What Is Fan-Out/Fan-In?
Fan-out/fan-in is a pipeline pattern where one stage spreads work across many parallel jobs (fan-out), and a later stage waits for all of them and combines their outputs (fan-in). It is how CI runs hundreds of test shards or build targets at once, then aggregates results into a single pass/fail or merged artifact.
Why it matters
Fan-out turns a long serial job into many short parallel ones, cutting wall-clock time. The fan-in step is the synchronization point that produces a single verdict, merged coverage report, or combined release artifact.
Related concepts
- Also called scatter-gather
- Job matrices are a declarative way to fan out
- The fan-in job depends on all fanned-out jobs completing
Related guides
What Is Scatter-Gather?Scatter-gather distributes a task across many workers in parallel (scatter) and then collects and merges thei…
What Is a Job Matrix?A job matrix expands a single job definition into many parallel jobs across combinations of variables like OS…
What Is Test Sharding?Test sharding splits a test suite into independent subsets that run on separate machines in parallel, cutting…