Case study: modernising a legacy monolith

A customer and billing platform built as a Java 8 and Spring Boot monolith, with a dated React frontend, needed to move to a modern cloud-native architecture. The constraint was clear: no downtime, no data loss and no dropped behaviour.

The challenge

Much of the platform’s business logic was undocumented and understood by only a few engineers. Rebuilding it meant first recovering what the system actually did, then proving the new platform did the same.

The approach

The work ran in six phases: discovery, architecture, development, testing, go-live and hypercare. Discovery came first. The team captured the lead engineers’ knowledge, documented the full database schema and audited the security of the existing stack before any new code was written.

Proving equivalence

A golden master test harness, built in .NET 8, compared the outputs of the legacy and new systems automatically. Any mismatch failed a test and had to be resolved before the work moved on.

The architecture

The monolith was broken into domain-specific serverless services on AWS Lambda, with a React frontend and a single API gateway. The gateway routed version one requests to the untouched legacy services and version two requests to the rebuilt ones, so old and new ran side by side. A separate development environment kept production safe throughout.

How success was defined

• Full functional replication of the legacy platform.
• Performance that met or beat the legacy benchmarks.
• A cutover with minimal downtime.
• Four weeks of hypercare with defined severity levels.

What the client received

The working platform, OpenAPI specifications, migration documentation and operational runbooks, so the client’s own team could run and extend the system after handover.

The lesson

The hardest part of modernisation is preserving what the old system knows. Thorough discovery, golden master testing and parallel running took the risk out of the move.