Legacy Modernization Pattern — Failures

Pattern: Legacy Modernization
Component: failures.md
Version: 1.1 | Updated: 2026-07-16

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Failure 1: Big-bang rewrite

Symptom: Multi-year rewrite; never cuts over; morale collapse.
Cause: No capability slices or seams.
Recovery: Freeze rewrite scope; pick one slice; strangler with parity.

Failure 2: Parity by demo

Symptom: Exec demo succeeds; month-end batch diverges.
Cause: No characterization harness.
Recovery: Capture legacy golden outputs; block ramp on harness.

Failure 3: Dual forever

Symptom: New and legacy both paid indefinitely.
Cause: No retirement Definition of Done.
Recovery: Add retirement checklist; calendar the decommission.

Failure 4: Seam churn

Symptom: Every sprint invents a new integration style.
Cause: No seam ADR.
Recovery: Lock one seam per slice; revisit only on hard failure.

Failure 5: Premature legacy delete

Symptom: Rolled to 40%; deleted batch job; cannot fall back.
Cause: Skipped retirement hooks.
Recovery: Restore legacy; reduce traffic; fix process gates.