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Advanced Fan-Out Patterns

After reviewing the basic fan_out broker, use these variants as unverified adaptation sketches.

Pattern 1: Failover with the try Broker​

While fan_out sends to all outputs, the try pattern sends to them in order, stopping at the first success. This is perfect for creating a primary/fallback setup.

Use Case: You want to send data to a primary Elasticsearch cluster, but if that is down, you want to fall back to a secondary cluster, and if that is also down, write to a local file.

Failover Routing with the 'try' Broker
output:
  broker:
    pattern: try # Tries each output in order until one succeeds
    outputs:
      # 1. Primary Destination
      - elasticsearch:
          urls: [ ${ES_PRIMARY_ENDPOINT} ]
          index: "primary-index"
          retries: 2 # Try a couple of times before failing over

      # 2. Secondary/Fallback Destination
      - elasticsearch:
          urls: [ ${ES_SECONDARY_ENDPOINT} ]
          index: "secondary-index"
          retries: 2

      # 3. Last Resort: Local File
      - file:
          path: /tmp/es_fallback_buffer.jsonl
          codec: lines

Pattern 2: Conditional Routing within a Fan-Out​

Sometimes a message should reach several branches while a conditional branch accepts a subset. Nest a switch inside the broker to express that selection.

Example: Send the authored logs to Kafka and S3, and select ERROR records for the alerting-service branch.

Conditional Fan-Out Routing
output:
  broker:
    pattern: fan_out
    outputs:
      # Destination 1: Kafka (all messages go here)
      - kafka:
          addresses: [ ${KAFKA_BROKERS} ]
          topic: "all-logs"

      # Destination 2: S3 (all messages go here)
      - aws_s3:
          bucket: "my-log-archive"
          path: "logs/${!timestamp_unix_date()}/${!uuid_v4()}.jsonl.gz"

      # Destination 3: Alerting Service (errors selected here)
      - switch:
          cases:
            - check: this.level == "ERROR"
              output:
                http_client:
                  url: "http://my-alerting-service/ingest"
                  verb: "POST"
            # If the check fails, the 'switch' does nothing,
            # effectively dropping the message for this branch.

This powerful combination allows you to build complex, business-aware routing logic while maintaining the core efficiency of the fan-out pattern.