Legacy Modernization Pattern — Problem

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

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Statement

Legacy replacement programs fail when teams fund a parallel rewrite without strangler seams, characterization/parity evidence, incremental traffic shift, or retirement criteria. The legacy system remains on the critical path while cost doubles.

Measurable symptoms

Symptom	How you detect it
Schedule blowout	Modernization >2× original estimate; legacy still critical
Parity gaps	Production edge cases only in legacy
Dual-run tax	Two stacks serving same capability indefinitely
Big-bang attempt	Single cutover weekend for entire monolith
SME bottleneck	Only one person understands batch rules; no tests

Root cause

Modernization is framed as “build the new system” instead of “shift traffic and retire the old path capability by capability.” Without seams and parity gates, cutover risk stays binary and teams delay forever.

Non-goals of this pattern

• Not greenfield product design
• Not a single schema migration (use migration-pattern inside a slice)
• Not dependency housekeeping on an already-modern service
• Not an excuse to rewrite working code for taste

Related failure modes

See failures.md.