Incremental Legacy Migration Plan
Get a staged legacy migration plan with dependency seams, API facades, data ownership, cutover gates, and rollback steps.
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
“We need to modernize this, but we can't stop shipping.” This skill turns an aging codebase into a sequence of bounded migrations, with explicit dependency seams, compatibility contracts, data transitions, and recovery steps for each release.
What it does
- Maps callers, shared tables, transactions, jobs, and external integrations to expose hidden migration dependencies.
- Selects a first migration slice by business value, coupling, observability, and recovery difficulty.
- Chooses between an in-process boundary, branch by abstraction, strangler routing, and a staged framework or language upgrade.
- Designs API facades that preserve legacy behavior while implementations change underneath them.
- Plans database ownership, backfills, synchronization, and the point where rollback requires data repair.
- Defines deployable phases with acceptance evidence, cutover gates, rollback actions, and retirement criteria.
What you get
A migration plan sized to your request: a focused fix, a boundary review, or a full roadmap with a dependency map, facade contract, data transition, and release gates. Unknowns are labeled, and each significant assumption comes with a concrete way to validate it.
Best for
Engineering leads and developers modernizing production monoliths, replacing brittle subsystems, or upgrading frameworks and runtimes while continuing feature delivery.
SKILL.md preview
---
name: incremental-legacy-migration-plan
description: Designs deployable legacy migrations using dependency and transaction mapping, compatibility facades, explicit data ownership, and gated cutovers with recovery steps. Use when the user says "modernize this legacy app", "break up our monolith", "plan a strangler migration", "upgrade our framework without downtime", "replace this subsystem incrementally", "our migration keeps getting stuck", or "review this migration plan".
---
# Incremental Legacy Migration Plan
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