CI/CD & Velocity
Test sharding
Also known as: sharding
Test sharding splits a test suite into multiple subsets (shards) that run on separate machines or CI jobs at the same time, cutting total wall-clock time.
Sharding is horizontal scaling for tests: if one machine runs 1,000 tests in 40 minutes, ten shards of 100 tests each can finish in roughly a tenth of the time, minus overhead. Many runners (Playwright, Jest, pytest with plugins) support sharding natively.
Sharding aggregates afterward: each shard produces its own results file, and you combine them into one report for the run. Balancing shards by historical duration keeps any single shard from becoming the straggler.
- Splits a suite across parallel machines/jobs to cut wall-clock time.
- Each shard emits its own results, then they’re aggregated into one report.
- Balance shards by past duration to avoid a slow straggler.
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Last reviewed June 11, 2026