code-modernization-expert

name	code-modernization-expert
description	Modernize legacy code using enterprise patterns, automated refactoring, technical debt analysis, and incremental migration with zero downtime

Modernize legacy codebases systematically using proven enterprise patterns and automated transformations.

Supported Migrations

• Languages: Python 2→3, Java 8→21, JavaScript ES5→ES6+, TypeScript
• Frameworks: React Class→Hooks, Vue 2→3, Spring Boot 2→3, Django, Rails, Angular
• Architecture: Monolith→Microservices, REST→GraphQL, Callbacks→Async/Await

**Assess Current State**

Use Glob to find all source files. Use Grep to identify:

• Framework versions (package.json, requirements.txt, pom.xml)
• Deprecated APIs and imports
• Code smells (TODO, FIXME, HACK)
• Security vulnerabilities

Generate Current State Report with:

• Languages, frameworks, versions
• Dependency health (outdated, vulnerable)
• Test coverage percentage
• Technical risks

See assessment-checklist.md for detailed items.

**Analyze Technical Debt**

Categorize debt by severity and effort:

• Code quality (God classes, long methods, duplicates)
• Dependencies (EOL frameworks, vulnerable libraries)
• Testing gaps (low coverage, missing tests)
• Architecture issues (tight coupling, monolithic design)

Use prioritization matrix in debt-prioritization.md to rank items.

Create inventory with: Severity, Effort, Risk, Impact on velocity.

**Select Modernization Pattern**

Choose based on context:

• Strangler Fig: Large monoliths, zero downtime required → patterns/strangler-fig.md
• Branch by Abstraction: Internal modules, service layers → patterns/branch-by-abstraction.md
• Seam-Based: Low test coverage, tightly coupled code → patterns/seam-based.md
• Parallel Run: Mission-critical, financial, healthcare systems → patterns/parallel-run.md

**Create Migration Plan**

Generate phased roadmap:

Preparation (1-2 weeks):

• Set up feature flags
• Implement monitoring/alerting
• Generate characterization tests
• Create rollback procedures (see rollback-template.yaml)

Execution (Iterative per module):

• Read source files
• Generate tests capturing current behavior
• Apply transformations with Edit/MultiEdit
• Update imports and dependencies
• Run tests to validate
• Deploy incrementally (1% → 5% → 25% → 100%)

Validation:

• Run test suite
• Perform shadow testing
• Monitor error rates and latency

Cleanup:

• Remove legacy code
• Remove feature flags
• Update documentation

**Execute Transformations**

Apply language-specific changes:

Python 2→3: See migrations/python.md

• print statements → print()
• dict.iteritems() → dict.items()
• Add type hints, dataclasses, f-strings

Java 8→21: See migrations/java.md

• javax.* → jakarta.*
• Date → java.time.*
• POJOs → Records
• Add pattern matching, text blocks

JavaScript ES5→ES6+: See migrations/javascript.md

• var → let/const
• callbacks → async/await
• prototypes → classes
• Add template literals, modules

React Class→Hooks: See migrations/react.md

• constructor+state → useState
• componentDidMount → useEffect
• lifecycle methods → hooks

**Generate Tests**

Create comprehensive coverage:

Characterization Tests: Capture current behavior (even if buggy) Golden Master Tests: Record production inputs/outputs Approval Tests: Snapshot complex outputs

See testing-strategies.md for examples.

Use Task tool to delegate test generation when needed.

**Manage Risks**

Implement mitigation:

Risk Registry: Track risks with likelihood, impact, mitigation, owners

Rollback Plan: Define triggers (error rate, latency, bugs), immediate actions, validation steps

Monitoring: Track error rates, latency, business metrics, set alerts

See rollback-template.yaml for complete template.

**Document Results**

Generate migration report with:

• Executive summary (current vs target, approach, timeline)
• Technical details (files modified, dependencies updated, tests added)
• Results (statistics, test results, performance impact)
• Lessons learned and next steps

Tool Usage

• Read: Load source files
• Glob: Discover files to migrate
• Grep: Find patterns, deprecated APIs
• Edit/MultiEdit: Apply transformations
• Write: Create reports, tests
• Bash: Run tests, install dependencies
• Task: Delegate to specialized agents

Configuration

Create .code-modernization-config.yml:

modernization:
  backup_enabled: true
  incremental: true
  generate_tests: true
  risk_threshold: medium

validation:
  run_tests: true
  run_linters: true
  type_check: true

monitoring:
  error_rate_threshold: 0.05
  rollback_on_threshold: true

feature_flags:
  enabled: true
  rollout_percentages: [1, 5, 10, 25, 50, 100]

Best Practices

DO: Start small, test first, use feature flags, monitor continuously, commit frequently DON'T: Big-bang rewrites, skip testing, ignore metrics, rush critical code, work in isolation