code-modernization-expert

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

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• Dependency health (outdated, vulnerable)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.

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.

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

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

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

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.

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.

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