Content-based routing (switch)

The switch structural endpoint picks a destination by the value of a metadata key. Output only.

output:
  switch:
    metadata_key: "country_code"
    cases:
      US: { kafka: { topic: "us_orders", url: "kafka-us:9092" } }
      EU: { nats:  { subject: "eu_orders", url: "nats-eu:4222" } }
    default: { file: { path: "/var/data/unroutable_orders.log" } }

A message whose key is missing or unmatched goes to default; without a default it is dropped.

Route on the payload, not just metadata

switch matches on metadata, not payload fields. To route on payload content, first promote the value into metadata. Two common ways:

• An endpoint that already emits a status key — e.g. http_status_code from an HTTP request, or mongodb.outcome from a Mongo upsert (see Upserts).
• transform with on_error: pass_through, which sets mqb.transform_error on failed records so you can shunt them aside:

output:
  middlewares:
    - transform: { schema_file: "schemas/order.json", on_error: pass_through }
  switch:
    metadata_key: "mqb.transform_error"
    cases: {}                                  # (no exact-match cases)
    default: { kafka: { topic: "orders", url: "localhost:9092" } }

Split HTTP responses by status

Pair switch with request, which forwards a response (or, on error/timeout, the original message) tagged with a status key:

output:
  request:
    to: { http: { url: "https://api.internal/score" } }
    forward_to:
      switch:
        metadata_key: "http_status_code"
        cases:
          "200": { nats: { subject: "ok", url: "nats://localhost:4222" } }
          "404": { file: { path: "not-found.jsonl" } }
        default: { file: { path: "other.jsonl" } }

See also Fan-out to send to all destinations instead of picking one.