Why Performance Testing Catches What Unit Tests Miss

Unit tests verify individual functions, but they can't simulate real-world load or concurrency issues. In this episode, Lucas and Luna explore a real case: a fintech startup whose checkout service passed every unit test but collapsed under Black Friday traffic because of a subtle database connection-pool leak. They break down why performance testing—specifically load testing with tools like k6 and Locust—caught a bug that unit tests missed entirely. Lucas explains the difference between latency spikes and throughput degradation, and why a single shared mutable state caused cascading failures. Luna pushes back on the common objection that performance tests are too expensive to run in CI, and they discuss practical strategies: running smoke-level load tests on every pull request, reserving full-scale tests for staging, and using APM tools to correlate response times with database query plans. The episode ends with a reflection on how performance testing forces developers to think about the production environment as a system, not just a collection of functions. #PerformanceTesting #LoadTesting #UnitTests #SoftwareTesting #QA #Fintech #DatabaseConnectionPool #ConcurrencyBugs #k6 #Locust #APM #CI #CICDPipeline #ProductionReadiness #Technology #SoftwareEngineering #FexingoBusiness #BusinessPodcast Keep every episode free: buymeacoffee.com/fexingo