Test Levels & Types

A developer runs unit tests on the `calculateDiscount()` function in isolation, with stubs for the pricing service.

A QA team verifies that the payments service and the invoicing service correctly exchange transaction data over their API contract, without test doubles for either side.

The performance team runs a load test with 5,000 concurrent users against the complete system deployed in staging, measuring response times under stress.

Before go-live, business representatives run through the complete purchase flow following the agreed acceptance criteria, to decide whether the system is ready for production.

After applying a hotfix in production, the team re-runs the exact test case that previously failed, against the complete deployed system, to confirm the defect has been fixed.

A developer writes tests based on a module's internal structure, designed to exercise every branch of its decisions (branch coverage), with direct access to the source code.

A UX team conducts an end-to-end usability testing session on the complete product, measuring how easily participants complete typical tasks.

After the backend team changes a payload's structure, contract tests are run between the mobile app and its backend to confirm the interface between them still works after the change.

A group of real customers tests a beta version of the product in their own everyday environment, evaluating whether the software meets their needs before general release.

A security specialist performs a penetration test on the complete, already-deployed application, looking for exploitable vulnerabilities across the system as a whole.

After refactoring the internal messaging bus, the team runs a regression suite focused on the boundaries between the modules that communicate through it, to detect side effects of the change.

A developer runs a performance micro-benchmark on an isolated parsing function, measuring whether its execution time stays within a defined budget.