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saitarrunsaitarrun/Devforge-ai/skills/legacy-modernization/SKILL.md

legacy-modernization

Skill4 starsChanged 5 months agoApache-2.0View on GitHub

Industrial legacy code modernization, strangler fig migration, monolith decoupling, schema evolution without downtime, and reverse-engineering undocumented codebases.

2 other projects use a version of this.

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The file

We split this skill into the rules it gives an agent

4 rules. About 0 are common, 4 are distinctive, and 0 are very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈4

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Never attempt a "Big Bang" rewrite of an industrial legacy application.

    line 15
  2. Distinctive:

    When dealing with 100k+ lines of undocumented legacy spaghetti code:

    line 36
  3. Distinctive:

    Before editing a single line, record input/output snapshots of the legacy system across hundreds of real-world scenarios.

    line 39
  4. Distinctive:

    When modifying legacy schemas touching millions of production records:

    line 50

≈ “Common” is an estimate: at least 1 in 5 of the 350,495 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

Industrial Legacy Code Modernization & Migration Skill

This skill defines the strategy for safely refactoring, decoupling, and migrating mission-critical legacy systems (e.g., COBOL backends, 15-year-old monoliths, stored-procedure-heavy databases, PHP/Java EE stacks, and undocumented C++/C# engines) without production outages.

1. The Strangler Fig Modernization Pattern

Never attempt a "Big Bang" rewrite of an industrial legacy application. Always use the Strangler Fig Pattern:

[Phase 1: Ingress Interception & Routing Proxy]
  ↳ Direct traffic through an API gateway / reverse proxy (Envoy / Cloudflare / Nginx).
          ↓
[Phase 2: Shadow Traffic & Dark Launching]
  ↳ Duplicate live production requests to both the Legacy System and the Modern Microservice.
  ↳ Compare responses; assert 100% data equality (diff assertions) without returning modern output to clients yet.
          ↓
[Phase 3: Incremental Traffic Shifting]
  ↳ Route 1% → 10% → 50% → 100% of read traffic to the new service.
          ↓
[Phase 4: Legacy Strangling & Decommissioning]
  ↳ Decommission the legacy code path once all mutations and reads are fully migrated.

2. Reverse-Engineering Undocumented Legacy Code

When dealing with 100k+ lines of undocumented legacy spaghetti code:

  1. Golden Master Characterization Testing:
    • Before editing a single line, record input/output snapshots of the legacy system across hundreds of real-world scenarios.
    • Lock down behavior with automated characterization test suites.
  2. Extract Seams & Anti-Corruption Layers (ACL):
    • Introduce an Anti-Corruption Layer (ACL) between legacy data structures and modern domain models to prevent legacy naming/type pollution.
  3. Database Stored-Procedure Extraction:
    • Extract raw SQL triggers, cursors, and stored procedures into explicit, version-controlled domain services.

3. Zero-Downtime Database Migration (Expand & Contract)

When modifying legacy schemas touching millions of production records:

[Step 1: Expand]
  ↳ Add new column/table in parallel; do NOT drop or alter existing legacy columns.
          ↓
[Step 2: Dual Write]
  ↳ Application writes to BOTH legacy and new schemas simultaneously; reads still from legacy.
          ↓
[Step 3: Backfill]
  ↳ Run background batch migration job to sync historical records into the new schema.
          ↓
[Step 4: Switch Reads]
  ↳ Switch application reads to the new schema.
          ↓
[Step 5: Contract]
  ↳ Stop writing to the old schema; safely drop deprecated columns/tables after 30 days.

4. Legacy Migration Checklist

  • Characterization snapshot tests written before refactoring
  • Anti-Corruption Layer (ACL) isolates legacy models from new code
  • Database migration uses Expand/Contract (zero table locking)
  • Shadow traffic diff comparison passes with 0 discrepancy
  • Rollback switch/feature flag active in API Gateway

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