Scatter-Gather

The Scatter-Gather is a Message Router from Hohpe and Woolf. It broadcasts a copy of the exchange to a fixed list of endpoints in parallel and gathers the responses into one aggregate exchange.

- id: scatter-gather-demo
  from: timer:tick?period=1000&repeatCount=3
  steps:
    - scatter_gather:
        endpoints:
          - direct:pricing
          - direct:inventory
          - direct:reviews
        aggregation: collect_all
    - to: log:aggregated?showBody=true

The scatter_gather step sends the same exchange to every endpoint listed under endpoints. All dispatches run in parallel. The aggregation field picks the strategy that merges the responses. The default is last_wins, which keeps the body of the last branch to complete. Set aggregation: collect_all to assemble every branch body into a JSON array. The step after scatter_gather receives the merged result.

Scatter-Gather is DSL sugar over Multicast. The YAML parser lowers the step to a multicast block with parallel: true and the chosen aggregation. No new processor or Rust builder method exists for it. Rust code calls .multicast() directly. Use Scatter-Gather when the broadcast-and-collect shape is the point. Use a raw multicast block when you need the extra knobs it exposes: parallel_limit, stop_on_exception, or timeout_ms.

The endpoints are fixed in the route definition. A route that must compute destinations at runtime uses a Recipient List. The gather is also stateless. No correlation key or completion condition exists. The step collects all parallel responses in one pass and moves on. Stateful accumulation across many exchanges is the Aggregator EIP.

Per ADR-0025, the lowered multicast compiles into a MulticastSegment that operates on PipelineOutcome. Per ADR-0001, the segment runs as a Service<Exchange> step in the Tower pipeline. The processor contract is documented in camel-processor/CONTEXT.md.