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Promote Inbound Metadata into the Body

This page collects four worked pipelines that use the metadata processor's promote block to copy input-supplied message metadata into the JSON body as fields. The driving use case is SCADA / IIoT — a Sparkplug B device stamps spb_metric_name, spb_datatype, spb_timestamp, … on every metric and emits a scalar value as the body. Without promote, every pipeline that lands these to a columnar store has to repeat the same mapping block per field. With promote, it's one declarative section.

The feature is fully general: anywhere an input stamps metadata you want as columns, promote removes the boilerplate.

For the full configuration surface, see the promote field reference.

1. Sparkplug B — scalar body, ~20 spb_* keys​

The canonical SCADA shape. A Sparkplug B input emits each metric as a scalar value (e.g. 23.5) and stamps the metric name, datatype, timestamp, sequence, alias, quality, source device, and a dozen other fields as spb_* metadata. Promote them all in one section, rename a few for column-friendly names, cast the numeric and boolean ones, and stamp Expanso provenance on top.

pipeline:
  processors:
    - metadata:
        include: [core, pipeline, node]
        target: body
        format: flat
        promote:
          match: "spb_"                 # every spb_* key the input stamped
          rename:
            spb_metric_name: metric
            spb_datatype: datatype
            spb_timestamp: timestamp
            spb_sequence: sequence
            spb_is_historical: is_historical
            spb_alias: alias
            spb_device_id: device_id
          cast:
            spb_timestamp: int           # epoch millis -> int64
            spb_sequence: int            # int alias counter
            spb_is_historical: bool      # "true"/"false" -> bool
          wrap_key: value                # 23.5 -> { "value": 23.5, ... }

output:
  stdout:
    codec: lines

Input message (the body is the scalar 23.5; metadata is set by the input):

body: 23.5
metadata:
  spb_metric_name:    "Temperature"
  spb_datatype:       "Float"
  spb_timestamp:      "1730000000000"
  spb_sequence:       "42"
  spb_is_historical:  "false"
  spb_alias:          "17"
  spb_device_id:      "plc-12"

Output:

{
  "value": 23.5,
  "metric": "Temperature",
  "datatype": "Float",
  "timestamp": 1730000000000,
  "sequence": 42,
  "is_historical": false,
  "alias": "17",
  "device_id": "plc-12",
  "run_id": "01HF...",
  "job_name": "scada-ingest",
  "job_namespace": "default",
  "event_time": "2026-04-30T12:34:56.789Z",
  "producer": "https://expanso.io/edge/v1.4.2",
  "pipeline_name": "scada-ingest",
  "pipeline_version": "v3",
  "node_id": "edge-eu-west-1-a",
  "hostname": "edge-01",
  "region": "eu-west-1",
  "environment": "production",
  "cluster_name": "edge-eu",
  "agent_version": "1.4.2"
}

Walking through the configuration:

  • target: body, format: flat — promote requires target: body. Flat puts every field at the JSON root so downstream Parquet / Delta sees them as top-level columns. Switch to format: nested (with body_key) if you want a nested structure.
  • match: "spb_" — a literal prefix match. match matches by prefix unless wrapped in slashes (/regex/).
  • rename: — keyed by the inbound metadata key, even when a cast for the same key is also configured.
  • cast: — int, float, or bool. A cast that fails to parse (or, for float, NaN / ±Inf) falls back to the string value and does not increment error_count — a recoverable cast fallback is not a pipeline error.
  • wrap_key: value — the body 23.5 is not an object, so promote wraps it as { "value": 23.5 } before adding fields. Without promote, a non-object body is passed through unchanged with a warning — wrapping only happens when promote is active.
  • Precedence: original body < promoted fields < resolved Expanso provenance. The resolved run_id, node_id, pipeline_name, … always win, so an inbound spb_node_id (if some input stamps one) cannot spoof Expanso's own provenance.

2. Kafka headers — promote selected headers as columns​

A Kafka input stamps every header as kafka_<header-name> metadata. Promote only the ones you want as columns, leave the noisy ones alone, and cast the timestamp.

input:
  kafka:
    addresses: [ "broker:9092" ]
    topics: [ "orders.events" ]
    consumer_group: "edge-orders"

pipeline:
  processors:
    - metadata:
        target: body
        format: flat
        promote:
          names:
            - kafka_tenant_id
            - kafka_event_type
            - kafka_correlation_id
            - kafka_ingest_timestamp
          rename:
            kafka_tenant_id: tenant
            kafka_event_type: event_type
            kafka_correlation_id: correlation_id
            kafka_ingest_timestamp: ingest_ts
          cast:
            kafka_ingest_timestamp: int

output:
  aws_s3:
    bucket: orders-events
    path: ${! @tenant }/${! timestamp_unix() }.json
    codec: lines

Why names and not match. Kafka stamps a lot of internal kafka_* keys (kafka_topic, kafka_partition, kafka_offset, …). A broad match: "kafka_" would pull all of them into the body. Listing the specific headers via names is more deliberate and keeps the columnar schema tight.

