SDET System Design Interview: How to Design Test Infrastructure for E-Commerce at Scale

“Design the test automation strategy for an e-commerce platform with 15 microservices, 100K daily orders, and weekly releases.” This is the SDET system design question. Here is how to ace it.

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The 5-Step Framework

1. Clarify (2 min): Team size? Current testing? Biggest pain point? Deploy frequency?
2. Test Pyramid (3 min): Draw layers, assign tools, estimate counts
3. Pipeline Design (5 min): PR gate → merge gate → nightly → release gate
4. Data Strategy (3 min): API factories, Testcontainers, cleanup
5. Observability (2 min): Allure reports, Grafana dashboards, Slack alerts

E-Commerce Test Architecture

Layer	What	Tool	Count
Unit	Business logic per service	Jest	2000+
Contract	Inter-service APIs	Pact	150
Integration	Service + DB	Testcontainers	500
E2E	Critical paths	Playwright	50
Performance	Checkout under load	k6	10 scenarios

Critical Path E2E Tests

• User registration and login
• Product search and filtering
• Add to cart and quantity update
• Checkout with payment (mocked Stripe)
• Order confirmation and email
• Order history and tracking

Pipeline Design

PR Gate (< 10 min):
  Unit + Contract + Lint + Type Check

Merge Gate (< 20 min):
  Integration + E2E smoke (10 critical paths)

Nightly (< 60 min):
  Full E2E + Performance + Security scan

Release Gate:
  Release confidence score > 85%
  Zero P1 open bugs
  Human sign-off

What Interviewers Evaluate

• Systems thinking: Do you consider all layers, not just E2E?
• Trade-off analysis: Why Playwright over Cypress? Why Pact over integration?
• Practical experience: Can you name real tools, real numbers?
• Communication: Did you draw diagrams? Explain decisions?