Lesson 8: Integration Testing, Error Handling, and Best Practices

Testing one program is hard; testing several programs talking to each other is harder, so the industry uses a testing pyramid. Unit tests check single functions in isolation and run in milliseconds. Integration tests check that components work together — does the API really write to the database? does the JSON parse into the right fields? End-to-end tests drive the whole system like a user would. A key technique for integration work is the mock (or stub): a fake stand-in for an external service. You cannot charge a real credit card in every test run, so you mock the payment gateway to return a scripted "success" or "declined" response. Most providers also offer a sandbox environment — a fake version of their real API with test credentials — which is why PayMongo, Maya, and similar services give developers test keys separate from live keys.

In an integrated system, failure is normal: networks drop, services time out, partners deploy bugs. Robust integration code plans for this. Patterns to know: timeouts on every external call (never wait forever); retries with exponential backoff — wait 1s, then 2s, then 4s before retrying, so a struggling service is not hammered; the circuit breaker — after repeated failures, stop calling the broken service for a while and fail fast, protecting your own app; and fallbacks — degrade gracefully, like showing cached shipping rates when the courier API is down. Equally important is logging: record every external call with a correlation id so you can trace one user's request across systems. Exam scenarios often describe a cascade ("the courier API slowed down and the whole store froze") and expect you to prescribe timeouts plus a circuit breaker.