# Modern SDLC Best Practices (2024-25)

This document outlines industry-standard software development lifecycle best practices based on 2024-25 research and modern engineering standards.

## Table of Contents
1. [Development Workflow](#development-workflow)
2. [Testing Strategy](#testing-strategy)
3. [Security (DevSecOps)](#security-devsecops)
4. [Code Quality](#code-quality)
5. [Performance](#performance)
6. [Documentation](#documentation)
7. [DevOps & CI/CD](#devops--cicd)
8. [DORA Metrics](#dora-metrics)
9. [Developer Experience](#developer-experience)
10. [Accessibility](#accessibility)

---

## Development Workflow

### Version Control (Git)

**Branching Strategy**:
- Main/master branch is always deployable
- Feature branches for new work: `feature/{component-name}`
- Bugfix branches: `bugfix/{issue-number}`
- Release branches for production releases
- No direct commits to main (use pull requests)

**Commit Messages**:
- Follow Conventional Commits format
- Structure: `type(scope): description`
- Types: feat, fix, docs, style, refactor, test, chore
- Example: `feat(auth): add OAuth2 social login`

**Code Review**:
- All changes require peer review
- Use pull request templates
- Automated checks must pass before merge
- Review within 24 hours for team velocity
- Focus on logic, security, and maintainability

### Test-Driven Development (TDD)

**RED-GREEN-REFACTOR Cycle**:
1. **RED**: Write failing test first
2. **GREEN**: Write minimum code to pass
3. **REFACTOR**: Improve code quality while tests pass

**Benefits**:
- Better design through testability
- Documentation through tests
- Confidence to refactor
- Fewer regression bugs

---

## Testing Strategy

### Testing Trophy (Kent C. Dodds)

**Philosophy**: "Write tests. Not too many. Mostly integration."

**Distribution**:
- **Integration Tests (70%)**: User workflows and component interaction
  - Test real user behavior
  - Test multiple units working together
  - Higher confidence than unit tests
  - Example: User registration flow end-to-end

- **Unit Tests (20%)**: Complex business logic only
  - Pure functions
  - Complex algorithms
  - Edge cases and error handling
  - Example: Tax calculation logic

- **E2E Tests (10%)**: Critical user journeys
  - Full stack, production-like environment
  - Happy path scenarios
  - Critical business flows
  - Example: Complete purchase flow

### What NOT to Test (Brittle Patterns)

**Avoid**:
- Emoji presence in UI elements
- Exact number of DOM elements
- Specific element ordering (unless critical)
- API call counts (unless performance critical)
- CSS class names and styling
- Implementation details over user behavior
- Private methods/functions
- Third-party library internals

### What to Prioritize (User-Focused)

**Prioritize**:
- User workflows and interactions
- Business logic and calculations
- Data accuracy and processing
- Error handling and edge cases
- Performance within acceptable limits
- Accessibility compliance (WCAG 2.1 AA)
- Security boundaries

### Semantic Queries (React Testing Library)

**Priority Order**:
1. `getByRole()` - Most preferred (accessibility-first)
2. `getByLabelText()` - Form elements
3. `getByPlaceholderText()` - Inputs without labels
4. `getByText()` - User-visible content
5. `getByDisplayValue()` - Form current values
6. `getByAltText()` - Images
7. `getByTitle()` - Title attributes
8. `getByTestId()` - Last resort only

### Coverage Targets

**Minimum Requirements**:
- Overall coverage: **80%**
- Critical paths: **100%** (auth, payment, data processing)
- Branch coverage: **75%**
- Function coverage: **90%**

**Tools**:
- Jest/Vitest for unit & integration tests
- Cypress/Playwright for E2E tests
- Istanbul/c8 for coverage reporting

---

## Security (DevSecOps)

### Shift-Left Security

**Principle**: Integrate security into every development stage, not as an afterthought.

**Cost Multiplier**:
- Fix in **design**: 1x cost
- Fix in **development**: 5x cost
- Fix in **testing**: 10x cost
- Fix in **production**: 30x cost

### OWASP Top 10 (2024)

1. **Broken Access Control**: Enforce authorization checks on every request
2. **Cryptographic Failures**: Use TLS, encrypt PII, avoid weak algorithms
3. **Injection**: Validate input, use prepared statements, sanitize output
4. **Insecure Design**: Threat modeling, secure design patterns
5. **Security Misconfiguration**: Harden defaults, disable unnecessary features
6. **Vulnerable Components**: Keep dependencies updated, scan for CVEs
7. **Authentication Failures**: MFA, rate limiting, secure session management
8. **Software Integrity Failures**: Verify integrity with signatures, SRI
9. **Security Logging**: Log security events, monitor for anomalies
10. **SSRF**: Validate URLs, whitelist allowed domains

