Migration Planner

Use this to plan a safe, phased migration with rollback using the strangler-fig pattern.

Tech profile — Technology: any · Language: n/a · Stack: n/a · Toolchain: n/a · Domain: architecture (Pure-reasoning skill — plans migrations between any two technologies/architectures.)

When to use this skill

• Migrating from a legacy system/component to a new one with minimal risk.
• Replatforming, re-architecting, or splitting a monolith incrementally.
• Any change too large/risky to do as a single cutover.

When NOT to use this skill

• Choosing what to migrate to — use architecture/tradeoff-analyzer first.
• A single reversible schema change — use the database-migration workflow.
• Reviewing a target design — use architecture/system-design-reviewer.

Prerequisites

• The source (current state) and the target (decided end state).
• Risk tolerance, downtime budget, and any hard deadline.

Inputs

Instructions

1. Confirm source, target, and "why". Restate both states and the driver. If the target isn't decided, stop and route to architecture/tradeoff-analyzer.
2. Inventory & assess. List the components/data/integrations in scope; rank by risk and coupling. Identify the seams where old and new can coexist.
3. Choose a strategy. Default to strangler-fig (incrementally route traffic from old to new behind a facade). Use big-bang only if explicitly justified and low-risk; record the rationale.
4. Define phases. Break the migration into independently shippable, independently reversible phases. Each phase has: scope, entry/exit criteria, and verification.
5. Plan data migration. Prefer expand/contract (dual-write/backfill/cut-read) for zero-downtime; specify integrity checks. (For the mechanics, the database-migration workflow applies.)
6. Define rollback per phase. Every phase must be reversible; state the rollback trigger and steps. A phase without a tested rollback is not ready.
7. Sequence & gate. Order phases by risk (de-risk early); define the go/no-go gate between phases.
8. List risks + assumptions. Top risks with mitigations; explicit assumptions.

Output

A phased migration plan: source/target/driver, scope inventory, chosen strategy (+why), ordered phases (scope, criteria, verification, rollback), the data-migration approach, a risk register with mitigations, and assumptions.

Constraints & safety

• Every phase is reversible. No phase ships without a stated, testable rollback (step 6). Aligns with the deployment-review checklist.
• De-risk early. Sequence the riskiest, most-uncertain phases first when feasible.
• No silent data loss. Data phases specify integrity verification.
• Conforms to /standards/prompt-engineering.md and /standards/security.md.

Examples

Minimal below; full plans in EXAMPLES.md.

Given: source = monolith auth, target = dedicated auth service, low risk tolerance. Produces: a strangler-fig plan — facade/routing phase → read-path migration → write-path migration → decommission, each with rollback and verification.