System Integration Testing
System integration testing verifies that an entire system works correctly with other external systems it depends on — third-party APIs, partner services, payment gateways, legacy platforms — as opposed to integration testing between modules built internally on the same team.
The distinction from ordinary integration testing is organizational as much as technical: system integration testing usually crosses a boundary the team doesn't fully control — a vendor's API, another company's system, infrastructure owned by a different internal team — which means test environments, data, and even uptime aren't fully within the testing team's control.
Because of that boundary, service virtualization and mocking are common tools here: simulating the external system's behavior so testing can proceed even when the real dependency is unavailable, rate-limited, or expensive to call repeatedly.
