Legacy Modernizer
Designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases.
Before & after
Same prompt, same agent — the only difference is the skill.
A Rails monolith from 2015 definitely sounds like it could be holding you back! You have a few options to think about. You could gradually refactor it, maybe extract some services, or even consider a full rewrite—though that's risky. The strangler fig pattern is popular because it lets you replace parts without everything breaking. You'd want to identify which services are most painful or highest-risk, and start there. Building new services alongside the old system takes more work up front but gives you a safety net. You'll need to decide on your tech stack for new services and think about how they'll communicate. Testing is critical throughout. It really depends on your team's capacity and how risk-averse you want to be. What parts of the monolith cause the most problems?
MODERNIZATION ROADMAP: Rails 2015 → Microservices (18-month strategy, zero downtime) DEPENDENCY ANALYSIS (criticality assessment) TIER 1 EXTRACT FIRST (high-pain, low-coupling): • File upload service (S3 integration) → used by 3 endpoints, independent logic • Notification service (email/SMS) → queued, fire-and-forget, no return dependency • Admin dashboard (reporting) → read-only, can mirror data, low risk TIER 2 EXTRACT MID-CYCLE (medium-coupling): • Payment processing (Stripe) → 8 endpoints depend, high financial impact • User auth (JWT) → coupled but stateless, good boundary TIER 3 KEEP LAST (core domain, tightly coupled): • Order/fulfillment logic → 40+ endpoints interconnected, extract after 1+2 stable STRANGLER FIG IMPLEMENTATION (Phase by phase) MONTH 1-2: File upload → Node.js microservice • Proxy all /upload requests to new service • Fallback route back to Rails if error • Monitor error rates; go full switch after 2 weeks clean • RISK: Low (isolated, can rollback instantly) MONTH 3-5: Notification service → Separate queue (BullMQ) • No return dependency; Rails publishes events, service consumes • Full separation in Month 5 • RISK: Low (eventual consistency acceptable) MONTH 6-10: Payment service → Node.js + Postgres replica • Circuit breaker: calls new service, fallback to Rails middleware • Test under load before cutover • RISK: Medium (financial data, monitor heavily) MONTH 11-18: Core order logic → Incremental refactoring in place • Don't extract; keep in Rails, refactor patterns • Add abstraction layer for next service (easier separation) • RISK: Medium (embedded in Rails DNA) KEY GUARDRAILS • Dual-write period (3 weeks per service): new service writes, Rails writes shadow copy • Metric tracking: latency, error rate, consistency checks • Rollback path defined before any cutover • Sales unaffected (A/B test routing if needed) SUCCESS: 3 services extracted by Month 10, core business logic still in Rails (acceptable), team ready to hire specialists for next stack.
About this skill
name: legacy-modernizer description: Use when Designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases.
Legacy Modernizer
Designs incremental migration strategies, identifies service boundaries, produces dependency maps and migration roadmaps, and generates API facade designs for aging codebases. Use when modernizing legacy systems, implementing strangler fig pattern or branch by abstraction, decomposing monoliths, upgrading frameworks or languages, or reducing technical debt without disrupting business operations.
What you get
- Public GitHub repo
- the skills/legacy-modernizer folder with SKILL.md and references.
Customize your output
- Fork the repo and extend the skill's reference files for your own stack conventions.
Example output
Activates on a matching request (e.g. building or reviewing Legacy Modernizer code) and can chain with other skills in the pack.
Best for
Full-stack developers and engineering teams using Claude Code.
SKILL.md preview
---
name: legacy-modernizer
description: Use this skill when modernizing a legacy system, decomposing a monolith, or planning a strangler-fig migration.
version: 1.0.0
category: Development / Engineering
author: AgentVolt
license: proprietary
tags:
- development
- engineering
---
# Legacy Modernizer
Designs incremental migration strategies for aging codebases — service boundaries, dependency maps, and migration roadmaps without disrupting the business.
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