### Dependency Management

**Best Practices**:
- Run `npm audit` / `yarn audit` weekly
- Update dependencies monthly
- Use Dependabot/Renovate for automated updates
- Pin dependency versions in production
- Check licenses for compliance
- Monitor CVE databases

### Secrets Management

**Rules**:
- NEVER commit secrets to version control
- Use environment variables for configuration
- Use secret management tools (Vault, AWS Secrets Manager)
- Rotate secrets regularly
- Scan git history for leaked secrets
- Use `.env.example` for documentation, not `.env`

---

## Code Quality

### Complexity Metrics

**Cyclomatic Complexity**:
- **1-10**: Simple, easy to test
- **11-20**: Moderate, consider refactoring
- **21-50**: High, should refactor
- **50+**: Very high, must refactor

**Tool**: ESLint `complexity` rule, SonarQube

### Code Duplication

**Thresholds**:
- **< 5%**: Excellent
- **5-10%**: Acceptable
- **10-20%**: Needs attention
- **> 20%**: Critical issue

**DRY Principle**: Don't Repeat Yourself
- Extract common code into functions/modules
- Use design patterns (Template Method, Strategy)
- Balance DRY with readability

### Code Smells

**Common Smells**:
- **God Object**: Too many responsibilities
- **Feature Envy**: Too much coupling to other classes
- **Long Method**: > 50 lines
- **Long Parameter List**: > 4 parameters
- **Dead Code**: Unused code
- **Magic Numbers**: Hard-coded values
- **Primitive Obsession**: Overuse of primitives vs objects

**Refactoring Techniques**:
- Extract Method
- Extract Class
- Introduce Parameter Object
- Replace Magic Number with Constant
- Remove Dead Code

### Static Analysis

**Tools**:
- **SonarQube**: Comprehensive code quality platform
- **ESLint**: JavaScript/TypeScript linting
- **Prettier**: Code formatting
- **TypeScript**: Type checking in strict mode
- **Checkmarx**: Security-focused analysis

---

## Performance

### Build Performance

**Targets**:
- Build time: < 2 minutes
- Hot reload: < 200ms
- First build: < 5 minutes

**Optimization**:
- Use build caching
- Parallelize builds
- Tree-shaking
- Code splitting
- Lazy loading

### Runtime Performance

**Web Vitals (Core)**:
- **LCP (Largest Contentful Paint)**: < 2.5s
- **FID (First Input Delay)**: < 100ms
- **CLS (Cumulative Layout Shift)**: < 0.1

**API Performance**:
- **P50**: < 100ms
- **P95**: < 500ms
- **P99**: < 1000ms

**Optimization Techniques**:
- Caching (Redis, CDN)
- Database indexing
- Query optimization
- Compression (gzip, Brotli)
- Image optimization (WebP, lazy loading)
- Code splitting and lazy loading

### Bundle Size

**Targets**:
- Initial bundle: < 200KB (gzipped)
- Total JavaScript: < 500KB (gzipped)
- Images optimized: < 200KB each

**Tools**:
- webpack-bundle-analyzer
- Lighthouse
- Chrome DevTools Performance tab

---

## Documentation

### Code Documentation

**JSDoc/TSDoc**:
- Document all public APIs
- Include examples for complex functions
- Document parameters, return types, exceptions

**Example**:
```typescript
/**
 * Calculates the total price including tax and discounts.
 *
 * @param items - Array of cart items
 * @param taxRate - Tax rate as decimal (e.g., 0.08 for 8%)
 * @param discountCode - Optional discount code
 * @returns Total price with tax and discounts applied
 * @throws {InvalidDiscountError} If discount code is invalid
 *
 * @example
 * const total = calculateTotal(items, 0.08, 'SUMMER20');
 */
function calculateTotal(items: CartItem[], taxRate: number, discountCode?: string): number {
  // ...
}
```

### Project Documentation

**Essential Files**:
- **README.md**: Project overview, setup instructions, quick start
- **CONTRIBUTING.md**: How to contribute, coding standards, PR process
- **CODE_OF_CONDUCT.md**: Community guidelines
- **CHANGELOG.md**: Version history and changes
- **LICENSE**: Legal license information
- **ARCHITECTURE.md**: High-level architecture overview
- **ADRs** (Architecture Decision Records): Document important decisions

---

## DevOps & CI/CD

### Continuous Integration

**Requirements**:
- Automated testing on every commit
- Build verification
- Code quality checks (linting, formatting)
- Security scanning
- Fast feedback (< 10 minutes)

**Pipeline Stages**:
1. Lint & Format Check
2. Unit Tests
3. Integration Tests
4. Security Scan
5. Build Artifacts
6. Deploy to Staging
7. E2E Tests
8. Deploy to Production (with approval)

### Continuous Deployment

**Strategies**:
- **Blue-Green**: Two identical environments, switch traffic
- **Canary**: Gradual rollout to subset of users
- **Rolling**: Update instances incrementally
- **Feature Flags**: Control feature visibility without deployment

