Modernizing Legacy Applications Safely

The risk of the big rewrite

Rewriting a core system from scratch rarely goes to plan. The business keeps changing while the new system is built, and the two drift apart.

The strangler-fig approach

Instead, replace the legacy system piece by piece. Route new functionality to modern services, and let the old system shrink until it can be retired safely.

Principles that keep it safe

• Keep behaviour: capture current behaviour with tests before changing it.
• Introduce seams: add APIs in front of legacy so you can swap implementations later.
• Ship incrementally: deliver value at every step, not only at the end.

Modernisation is not a project — it is a way of evolving the system continuously.