`JSON-cppagent-mqtt`: `Header.testIndicator` absent on MQTT Streams Header

[ottobolyos]

Summary

Under the JSON-CPPAGENT-MQTT document format, the MTConnectStreams.Header object omits testIndicator entirely, while cppagent's JSON v2 printer emits testIndicator: false (the XSD-optional attribute's default-false value) on every Header. The XSD marks testIndicator as optional, so the absence is not an XSD violation — but the JSON-CPPAGENT-MQTT format label promises cppagent-JSON-v2 compatibility, and cppagent consistently emits the field.

Environment

• MTConnect.NET-Applications-Agents + MTConnect.NET-JSON-cppagent at v6.9.0 (official Docker image trakhound/mtconnect.net-agent:6.9.0, published 2025-10-16 — binary-equivalent to v6.9.0.2).
• Broker: eclipse-mosquitto:2.0.22.
• Comparison agent: mtconnect/agent:latest = cppagent v2.7.0.7 with JsonVersion = 2.
• Format ID: JSON-CPPAGENT-MQTT.

Reproduction

Minimum rig — mqtt-relay module pointed at local Mosquitto:

modules:
- mqtt-relay:
    server: localhost
    port: 1883
    documentFormat: JSON-CPPAGENT-MQTT
    topicPrefix: MTConnect/Document
    topicStructure: Document

Run the agent, capture a Current payload, and inspect the Header:

mosquitto_sub -h localhost -t 'MTConnect/Document/Current/<uuid>' -C 1 \
  | jq '.MTConnectStreams.Header'

Observed — MT.NET

{
  "instanceId": 1776717208,
  "version": "6.9.0.0",
  "sender": "9e17047e9163",
  "bufferSize": 150000,
  "firstSequence": 1,
  "lastSequence": 36,
  "nextSequence": 37,
  "deviceModelChangeTime": "2026-04-20T20:33:28.8491672Z",
  "creationTime": "2026-04-20T20:33:39.9655145Z"
}

No testIndicator property.

Expected — cppagent JSON v2 reference

Run mtconnect/agent:latest with JsonVersion = 2 and the same MqttService sink, capture the same topic:

{
  "version": "2.7.0.7",
  "creationTime": "2026-04-20T19:28:33Z",
  "testIndicator": false,
  "instanceId": 1776713243,
  "sender": "localhost",
  "schemaVersion": "2.7",
  "deviceModelChangeTime": "",
  "bufferSize": 131072,
  "nextSequence": 104,
  "lastSequence": 103,
  "firstSequence": 1
}

cppagent emits testIndicator: false even when the agent is not in test mode — the field is always present so consumers don't need hasOwnProperty checks.

Evidence files: current.json + cppagent-current.json.

Authority

• XSD: MTConnectStreams_2.7.xsd (and every v2.x back to v1.3) declares testIndicator on HeaderAttributesType with use="optional":

  <xs:attribute name='testIndicator' type='xs:NMTOKEN' use='optional'/>

  XSD optionality means an emitter is not required to write the attribute. It does not forbid writing it with a default value.

• cppagent JSON v2 reference: the printer source and test_package/json_printer_stream_test.cpp both show testIndicator always emitted. The JSON-CPPAGENT-MQTT format label is an explicit compatibility promise with this output; consumers that select the format ID are entitled to expect the field present.

• XMI: testIndicator is defined on HeaderAttributesType (and on every concrete Header complex type) in MTConnectSysMLModel.xml. It has no normative multiplicity > 0, confirming optionality.

Root cause — library source

libraries/MTConnect.NET-JSON-cppagent/Streams/JsonStreamsHeader.cs does declare TestIndicator with [JsonPropertyName("testIndicator")]:

[JsonPropertyName("testIndicator")]
public bool TestIndicator { get; set; }

…and it is populated from the source header. But the System.Text.Json serialiser options in use apparently skip default-value booleans (false is the zero-value for bool), so testIndicator never reaches the wire. Candidates to verify:

• JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault somewhere in the formatter pipeline.
• Or the property is written via a custom converter that skips defaults.

Impact

• Low on its own. testIndicator is XSD-optional and consumers that tolerate its absence work fine.
• Tightens the JSON-CPPAGENT-MQTT format-label contract: the cppagent reference always emits this field, so strict cppagent-compat consumers may expect it present.
• Slightly raises the bar for cross-implementation test fixtures — diff-based integration tests between MT.NET and cppagent payloads need to whitelist this difference.

Suggested fix

1. Ensure testIndicator serialises unconditionally — either by changing the serialiser's ignore-condition for this property to Never, or by wrapping the property in a nullable form and always assigning false explicitly. Match cppagent's "always emit" behaviour.
2. Same treatment for JsonDevicesHeader.TestIndicator and JsonAssetsHeader.TestIndicator if they exist.
3. Add a unit test that round-trips a Header with TestIndicator = false and asserts the serialised JSON contains "testIndicator": false.

Stability across MTConnect versions

testIndicator has been optional on HeaderAttributesType since v1.3+ with no semantic change. cppagent's "always emit" behaviour has been stable across its JSON v2 lineage. Fix is version-agnostic.

Related issues

• JSON-cppagent-mqtt: Header.schemaVersion absent on MQTT Streams Header #130 — Header.schemaVersion absent — another missing Header attribute from the same class.
• Header.version reports library assembly version instead of MTConnect release #127 — Header.version reports library assembly version — adjacent Header defect.
• MTConnectStreams.schemaVersion / MTConnectDevices.schemaVersion hardcoded to "2.0" in JSON-cppagent-mqtt formatter #128 — MTConnectStreams.schemaVersion / MTConnectDevices.schemaVersion hardcoded to "2.0" — parallel Header-semantic bug; the JSON-cppagent-mqtt formatter owns all three.

References

• Source: libraries/MTConnect.NET-JSON-cppagent/Streams/JsonStreamsHeader.cs
• cppagent reference: test_package/json_printer_stream_test.cpp
• XSD: MTConnectStreams_2.7.xsd — HeaderAttributesType.testIndicator declared use="optional".
• Evidence captures: current.json + cppagent-current.json.