System Design for SDET Interview Questions

Understand the service, its users and dependencies, then rank risks. Map each risk to the cheapest test layer that catches it (unit, component, contract, integration, a few end-to-end), plan environments, data and non-functional testing, wire tests into CI with quality gates, and monitor in production.

Focus on money-correctness risks: idempotency and duplicate charges, state machine transitions, amounts and currency rounding, failure and timeout handling with the payment provider, reconciliation, security and compliance. Use provider stubs and sandboxes, contract and concurrency tests, and production reconciliation.

Test each service in isolation against its events (consume and produce), use contract or schema compatibility checks for messages, run integration tests with a real broker in containers, and keep a few end-to-end flows that poll for outcomes. Test duplicates, ordering, retries, dead-letter queues and observability.

It means verifying behaviour in the real environment with controls that limit blast radius: feature flags, canary and progressive rollouts gated on metrics, synthetic monitoring of critical journeys, post-deploy smoke checks, and observability. It complements pre-production testing; it never replaces it.