Legacy Modernization: Strangler Fig at Scale
How teams replaced ageing systems without stopping the business — containers, .NET 9, and phased cutovers in 2025.
Legacy modernisation dominated IT backlogs in 2025. COBOL and early .NET monoliths still ran critical processes, but maintenance costs, security exposure, and shrinking skill pools pushed leaders to act — carefully.
The strangler fig pattern
Rather than big-bang rewrites, many programmes routed new functionality through modern services while legacy cores handled stable workloads. API facades wrapped old databases, giving frontends a clean contract while data migration happened in the background. Event outboxes bridged legacy batch jobs with real-time downstream consumers.
Technical targets
Containers and Kubernetes provided a consistent deploy target for refactored modules. .NET 9 migration paths helped Windows-centric estates move forward. Where appropriate, selected modules moved to managed cloud services to reduce patching burden. Teams paired modernisation with observability upgrades so regressions surfaced before users reported them.
People and process
Modernisation failed when treated as pure technology. Training, parallel running, and clear rollback plans kept operations teams confident. Knowledge capture from senior maintainers was as valuable as the new codebase. Incremental delivery let sponsors see value each quarter instead of waiting years for a switch-over weekend.
PrequaliQ modernises legacy systems in phases — improving security and agility while protecting daily operations.