Jun 13, 2025
Case Study: Modernising a Legacy Monolith
Paradym Switch was engaged to replace an ageing, business-critical platform: a Java 8 and Spring Boot monolith with a dated frontend, carrying years of accumulated business logic the organisation could not afford to lose. The brief was to rebuild it on a modern, serverless architecture while the existing system kept running. This is how we structured the work, and why.
The challenge: a critical system no one could afford to break
The platform sat at the centre of the client’s customer and billing operations. Its logic had been refined over many years, much of it undocumented and held in the minds of a small number of engineers. The technology had aged into a single Java 8 and Spring Boot monolith with an outdated React frontend and a rising maintenance burden. The organisation wanted the agility of a cloud-native platform, but could not tolerate downtime, data loss, or a rebuild that quietly dropped behaviour customers relied on.
The approach: discovery first, equivalence throughout
We structured the work in six phases, from discovery and architecture through development, testing, go-live and a period of hypercare.
We began with discovery rather than code. Before designing the replacement, the team captured the institutional knowledge held by the lead engineers, documented the full database schema, and ran a security audit of the existing stack. The aim was to surface the business logic that the source code alone does not explain.
We also made functional equivalence a measurable target rather than an aspiration. A golden master testing harness, built in .NET 8, captured the behaviour of the legacy application and compared it automatically against the modern services as they were built. Where old and new disagreed, the test failed, and the difference was resolved before the work moved on.
Rebuilding without switch off
The new architecture decomposed the monolith into domain-specific serverless microservices on AWS Lambda, paired with a modern React frontend and a unified API gateway as the single point of entry. The gateway was the key to a low-risk transition. It routed traffic to the untouched legacy services under a version-one interface while serving the rebuilt services under a version-two interface, so old and new ran side by side. A separate development environment preserved the integrity of the live system throughout, allowing parallel development without putting production at risk.
What the engagement was built to deliver
Each phase carried explicit success criteria. Discovery had to produce a complete account of the critical business logic and a prioritised catalogue of security vulnerabilities. Development had to replicate the platform’s functionality in full, with performance meeting or exceeding the legacy benchmarks and golden master tests confirming equivalence. Go-live was designed for a minimal-downtime cutover, followed by a four-week hypercare period with defined severity levels and response times to stabilise the system before handover. The deliverables ran from the working platform itself to OpenAPI specifications, migration documentation and operational runbooks, so the client’s own team could own and extend the system afterwards.
Why this approach de-risks modernisation
The hardest part of modernising a legacy system is not writing new code; it is preserving what the old system knows. Beginning with discovery captured the knowledge that lived only in people’s heads, and golden master testing turned the vague goal of equivalence into a test that either passed or failed. Running old and new in parallel meant the business was never asked to trust a wholesale switch. The result was a modern platform delivered against a clear, evidence-based definition of done.
Frequently asked questions
How do you modernise a legacy system without losing its business logic?
Start with discovery, capturing the knowledge held by the people who built and run the system, then make functional equivalence measurable. In this engagement a golden master test harness compared the modern platform’s outputs against the legacy application automatically, so any divergence was caught and resolved before go-live.
How do you avoid downtime when replacing a monolith?
Run the old and new systems in parallel behind a unified API gateway. Here the gateway routed to legacy services under one API version and to rebuilt services under another, with a separate development environment protecting the live system, enabling a phased, minimal-downtime cutover.
About this case study: details have been generalised to protect client confidentiality. The engagement modernised a legacy Java monolith into a serverless, cloud-native architecture, delivered across discovery, architecture, development, testing, go-live and hypercare.
