Modernize Legacy Applications
Make an ageing system safe to change, without stopping the business
A critical system is expensive to change, hard to hire for, and holding the roadmap back.
What we deliver
- Assessment report with risk-ranked modernization roadmap
- Characterisation test suite documenting current behaviour
- Routing layer enabling gradual traffic migration
- Migrated capabilities running in production with rollback available
- Data migration with reconciliation reports
- Updated architecture and operational documentation
Common challenges
What tends to go wrong
These are the failure patterns we see most often in this situation — and what our approach is designed to avoid.
Change has become disproportionately risky
No tests, unclear boundaries and undocumented behaviour mean a small change can break something unrelated. Teams respond by changing less, which makes it worse.
The rewrite that never lands
Parallel rebuilds routinely stall before reaching feature parity, leaving two systems, two costs and no benefit.
Hiring and support are getting harder
Unsupported runtimes and niche frameworks shrink the pool of people who can work on it, and remove your security update path.
Data is entangled with everything
Reporting jobs, integrations and batch scripts read the production schema directly, so the database cannot change without breaking things nobody has catalogued.
Recommended approach
How we would run it
Assess before touching anything
Codebase, dependencies, data model, integrations and operational history — producing a risk-ranked plan and, often, a list of things that can simply be retired.
Build the safety net first
Characterisation tests capture what the system does today. This is what makes every later change verifiable rather than hopeful.
Strangle, do not replace
New implementations run alongside the old behind a routing layer. Traffic shifts gradually, output is compared, and the legacy path is removed only once the replacement is proven.
Decouple the data last and carefully
Change data capture and an anti-corruption layer let consumers migrate at their own pace, with reconciliation proving both sides agree before anything is switched off.
Deliverables
What you receive
- Assessment report with risk-ranked modernization roadmap
- Characterisation test suite documenting current behaviour
- Routing layer enabling gradual traffic migration
- Migrated capabilities running in production with rollback available
- Data migration with reconciliation reports
- Updated architecture and operational documentation
- Team enablement so your engineers can continue the programme
Strangler-pattern migration
A routing layer sits in front of both implementations, so each capability can be moved, verified and rolled back independently.
- Routing layerPer-capability traffic control and rollback
- Legacy applicationRemains live until replacement is proven
- Migrated servicesNew implementations, one capability at a time
- Anti-corruption layerTranslation between old and new models
- Change data captureKeeps both data stores consistent
- VerificationOutput comparison and reconciliation reports
Delivery roadmap
The sequence of work
Phase 1 — Assessment
Understand the system, quantify risk, identify retirement candidates and sequence the work.
Phase 2 — Stabilise
Add monitoring and characterisation tests; fix anything actively unsafe before migration begins.
Phase 3 — First slice
Migrate one bounded, valuable capability end to end to prove the pattern and the tooling.
Phase 4 — Scale the programme
Repeat slice by slice, with your team increasingly leading the work.
Phase 5 — Decommission
Retire legacy components once no traffic remains, and remove the routing layer.
Indicative timeline
Roughly how long this takes
Indicative only. Timings assume reasonable availability for decisions and access to the systems involved — we confirm a specific plan after discovery.
| Phase | Indicative duration | What affects it |
|---|---|---|
| Assessment | About 2–4 weeks | Scales with codebase size and available documentation |
| Stabilisation | Typically 4–6 weeks | Depends on existing test coverage, which is often none |
| Per migration slice | Often 4–8 weeks each | Slices run sequentially or in parallel depending on capacity |
Engagement model
How this work is usually structured
Company-managed delivery pod, or team extension where your engineers lead and we add specialist capacity.
Compare engagement modelsRelated
Where to go next
- LogisticsSample content
Modernizing a logistics portal without pausing operations
Incremental migration of a business-critical customer portal and driver workflow using the strangler pattern, with no cutover event.
- Next.js
- TypeScript
- Node.js
- PostgreSQL
Modernization & Quality
Legacy modernization, quality engineering, test automation, application security and managed support — delivered incrementally, without a big-bang rewrite.
Explore Modernization & QualityCloud, Data & DevOps
Cloud architecture, delivery pipelines, data platforms and observability — engineered for reliability, cost control and safe, frequent releases.
Explore Cloud, Data & DevOps
Questions
Modernize Legacy Applications — questions we are asked
Modernize Legacy Applications
Book a consultation
Thirty minutes with an engineer who has done this before. You leave with an approach, whether or not you engage us.
Prefer email? contact@xalicon.co