hodgson)
- **Release toggle**: dark launch, gradual rollout (short-lived).
- **Experiment toggle**: A/B/n test (medium-lived).
- **Ops toggle**: kill switch, circuit breaker (medium-lived).
- **Permission toggle**: per-user/tenant entitlement (long-lived).

### 매 axis
- Lifetime: hours ↔ years.
- Dynamism: build-time → runtime → request-time.
- Scope: global → user → request.

### 매 응용
1. Trunk-based development + dark launch.
2. Canary / progressive rollout (1% → 100%).
3. Kill switch (incident mitigation).
4. A/B testing & holdout group.
5. Tier-based feature gating (free/pro/enterprise).

## 💻 패턴

### OpenFeature SDK (vendor-neutral)
```typescript
import { OpenFeature } from '@openfeature/server-sdk';
import { LaunchDarklyProvider } from '@openfeature/launchdarkly-server-provider';

await OpenFeature.setProviderAndWait(new LaunchDarklyProvider(SDK_KEY));
const client = OpenFeature.getClient();

const showNew = await client.getBooleanValue('new-checkout', false, {
  targetingKey: user.id,
  email: user.email,
  plan: user.plan,
});
return showNew ? renderNewCheckout() : renderOldCheckout();
```

### Percentage rollout (deterministic hash)
```go
import "hash/fnv"
func rollout(flagKey, userID string, percent int) bool {
    h := fnv.New32a()
    h.Write([]byte(flagKey + ":" + userID))
    return int(h.Sum32()%100) < percent
}
// Same user → same bucket across calls (sticky).
```

### Kill switch (ops toggle)
```typescript
async function fetchRecommendations(userId: string) {
  if (await flags.getBooleanValue('reco-kill-switch', false)) {
    return []; // disable the feature instantly during incident
  }
  return recoService.fetch(userId);
}
```

### Multivariate experiment
```typescript
const variant = await client.getStringValue('checkout-variant', 'control', ctx);
switch (variant) {
  case 'one-click': return <OneClickCheckout />;
  case 'wallet':    return <WalletCheckout />;
  default:          return <StandardCheckout />;
}
metrics.track('checkout_view', { variant, userId });
```

### Local override (dev/test)
```typescript
if (process.env.NODE_ENV === 'development') {
  OpenFeature.setProvider(new InMemoryProvider({
    'new-checkout': { defaultVariant: 'on', variants: { on: true, off: false } },
  }));
}
```

### Flag cleanup (technical debt)
```bash
# Find old flags
rg "getBooleanValue\\('old-checkout'" --type ts
# Provider audit: list flags > 90 days old, no recent eval, 100% rollout → delete.
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Dark launch / gradual | Release toggle, percentage rollout |
| Hypothesis test | Experiment toggle + analytics |
| Incident mitigation | Ops toggle (always available) |
| Tier / paid features | Permission toggle (long-lived) |
| Build-time only | Compile flag (no runtime cost) |

**기본값**: OpenFeature SDK + managed provider (LaunchDarkly/Unleash). Kill switch 모든 critical path 에 매.

## 🔗 Graph
- 부모: [[Continuous Delivery]] · [[Progressive Delivery]]
- 변형: [[Kill Switch]]
- 응용: [[Trunk-based Development]]
- Adjacent: [[LaunchDarkly]] · [[Unleash]]

## 🤖 LLM 활용
**언제**: Release strategy 설계, incident playbook, experiment platform 평가.
**언제 X**: Static config — env var / config file 충분.

## ❌ 안티패턴
- **Flag debt**: 100% rolled-out flag 를 매 안 지움 → cyclomatic complexity 폭발.
- **Nested flags**: A&&B&&C — 매 test space 매 explosion.
- **Flag in hot loop**: per-request eval 의 매 latency — cache locally.
- **No fallback**: provider 다운 시 feature 깨짐 — default + cached value.
- **Flag = config 오용**: 진짜 config 는 매 config service 로.
- **No analytics linkage**: experiment 인데 evaluation 안 records.

## 🧪 검증 / 중복
- Verified (martinfowler.com Feature Toggles, OpenFeature spec, LaunchDarkly docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — 4-axis taxonomy + OpenFeature/percentage/kill-switch 패턴 |