PrincipalAPI & Integration Testing
How would you design an API test suite for a payment processing system?
- payment testing
- idempotency
- financial systems
Answer it first, then compare. Nothing here is saved or scored.
Model answer
Critical areas: (1) Happy path, successful charge, refund, subscription creation. (2) Payment failures - declined cards, insufficient funds, expired cards (use test card numbers). (3) Idempotency - duplicate charge requests don't double-charge. (4) Webhooks - verify event delivery for charge.succeeded, charge.failed, subscription.canceled. (5) Currency handling - decimal precision, conversion. (6) PCI compliance - ensure no raw card data in logs or responses. (7) Concurrency - race conditions on balance updates. Use sandbox/test mode exclusively.
What interviewers listen for
- Schema-first thinkingYou validate response shape before you assert on values, every time.
- Status-code literacyYou can defend why 401 vs 403 vs 422 matters for the consumer.
- Contract awarenessYou know when consumer-driven contract tests beat end-to-end checks.
- Auth fluencyYou can describe rotating tokens across parallel workers without secrets in logs.