Dev Team Cuts CI Time from 43 to 4 Minutes Using Per-Test Coverage Mapping

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A development team reduced their continuous integration pipeline runtime by roughly 90% by leveraging per-test code coverage data to run only tests relevant to changed files. Instead of executing all tests or relying on filename pattern matching, they built a reverse map linking each source file to the specific tests that execute it. Using Git diff output, the system queries this map to identify and run only the affected tests after any code change. The coverage map is built using standard tools like Istanbul, coverage.py, SimpleCov, and JaCoCo, with roughly 500 lines of instrumentation code across any language. The approach also reduced CI costs from around $12 to $1.20 per run and improved test selection accuracy from 15% to over 95%.

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