Integration Testing

Integration testing verifies that different modules or components of an application work together correctly once combined — catching errors that arise specifically from integration, which neither isolated unit testing nor whole-system testing targets as directly.

Integration strategies

• Big Bang — integrate all components at once and test the whole combination together; simplest to set up, hardest to isolate a failure in when something breaks.
• Top-Down — start from the topmost module, progressively adding lower-level modules (often using stubs for not-yet-integrated lower components).
• Bottom-Up — start from the lowest-level modules, progressively adding higher-level ones (often using drivers to simulate not-yet- integrated callers).
• Hybrid (Sandwich) — a combination of top-down and bottom-up, meeting in the middle.

Top-down and bottom-up isolate integration failures more precisely than Big Bang, at the cost of needing stubs/drivers and a more staged test plan.

Where it fits in the sequence

Integration testing typically follows Unit Testing (individual units already verified in isolation) and precedes System Testing (the whole system verified as one), catching interaction defects before they reach the more expensive, harder-to- diagnose whole-system testing stage.

Automation

Preferred when an application updates frequently and has many modules — automated integration tests can be re-run on every change far more practically than repeated manual testing of the same interaction points.

Benefits

Earlier defect detection (before reaching end-to-end or production), reduced overall development cost (an integration bug found here is cheaper to fix than the same bug found in system testing or production), and improved confidence that the pieces actually cooperate, not just that each piece individually works.

Common pitfalls

• Skipping straight to Big Bang integration for a large system — makes isolating which interaction actually failed much harder than a staged top-down/bottom-up approach would.
• No stubs/drivers plan for top-down or bottom-up integration — without them, a staged integration strategy can't actually test components before their real counterparts exist.
• Treating integration tests as a substitute for unit tests — a passing integration suite with no unit coverage underneath it makes failures much harder to localize to a specific root cause.

Learn more

• Unit Testing for the isolated-component layer below integration testing.
• System Testing, End to End Testing for the whole-system layer above it.