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QA Foundations

System integration testing

Also known as: SIT

System integration testing (SIT) verifies that separately built modules or systems work correctly together, focusing on the interfaces and data flow between components.

SIT runs after unit testing and before user acceptance testing. It targets the seams of a system (APIs, databases, message queues, and third-party services) where mismatches only appear once independently developed parts are combined.

Integration can proceed big-bang (everything at once), top-down, bottom-up, or sandwich. Common failures are contract mismatches, data-format errors, and timing or async issues, the last of which are a frequent source of flaky tests.

  • Tests the interfaces and data flow between components, not the components alone.
  • Sits between unit testing and user acceptance testing.
  • Approaches: big-bang, top-down, bottom-up, sandwich.

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Related terms

Smoke testing User acceptance testing Test pyramid Flaky test

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Last reviewed August 2, 2026