Monolith-to-Modern Strangler Migration
Monolith-to-Modern Strangler Migration — Defense Client Web Suite
Stabilized an 8-year-old .NET + Angular monolith and won approval for a multi-million-CHF modernization roadmap.
- Date: Jun 2023 - Jan 2024
- Roles: Tech Lead · Software Architect · Lead Software Engineer
- Team size: 15
- Customer: Confidential Defense Client (Switzerland)
- Core Stack:
Backend: C# / . NET 6 · EF Core · Vertical-Slice CQRS · API Gateway (YARP)
Frontend: Angular 16 · TypeScript
Cloud/DevOps: Docker · Nginx Reverse Proxy · Azure DevOps Pipelines · Nuke Build · Terraform IaC
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1. Problem
The web suite manages finance, HR, warehousing and logistics for the defense organization. After eight years and multiple vendor hand-overs, the codebase had decayed into a spaghetti monolith:
- frequent production bugs revealed only after deployment
- lead-time for changes measured in weeks, driving up cost
- developers avoided the project, causing high turnover
- E2E test job exceeded 60 min, delaying releases
2. Solution Overview
I performed an end-to-end architecture assessment, identified root causes, and led the client through four strategic options—ultimately selecting a Strangler-Fig migration via API Gateway:
| Track | Purpose | Key Tech / Notes |
|---|---|---|
| Containerisation | Package entire legacy app for predictable rollout | Docker 🠒 customer DC |
| Gateway / Proxy | Route requests to legacy or new services | YARP · Nginx |
| Vertical-Slice Backend | Rebuild modules behind clean boundaries | .NET 6 · MediatR |
| CI/CD & Tests | Speed up feedback loops | Nuke · Azure Pipelines · +20 min test cut |
Security: Runs inside restricted defense networks; reverse-proxy hardened (CIS Level 1) and pentest cleared.
3. Impact
- Client funded CHF > 3 M roadmap based on my architecture dossier
- Regression defects -70 % per release after proxy cut-over (Q4 2024)
- Test pipeline reduced from 60 → 40 min (-33 %) via Nuke-based re-write
- Developer satisfaction survey rose from 2.1 / 5 to 4.0 / 5 within three quarters
4. My Contributions
Diagnosed root-cause architecture flaws; authored 10-page strategy paper & roadmap; implemented containerisation, gateway PoC and vertical-slice template; rewrote CI pipeline in Nuke; mentored 8 engineers on modern .NET patterns.
5. Key Challenges & Mitigations
| Challenge | Mitigation & Result |
|---|---|
| Tight coupling of DB & business logic | Strangler proxy → new bounded contexts w/ vertical slices — decoupled deployments |
| Failing "Onion" layers | Re-established Application layer, moved 42 mixed repositories out of Infrastructure |
| Slow test suite | Parallelised fixtures + in-memory DB ⇒ -20 min runtime |
| Low dev morale & churn | Introduced roadmap, guild sessions, code radar — turnover halted |
6. Degraded Legacy Backend Onion Architecture
