Fan-out

The fanout structural endpoint publishes each message to every listed endpoint. Output only. The value is a plain list — each branch may have its own middleware and may itself be structural.

output:
  fanout:
    - kafka: { topic: "audit", url: "localhost:9092" }
    - file: { path: "audit.jsonl" }
    - nats: { subject: "audit", url: "nats://localhost:4222" }

All branches receive the same message. Use it to mirror a stream to an archive while it also flows to its primary sink, to tee traffic to an audit log, or to emulate a static subscriber set (one branch per subscriber).

Fan-out vs. switch

• fanout → every destination gets the message.
• switch → one destination, chosen by a metadata key.

Give one branch its own reliability

Because each branch is a full endpoint, you can wrap just the fragile one with retry/dlq while the others stay plain:

output:
  fanout:
    - kafka: { topic: "orders", url: "localhost:9092" }
    - middlewares:
        - retry: { max_attempts: 5 }
        - dlq: { endpoint: { file: { path: "audit-failed.jsonl" } } }
      http: { url: "https://audit.internal/ingest", method: "POST" }