3. MQTT topic parts via regex​

An MQTT input stamps the full topic as mqtt_topic. To break a topic like building/floor-3/zone-12/temperature into building, floor, zone, and metric columns, use a mapping step to split the topic into per-part metadata keys and then promote them with a regex match:

input:
  mqtt:
    urls: [ "tcp://broker:1883" ]
    topics: [ "building/+/+/+" ]

pipeline:
  processors:
    # First, split the topic into per-part metadata keys.
    - mapping: |
        let parts = metadata("mqtt_topic").split("/")
        meta topic_building = $parts.index(0).or("")
        meta topic_floor    = $parts.index(1).or("")
        meta topic_zone     = $parts.index(2).or("")
        meta topic_metric   = $parts.index(3).or("")

    # Then promote everything that starts with topic_ into the body.
    - metadata:
        target: body
        format: flat
        promote:
          match: "/^topic_/"            # regex: leading slashes are the regex marker
          rename:
            topic_building: building
            topic_floor: floor
            topic_zone: zone
            topic_metric: metric
          wrap_key: payload             # MQTT body is opaque bytes — wrap as "payload"

output:
  parquet:
    path: /var/lib/expanso/parquet/${! @building }-${! timestamp_unix() }.parquet

/^topic_/ is a regex because it is wrapped in slashes. A bare topic_ would be a literal prefix; both forms work here, but regex gives you the full power of Go's regexp package when you need it (lookahead, character classes, alternation). The empty regex // is rejected at submission — it would match every metadata key.

4. Promote file_path from the tail input​

The tail input stamps every line with file_path (absolute) and file_name (basename) metadata. Promote them into the body so downstream stores see them as columns, and use the promote block alongside lineage stamping:

input:
  tail:
    paths: [ /var/log/myapp/*.log ]

pipeline:
  processors:
    - mapping: |
        # Try to parse JSON logs, fall back to raw text.
        let parsed = this.parse_json_object()
        root = if parsed.exists() { parsed } else { { "message": this } }

    - metadata:
        include: [core, pipeline, node]   # also stamp Expanso provenance
        target: body
        format: nested
        body_key: provenance
        promote:
          names: [file_path, file_name]
          rename:
            file_path: source_file
            file_name: source_basename

output:
  http_client:
    url: https://ingest.example.com/v1/logs
    verb: POST

What you get. Every emitted message has source_file and source_basename at the JSON root (alongside whatever the upstream mapping step produced), plus a provenance object carrying run_id, node_id, pipeline_name, etc. The nested-format precedence rule still applies inside provenance (promoted < resolved); the root-level fields from promote sit outside the nested object.

Common variations​

Promote everything except reserved fields​

match: "/.+/" selects every metadata key. The runtime drops any key whose source name or rename target is reserved (node_id, pipeline_id, node_label_*, …) — see the reserved-name guard — so a permissive match cannot leak Expanso's own provenance.

promote:
  match: "/.+/"

A bare empty regex (match: "//") is rejected at submission instead of silently being a "match-everything" pattern — that ambiguity was deliberately closed in the schema.

Promote with format: nested​

If your downstream consumer expects a nested structure rather than flat columns, set format: nested and body_key. Promoted fields go inside the nested object alongside the resolved Expanso fields:

- metadata:
    include: [core, pipeline]
    target: body
    format: nested
    body_key: scada
    promote:
      match: "spb_"
      rename: { spb_metric_name: metric }

A scalar body 23.5 becomes:

{
  "value": 23.5,
  "scada": {
    "metric": "Temperature",
    "datatype": "Float",
    "run_id": "01HF...",
    "node_id": "edge-eu-west-1-a"
  }
}

wrap_key defaults to value. In nested format, wrap_key must not equal body_key — the nested metadata object would otherwise clobber the wrapped body value, and the validator rejects this at submission.

Cast a metadata value to float, with NaN-safe fallback​

A device that occasionally emits "NaN" or "-" for an unset reading would otherwise drop the entire record (a non-finite float fails the body marshal). The float cast deliberately treats NaN / ±Inf / out-of-range as a parse failure and falls back to the original string:

promote:
  match: "spb_"
  cast:
    spb_value: float

A reading of "23.5" becomes the float 23.5; a reading of "NaN" is kept as the string "NaN". Cast fallbacks are logged at debug only — they do not increment error_count, so the false-positive alert risk on a noisy source is zero.

Next Steps​