**Rollback**:
- Automated rollback on failure detection
- Keep last 3-5 versions deployable
- Database migrations reversible
- Monitor key metrics post-deployment

### Infrastructure as Code

**Tools**:
- Terraform, CloudFormation, Pulumi
- Ansible, Chef, Puppet
- Docker, Kubernetes

**Benefits**:
- Version-controlled infrastructure
- Reproducible environments
- Disaster recovery
- Automated provisioning

---

## DORA Metrics

**Four Key Metrics** (DevOps Research and Assessment):

### 1. Deployment Frequency

**How often code is deployed to production**

- **Elite**: Multiple times per day
- **High**: Once per day to once per week
- **Medium**: Once per week to once per month
- **Low**: Less than once per month

### 2. Lead Time for Changes

**Time from commit to production**

- **Elite**: Less than 1 hour
- **High**: 1 day to 1 week
- **Medium**: 1 week to 1 month
- **Low**: More than 1 month

### 3. Change Failure Rate

**Percentage of deployments causing failures**

- **Elite**: < 1%
- **High**: 1-5%
- **Medium**: 5-15%
- **Low**: > 15%

### 4. Time to Restore Service

**Time to recover from production incident**

- **Elite**: < 1 hour
- **High**: < 1 day
- **Medium**: 1 day to 1 week
- **Low**: > 1 week

**Tracking**: Use CI/CD tools, APM (Application Performance Monitoring), incident management systems

---

## Developer Experience

### Why It Matters

**Statistics**:
- 83% of engineers experience burnout
- Developer experience is the strongest predictor of delivery capability
- Happy developers are 2x more productive

### Key Factors

**Fast Feedback Loops**:
- Quick build times
- Fast test execution
- Immediate linting/formatting feedback
- Hot module reloading

**Good Tooling**:
- Modern IDE with autocomplete
- Debuggers and profilers
- Automated code reviews
- Documentation generators

**Clear Standards**:
- Coding style guides
- Architecture documentation
- Onboarding guides
- Runbooks for common tasks

**Psychological Safety**:
- Blameless post-mortems
- Encourage experimentation
- Celebrate learning from failure
- Mentorship programs

---

## Accessibility

### WCAG 2.1 Level AA Compliance

**Four Principles (POUR)**:

1. **Perceivable**: Information must be presentable to users
   - Alt text for images
   - Captions for videos
   - Color contrast ratios

2. **Operable**: UI components must be operable
   - Keyboard navigation
   - Sufficient time to read content
   - No seizure-inducing content

3. **Understandable**: Information must be understandable
   - Readable text
   - Predictable behavior
   - Input assistance (error messages)

4. **Robust**: Content must be robust across technologies
   - Valid HTML
   - ARIA attributes
   - Cross-browser compatibility

### Testing Tools

**Automated**:
- axe DevTools
- Lighthouse
- WAVE
- Pa11y

**Manual**:
- Keyboard navigation testing
- Screen reader testing (NVDA, JAWS, VoiceOver)
- Color contrast checkers
- Zoom testing (200%+)

---

## Modern Trends (2024-25)

### AI-Assisted Development

**Tools**:
- GitHub Copilot
- ChatGPT / Claude
- Tabnine
- Amazon CodeWhisperer

**Best Practices**:
- Review all AI-generated code
- Write tests for AI code
- Understand before committing
- Train team on effective prompting

### Platform Engineering

**Concept**: Internal developer platforms to improve developer experience

**Components**:
- Self-service infrastructure
- Golden paths (templates)
- Developer portals
- Observability dashboards

### Observability (vs Monitoring)

**Three Pillars**:
1. **Logs**: What happened
2. **Metrics**: Quantitative data
3. **Traces**: Request flow through system

**Tools**:
- Datadog, New Relic, Grafana
- OpenTelemetry for standardization
- Distributed tracing (Jaeger, Zipkin)

---

## Industry Benchmarks (2024-25)

### Code Quality
- Tech debt ratio: < 5%
- Duplication: < 5%
- Test coverage: > 80%
- Build time: < 2 minutes

### Security
- CVE remediation: < 30 days
- Security training: Quarterly
- Penetration testing: Annually

### Performance
- Page load: < 3 seconds
- API response: P95 < 500ms
- Uptime: 99.9%+

### Team Metrics
- Pull request review time: < 24 hours
- Deployment frequency: Daily+
- Incident MTTR: < 1 hour
- Developer onboarding: < 1 week

---

**References**:
- DORA State of DevOps Report 2024
- OWASP Top 10 (2024 Edition)
- WCAG 2.1 Guidelines
- Kent C. Dodds Testing Trophy
- SonarQube Quality Gates
- Google Web Vitals

**Last Updated**: 2024-25
**Version**: 1.0