Legacy
Modernizing legacy without stopping the business.
The system that is live keeps serving while the new one is built beside it.
The method
Read before writing.
The people who wrote the code have left and the documentation has aged. What still holds of the business rules is in the system that runs today.
- 01 Read
- All the existing code and all the existing documentation enter the knowledge base before a single new line is written.
- 02 Build beside it
- The new system is born AI-native, typed and instrumented.
- 03 Check
- The rules taken out of the old system are checked against its behaviour, not against the documentation.
- 04 Switch
- The replacement is planned with your team and happens in parts. Each part goes in only after it has been checked.
The line does not leave after the switch. Sustainment is a station.
The deployment
A subscription platform with eight years in production.
Deployment 01
Dr. Consulta · subscription platform
Eight years in production · Challenge B, refactor legacy
InheritedOn the line
A featureInheritedtwo weeksOn the lineup to three days[one representative feature, not a measurement of the whole backlog]
A text changeInheriteda developerOn the linea product manager
MaintenanceInheritedby handOn the linesupervising the AI
Infrastructure costInheritedwhat the platform costOn the line−75%
The third gain
Speed and cost show up first. Correctness decides whether they are worth anything.
A refactor cannot change the behaviour of the old system.
Where to start
Not with the whole system.
The line comes in through one part that can be checked end to end, usually the part that eats the most maintenance today. That part is refactored and put into production before the next one begins.