Microservices Decision Framework — UK Regulated Software
| Criteria | Microservices | Modular Monolith | ClickMasters Guidance |
|---|---|---|---|
| Team size | 3+ independent teams (20+ engineers) | 1–10 engineers | Monolith under 10 engineers — every time |
| FCA PS21/3 IBSs | Each IBS = natural service boundary | Single deployment — all IBSs in one system | FCA IBS mapping often justifies microservices post-Series B |
| NHS DTAC data isolation | Database-per-service — strongest isolation | PostgreSQL RLS — equally strong isolation | PostgreSQL RLS achieves DTAC Domain 2 in a monolith |
| Independent deployment cadence | Different services deploy at different rates | All modules deploy together | Need independent deployment? Go microservices |
| Scaling requirements | Scale individual services independently | Scale the whole application | Genuine 10× scaling disparity? Microservices help |
| Operational complexity | High — service mesh, distributed tracing | Low — single deployment, single database | Operational cost is real — do not underestimate |
| UK GDPR data isolation | Database-per-service — clearest isolation | PostgreSQL RLS — equally effective | Both achieve UK GDPR isolation — monolith is simpler |
UK Regulatory Benefits of Microservices (Real, Not Imagined)
Warning: "Microservices are more secure / more GDPR compliant" is a myth for most UK projects. PostgreSQL Row Level Security achieves equivalent data isolation in a monolith. Do not migrate to microservices for GDPR compliance alone — the benefit is marginal and the operational cost is real.
Microservices Operational Requirements — UK Cost Implications
| Operational Requirement | Monthly Cost (AWS eu-west-2) | Skill Required | UK Compliance Value |
|---|---|---|---|
| Service mesh (AWS App Mesh) | £50–£200/month | Platform engineering | mTLS between services — Cyber Essentials network access control |
| Distributed tracing (AWS X-Ray) | £20–£80/month | DevOps | FCA audit trail — cross-service request tracing |
| Per-service CI/CD pipelines | GitHub Actions: additional minutes | DevOps | FCA PS21/3 — independent deployment audit trail |
| Container orchestration (ECS Fargate) | £80–£400/service/month | Platform engineering | Cyber Essentials — managed OS patching |
| API Gateway (AWS API Gateway) | £3.50/million API calls | Backend engineering | FCA COBS — API rate limiting and authentication |
| Service registry and discovery | Route 53 + ECS Service Discovery: negligible | DevOps | Operational resilience |