Modernizing Legacy Applications Without Stopping the Business

Software Development · 2026-07-29 · 5 min read

The strangler-fig approach in practice: replacing the system everyone is afraid of, one piece at a time.

Every organization has one: a system that runs something important, that nobody fully understands, and that everyone is afraid to touch. Rewriting it in one big project is how most modernization efforts fail.

Modernize at the seams

Find the system's natural seams, such as reporting, a specific workflow or an integration point, and rebuild one seam at a time on the new stack, routing traffic across gradually. The old core keeps running until, eventually, nothing depends on it.

Capture the logic, not just the code

The real asset in an old system is the business logic that has accumulated over years. Document its behaviour with tests before changing anything. The tests become both a safety net and the specification.

Deliver something at every step

Each rebuilt piece should be useful on its own: faster reporting this quarter, a self-service portal the next. The business sees progress all the way through, which is what keeps the work funded.