CI/CD & Velocity
Test parallelization
Also known as: parallel testing
Test parallelization runs multiple tests at the same time — across threads, processes, or machines — instead of one after another, to shorten the total run.
Parallelization covers any concurrency: multiple workers on one machine (for example pytest-xdist or Jest workers) and multiple machines via sharding. It is the single biggest lever for fast CI feedback as suites grow.
The catch is isolation. Tests that share state, fixtures, ports, or data will fail unpredictably when run in parallel — which itself shows up as flakiness. Parallel-safe tests must be independent and not assume execution order.
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Last reviewed June 11, 2026