Does Lambda cold start affect NHS patient-facing applications?

Lambda cold starts are a clinical safety consideration for NHS patient-facing applications. Cold start: when a Lambda function is invoked after a period of inactivity, AWS must initialise the execution environment — adds 0.5–3 seconds to first response. For NHS applications: a 3-second delay on a medication alert or clinical decision support response is unacceptable. ClickMasters uses ECS Fargate for all NHS patient-facing applications — persistent container, no cold start, consistent sub-100ms API response time. Lambda is used for NHS event-driven background jobs (FHIR resource processing, batch analytics, scheduled reports) where cold start latency does not affect patient care. Lambda Provisioned Concurrency (eliminates cold start — pre-warmed instances) is an option for Lambda patient-facing functions, but adds cost and complexity.

What is the cost difference between Fargate and Lambda for a UK SaaS?

Cost comparison for a typical UK SaaS (1,000 daily active users, 100,000 API requests/day): (1) ECS Fargate (2 × 0.5vCPU, 1GB tasks, eu-west-2): approximately £180–£250/month. (2) Lambda (100K invocations/day × 200ms average, 256MB): approximately £15–£25/month. Lambda is 10× cheaper at this scale. Cost crossover: Fargate becomes cost-competitive at > 2M requests/day (Lambda pricing per invocation becomes significant). ClickMasters recommendation: start with Lambda for non-regulated UK SaaS (cost advantage is substantial at startup scale), migrate to Fargate if FCA or NHS regulatory requirements demand consistent latency, or if request volume makes Lambda expensive.

Lambda Vs Fargate Vs App Runner Uk 2026

AWS Lambda vs ECS Fargate vs App Runner — UK Serverless Compute (2026)

Updated: March 20269 min read

🇬🇧 UK💷 GBP📊 Comparison🔒 UK GDPR⚖️ UK Law

Direct Answer

ECS Fargate is ClickMasters' default for UK regulated applications (NHS, FCA): predictable latency, persistent container, and full VPC control (private subnets, Security Groups). Lambda is right for event-driven workloads (S3 triggers, SQS consumers, scheduled jobs). App Runner is right for simple public APIs where you want zero infrastructure configuration. Lambda cold starts disqualify it from NHS patient-facing applications where consistent response time is required. Compute platform choice affects UK GDPR data residency, NHS DTAC Domain 3 security controls, FCA PS21/3 availability, and operational cost.

Factor	ECS Fargate	AWS Lambda	AWS App Runner
UK GDPR data residency (eu-west-2)	✅ VPC private subnet — stays in account	✅ VPC Lambda — private subnet available	No VPC — public endpoint
NHS DTAC Domain 3 (network isolation)	✅ Security Groups, private subnet — strongest	✅ VPC Lambda — available	Not suitable for clinical data
FCA PS21/3 (consistent latency)	✅ No cold start — persistent container	Cold start (up to 3s) — unsuitable for IBS	Warm start — better than Lambda
FCA PS21/3 (availability measurement)	✅ ALB health check — continuous IBS monitoring	Lambda concurrency limits — less predictable	App Runner health check available
Cost at medium scale (UK startup)	~£200–£400/month (2 tasks)	✅ £0 to ~£100/month (< 1M invocations free tier)	~£80–£200/month
Container image (Docker)	✅ Full Docker support	Lambda container images — supported	✅ Full Docker support
Auto-scaling	✅ ECS Service Auto Scaling — fine-grained	✅ Automatic concurrency scaling	✅ Automatic scaling
Deployment complexity	Medium — CDK task definition	Low — Lambda function zip/image	✅ Lowest — just push Docker image
ClickMasters default	✅ All NHS/FCA/GDS regulated workloads	S3/SQS/scheduled event-driven jobs	Non-regulated APIs (demo, public docs)