Legacy system modernization, carried out while the system stays in production
Modernisation for systems that are too risky to touch and too important to freeze: reverse engineering, monolith decomposition and platform recovery, carried out while the system stays live. Core banking and finance systems, FMCG content platforms, and retail commerce estates. Production systems taken over and stabilised where other approaches couldn't, including a public rescue of Royal Canin's high-traffic CMS platform.
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Signs a system is a candidate for modernisation
Not every old system needs the same intervention, and some of what feels unfixable is more fixable than it looks. These are the symptoms worth checking for first.
TEAM
The people who built it have gone. There is no documentation, nobody left who remembers why it works the way it does, and the record of the decisions went with them.
HISTORY
A rewrite was attempted and quietly stopped. The replatforming budget was spent, some of it still runs in parallel, and nobody wants to propose the same thing twice.
RISK
Releases are frozen or close to it. Every change carries production risk, every change needs sign off, and the safest option each quarter is to do nothing.
COST
Maintenance rises while delivery slows. Small features take months, and most of the effort goes into not breaking what is already there.
KNOWLEDGE
One or two people hold the system in their heads. That is a delivery bottleneck on a normal week, and a finding the first time a regulated review asks who could operate it if they left.
PLAN
A full replacement is on the table and everyone is nervous. The platform cannot take the downtime a replacement would need, and the last estimate was the reason the project stalled.
Recognising two or three of those is where an assessment starts. What follows is the order we work through them in, chosen to remove risk before anything structural gets touched.
The stages we work through to reduce risk
Each stage removes a specific risk rather than moving the project along. They are ordered so nothing structural gets touched until the things that would make it dangerous are gone. Learn more during a conversation with our engineers.
1
Map it (read-only)
Real behaviour, dependencies and data flows, read from the code and observed in production rather than trusted from old diagrams. This includes how change is approved today and who can access what, because both constrain everything that follows. risk removed - guessing what will break.
2
Prove it's safe
Characterisation tests pin the existing behaviour before anything is refactored, starting with the most acute risks. The tests double as evidence that behaviour did not change, which is what a release board or an auditor asks for. risk removed - changes that can only be hoped for, not verified.
3
Cut it in pieces
The strangler pattern, one capability at a time, old and new running in parallel until each slice is proven. Traffic moves per capability, so the system keeps trading while the migration is underway. risk removed - a single moment where everything has to work at once.
4
Hand back ownership
Documentation, runbooks and a team that understands the system again, operated long term or handed over. Written in a form that survives an audit or a change of supplier, not just a handover meeting. risk removed - the knowledge that leaves when the engagement ends.
Each stage above leans on a specific set of techniques. None of them are exotic. They get applied deliberately, and in the right order.
Reverse engineering
Code analysis, dependency and data flow mapping, and behavioural reconstruction where documentation does not exist.
Characterization testing
Tests that capture existing behaviour before refactoring, so modernisation does not silently change the business depends on. The same tests serve as evidence that behaviour did not change.
Strangler-pattern migration
Incremental replacement, traffic routed per capability as each part is verified. The usual approach where a platform cannot take a maintenance window.
Monolith decomposition
Domain driven boundaries, service extraction, anti-corruption layers and careful contract design.
Data migration
Schema evolution, dual write and backfill strategies, continuous reconciliation and minimal downtime cutovers, with reconciliation evidence retained for audit.
System recovery
Stabilising failing or unmaintained platforms, including systems taken over mid incident from a previous supplier.
Technical debt reduction
Targeted remediation prioritised by business risk and change frequency, starting with the parts that block releases.
Migration planning
Sequencing, rollback strategy, business continuity planning, and change windows that fit release approval cycles.
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Systems modernised and operated for banking, FMCG and retail clients since 2008
How engagements are structured, from a single audit to full ownership
Most modernisation work begins at Project or Managed Service. The rest of the ladder is there when it is wanted, not required upfront.
Have questions about Legacy System Modernizations?
Discuss a modernization roadmap for a system that cannot go offline and cannot stay as it is.
A conversation about the system that's difficult to change, and a realistic, incremental path forward.
