legacy-modernization

# Legacy Modernization Expert guidance for safe, incremental modernization of legacy systems, frameworks, and dependencies using proven migration patterns and risk mitigation strategies. ## When to Use This Skill - Planning framework migrations (jQuery to React, Java 8 to 17, Python 2 to 3) - Decomposing monoliths into microservices or modular architectures - Modernizing databases (stored procedures to ORMs, schema migrations) - Reducing technical debt with a phased, low-risk approach - Updating outdated dependencies with backward compatibility concerns - Establishing test coverage for untested legacy code before refactoring - Designing rollback procedures for migration phases - Implementing feature flags for gradual rollout of modernized components ## Quick Reference | Task | Load reference | | --- | --- | | Strangler fig, feature flags, migration checklists, rollback procedures | `skills/legacy-modernization/references/modernization-patterns.md` | ## Workflow ### 1. Assessment Inventory legacy components, risks, and dependencies before changing anything. - Map the dependency graph and identify high-risk areas - Define modernization goals and phased milestones - Establish success metrics (test coverage, performance, defect rate) - Prioritize based on business value and risk ### 2. Safety Net Setup Establish guardrails before any migration work begins. - Add characterization tests for existing behavior - Set up feature flags for gradual rollout - Create compatibility layers and adapter interfaces - Document current behavior and integration points ### 3. Incremental Execution Apply the strangler fig pattern: replace components one at a time. - Route traffic gradually to new implementations - Maintain backward compatibility at every step - Run old and new paths in parallel where possible - Monitor for regressions continuously ### 4. Stabilization Validate the migration and retire legacy paths. - Run full regression suites against new implementations - Monitor adoption metrics and error rates - Deprecate and remove legacy code paths - Document the new architecture and migration decisions ## Common Mistakes - Attempting big-bang rewrites instead of incremental migration - Refactoring without tests covering existing behavior - Removing backward compatibility before all consumers migrate - Skipping rollback planning for each migration phase - Ignoring data migration complexity and state synchronization - Not involving stakeholders in deprecation timelines