Legacy modernisation with staged cutover

The challenge

A critical business system — over fifteen years old — was becoming increasingly difficult to maintain. The original development team had moved on, documentation was sparse, and the technology stack was no longer supported by vendors. The system processed millions of transactions daily, and any downtime had direct revenue impact. Two previous modernisation attempts had failed: one ran out of budget, and the other introduced regressions that forced a rollback.

Leadership wanted the system replaced but could not accept the risk of a big-bang migration. The team needed an approach that delivered incremental value while managing the risk of changing a system that many downstream processes depended on.

What we did

Phase 1 — Interface stabilisation. Before changing any internals, we built a thorough integration test suite that captured the system's actual behaviour — not its documented behaviour. We wrapped the legacy system with stable API interfaces that downstream consumers could migrate to, decoupling them from the internal implementation.

Phase 2 — Strangler fig pattern. We implemented new capabilities alongside the legacy system using the strangler fig pattern. New requests were routed to the modern implementation while existing traffic continued to flow through the legacy system. Both systems ran in parallel with continuous comparison to verify correctness.

Phase 3 — Data migration. We designed an incremental data migration strategy that moved data in stages rather than in a single cutover event. Each stage was reversible, with automated rollback procedures tested before every migration window. Data consistency was verified continuously using reconciliation checks.

Phase 4 — Staged cutover. We migrated traffic in controlled stages — starting with low-risk transaction types and gradually expanding to higher-value flows. Each stage ran for a minimum soak period with enhanced monitoring before proceeding. The entire cutover took twelve weeks rather than a single weekend.

Measurable results

The modernisation was completed without a single customer-impacting incident. The new system is now maintained by the existing team using modern tooling, and the organisation has adopted the strangler fig approach as its standard pattern for future modernisation efforts.