Facade/router layer

Feature flag routing config

User-hash canary routing

Phased migration plan

Error rate thresholds per phase

Rollback mechanism

Dual-write new-DB-first

Legacy sync non-blocking

Lazy read migration

Legacy cleanup timing

No direct big-bang switch

Characterization tests first

Tests capture current behavior

Abstract interface created

Legacy adapter wraps old code

New implementation alongside legacy

Dependency injection switch

Parallel run comparison

Legacy code retained

Characterization tests still pass

Coverage target noted

No call-site modification before abstraction

Lines of code analysis

Dependency analysis

Code smell detection

Test coverage estimate

Hotspot identification

SQALE-style tech debt

Risk matrix with scores

Risks prioritized by score

Mitigation per risk

Phased roadmap

Success metrics per phase

Rollback plan per phase

Incremental not big-bang

V1 endpoint preserved

V2 endpoint added

Content negotiation or versioned routing

X-API-Deprecation header

X-API-Sunset header

V1 response structure unchanged

V2 response structure differs

Feature flag or gradual routing

No big-bang removal

Migration documentation

Code output present

EXPAND step: nullable new column

WRITE BOTH step: dual writes

MIGRATE step: backfill script

READ NEW step: prefer new column

CONTRACT step: drop old column

Steps are separate and ordered

No downtime requirement met

Old column not dropped early

Application model reflects dual state

Migration steps labeled

Error rate metric tracked

Latency metric tracked

Per-phase thresholds defined

Automated rollback trigger

Rollback reduces percentage not kills service

Phase advancement validation

Monitoring dashboard or report

One release cycle at 100% before cleanup

Business behavior validation

Migration decisions documented

Alert notification defined