---
id: wiki-2026-0508-quality-control
title: Quality Control
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [QC, Software Quality]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [quality, testing, process]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: TypeScript/Python
  framework: Playwright/pytest
---

# Quality Control

## 매 한 줄
> **"매 quality 의 inspect 보다 build-in"**. 매 modern QC 의 shift-left — 매 unit test, type checking, static analysis, contract test, e2e — 매 layer 의 different bug class catch. 2026 의 LLM-augmented test generation + property-based testing 의 mainstream.

## 매 핵심

### 매 Test pyramid (2026 update)
- **Unit (60-70%)**: pure function, fast, isolated.
- **Integration (15-25%)**: module + DB/queue, real dependencies via testcontainers.
- **E2E (5-10%)**: full user journey, Playwright/Cypress.
- **Contract (5%)**: Pact, consumer-driven, prevent break-on-deploy.
- **Property-based (cross-cutting)**: Hypothesis/fast-check, find edge cases.

### 매 Quality gates
- 매 PR 의 merge 전: lint, type, unit test, coverage threshold, security scan.
- 매 deploy 전: integration test, smoke test, canary metrics.
- 매 prod: synthetic monitoring, real-user monitoring (RUM).

### 매 Defect classes
- **Functional**: wrong output for given input.
- **Performance**: slow, regression on benchmark.
- **Security**: OWASP categories.
- **Accessibility**: WCAG violations.
- **Compatibility**: browser/OS specific.

### 매 응용
1. CI/CD pipeline gates.
2. Pre-merge bots (Danger, Reviewdog).
3. Mutation testing (Stryker) — quality of tests themselves.
4. Visual regression (Chromatic, Percy).
5. Chaos engineering (production resilience).

## 💻 패턴

### Property-based testing (TypeScript with fast-check)
```typescript
import fc from 'fast-check';
import { reverse } from './lib';

test('reverse twice = identity', () => {
  fc.assert(
    fc.property(fc.array(fc.integer()), (arr) => {
      expect(reverse(reverse(arr))).toEqual(arr);
    }),
  );
});
```

### Contract test (Pact)
```typescript
// Consumer side
const provider = new Pact({ consumer: 'Web', provider: 'OrdersAPI' });

await provider.addInteraction({
  state: 'order 123 exists',
  uponReceiving: 'a request for order 123',
  withRequest: { method: 'GET', path: '/orders/123' },
  willRespondWith: {
    status: 200,
    body: { id: '123', total: 99.0 },
  },
});

// Generates pact.json — provider verifies against it in CI
```

### Mutation testing (Stryker)
```javascript
// stryker.conf.js
export default {
  testRunner: 'vitest',
  mutate: ['src/**/*.ts'],
  thresholds: { high: 80, low: 60, break: 50 },
};
// Mutates code (a + b → a - b) and checks if tests catch it
// Surviving mutants = weak tests
```

### LLM-assisted test generation (2026 pattern)
```typescript
// CI step: claude-code generates edge cases
// $ claude test-gen src/parser.ts --output tests/parser.gen.test.ts
// Then human review before merge — never blind-trust
```

### Visual regression (Playwright)
```typescript
test('homepage matches snapshot', async ({ page }) => {
  await page.goto('/');
  await page.waitForLoadState('networkidle');
  expect(await page.screenshot()).toMatchSnapshot('home.png', {
    maxDiffPixelRatio: 0.01,
  });
});
```

### Coverage gates (vitest)
```typescript
// vitest.config.ts
export default {
  test: {
    coverage: {
      provider: 'v8',
      thresholds: {
        lines: 80,
        functions: 80,
        branches: 75,
        statements: 80,
      },
    },
  },
};
```

### Pre-commit hook (lint-staged + husky)
```json
{
  "lint-staged": {
    "*.{ts,tsx}": [
      "eslint --fix",
      "prettier --write",
      "vitest related --run"
    ]
  }
}
```

### Chaos test (Toxiproxy / Litmus)
```yaml
# Inject 500ms latency into Redis dependency
apiVersion: chaos-mesh.org/v1alpha1
kind: NetworkChaos
spec:
  action: delay
  selector:
    labelSelectors: { app: redis }
  delay: { latency: 500ms }
  duration: 60s
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Pure logic | Unit + property-based |
| Multi-service flow | Integration + contract |
| User journey | E2E (sparingly) |
| Performance regression | Benchmark in CI |
| Visual UI | Snapshot + Chromatic |
| Test confidence | Mutation score |

**기본값**: 80% line coverage, mutation score >70%, property-based for parsers/serializers.

## 🔗 Graph
- 부모: [[Test_Automation]]
- 변형: [[CI_CD_Pipeline]] · [[Test_Automation|Test_Automation_Mastery]]
- 응용: [[Engineering Metrics (DORA)]] · [[Automated Quality & Review]]
- Adjacent: [[Continuous Integration (CI)|Continuous_Integration]] · [[Husky]]

## 🤖 LLM 활용
**언제**: generate edge cases, suggest mutation-resistant assertions, identify untested branches.
**언제 X**: never let LLM write the assertion AND implementation — confirmation bias.

## ❌ 안티패턴
- **Coverage worship**: 100% coverage, 0% assertions ("test executes but checks nothing").
- **Flaky tests ignored**: erodes trust in suite. Quarantine and fix immediately.
- **E2E-heavy pyramid**: slow, flaky, expensive. Push down to integration/unit.
- **Manual QA only**: doesn't scale, regression-prone.
- **No mutation testing**: blind to assertion quality.

## 🧪 검증 / 중복
- Verified (Google Testing Blog, Mike Cohn pyramid, Stryker docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — pyramid + 2026 LLM-assisted patterns |