openprose__prose/packages/std/patterns/fan-out.prose.md

name, kind

name	kind
fan-out	pattern

Fan-Out

Parallel delegation without reduction. Send briefs to N delegates, collect all results. The instantiating system decides how to use them.

Metadata

• version: 0.1.0
• role: coordinator

Slots

• delegates

Config

• None. The instantiating system supplies delegate bindings with with: and briefs as system inputs.

Invariants

• Each delegate receives exactly one brief
• Delegates remain independent and do not see each other's outputs
• Result order matches the order of the delegates slot instances
• The pattern never merges, ranks, or synthesizes delegate outputs

Shape

• self: dispatch briefs to delegates in parallel, collect all results, return collection
• delegates:
  • delegate_1..delegate_N: execute assigned brief
• prohibited: merging or synthesizing results — that is the instantiating system's job

Requires

• Pattern instance receives: delegates: string[] -- service or system names for each delegate briefs: string[] -- one brief per delegate (same length as delegates) OR a single string applied to all delegates

Ensures

• Each delegate receives exactly one brief
• All delegates execute in parallel
• No delegate knows other delegates exist
• Results are returned as an ordered array matching the input delegates
• No merging or synthesis — the raw results are the output
• result: array of all delegate outputs
• results: same array, provided as a keyed alias

Delegation

let results = parallel for delegate, index in delegates:
  call delegate
    brief: briefs[index]
    fallback_brief: task_brief

return {
  result: results,
  results: results
}

Notes

No delegate knows other delegates exist. Each receives a brief and returns a result. The fan-out pattern collects results but does not interpret them — interpretation is the instantiating system's responsibility.

Different from map-reduce: map-reduce includes a reducer that merges results into a single output. Fan-out returns the raw collection. Use fan-out when the instantiating system needs to see all results individually (e.g., to compare, to select, to present side-by-side). Use map-reduce when the goal is a single merged artifact.

Different from race: fan-out waits for ALL delegates. Race returns the FIRST acceptable result and cancels the rest.