Technical guide
Modernizing legacy systems without stopping the business
The safe path does not start with a rewrite. It starts with observable behavior, stable boundaries and a continuously available way back.
Marvin Kamp2026-08-11
1. Preserve behavior
Characterization tests, real but sanitized sample data and immutable comparison output document what the legacy system actually does.
2. Build one narrow boundary
A CLI or API initially wraps one clear read or validation path. The new system does not replace anything yet; it first observes reproducibly.
3. Compare in parallel
Old and new process the same business keys. Differences are classified, not overwritten. Only stable parity permits the next slice.
4. Keep the way back
Feature flags, reversible data migration, audit and a tested rollback remain active beyond business acceptance.