# Fan-out

**Source:** https://promtable.com/glossary/fan-out

> Fan-out is the workflow pattern where one event spawns many parallel jobs / steps — process N records in parallel, call N APIs concurrently, generate N variants. The complementary 'fan-in' joins them back.

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Fan-out is the workflow pattern where one event spawns many parallel jobs / steps — process N records in parallel, call N APIs concurrently, generate N variants. The complementary 'fan-in' joins them back.

Fan-out unlocks parallelism without writing concurrency code. A single event ('user uploaded 50 images') spawns 50 parallel jobs that all run; fan-in waits for all 50 to complete + aggregates. Workflow engines (Inngest, Temporal, AWS Step Functions, Trigger.dev v3) provide first-class fan-out primitives that handle retries per-branch, partial-failure aggregation, and concurrency limits. AI use cases in 2026: parallel image generation (10 variants of one prompt), bulk classification (10K records at 50 concurrent), agent self-consistency (run the same agent 5 times, take majority vote). Trade-offs: fan-out can overwhelm downstream APIs without concurrency caps; debugging gets harder as parallel branches diverge.

## When to use

- Parallel batch work, independent jobs.
- AI variant generation, self-consistency sampling.

## Common mistakes

- No concurrency limit — fan-out hammers downstream APIs.
- Missing fan-in — workflow proceeds without waiting for all branches.

## Related terms

- [self-consistency](https://promtable.com/glossary/self-consistency)
- [background-job](https://promtable.com/glossary/background-job)
- [workflow-engine](https://promtable.com/glossary/workflow-engine)

*Last updated: 2026-06-01*
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Original page: https://promtable.com/glossary/fan-out
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