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The connector for OPC UA publishes messages from OPC UA servers to the MQTT broker in JSON format. Each message has a payload and a collection of properties that are a part of the MQTT user properties section. The payload contains the messages from the OPC UA server, and the properties provide metadata about the message.
Payload
The payload of an OPC UA message is a JSON object that contains the messages from the OPC UA server. The following example shows the payload of a message from the sample thermostat asset used in the quickstarts. Use the following command to subscribe to messages in the azure-iot-operations/data topic:
mosquitto_sub --host aio-broker --port 18883 --topic "azure-iot-operations/data/#" -v --debug --cafile /var/run/certs/ca.crt -D CONNECT authentication-method 'K8S-SAT' -D CONNECT authentication-data $(cat /var/run/secrets/tokens/broker-sat)
The output from the previous command looks like the following example:
Client $server-generated/05a22b94-c5a2-4666-9c62-837431ca6f7e received PUBLISH (d0, q0, r0, m0, 'azure-iot-operations/data/thermostat', ... (152 bytes))
{"temperature":{"SourceTimestamp":"2024-07-29T15:02:17.1858435Z","Value":4558},"Tag 10":{"SourceTimestamp":"2024-07-29T15:02:17.1858869Z","Value":4558}}
Client $server-generated/05a22b94-c5a2-4666-9c62-837431ca6f7e received PUBLISH (d0, q0, r0, m0, 'azure-iot-operations/data/thermostat', ... (152 bytes))
{"temperature":{"SourceTimestamp":"2024-07-29T15:02:18.1838125Z","Value":4559},"Tag 10":{"SourceTimestamp":"2024-07-29T15:02:18.1838523Z","Value":4559}}
Client $server-generated/05a22b94-c5a2-4666-9c62-837431ca6f7e received PUBLISH (d0, q0, r0, m0, 'azure-iot-operations/data/thermostat', ... (152 bytes))
{"temperature":{"SourceTimestamp":"2024-07-29T15:02:19.1834363Z","Value":4560},"Tag 10":{"SourceTimestamp":"2024-07-29T15:02:19.1834879Z","Value":4560}}
Client $server-generated/05a22b94-c5a2-4666-9c62-837431ca6f7e received PUBLISH (d0, q0, r0, m0, 'azure-iot-operations/data/thermostat', ... (152 bytes))
{"temperature":{"SourceTimestamp":"2024-07-29T15:02:20.1861251Z","Value":4561},"Tag 10":{"SourceTimestamp":"2024-07-29T15:02:20.1861709Z","Value":4561}}
Client $server-generated/05a22b94-c5a2-4666-9c62-837431ca6f7e received PUBLISH (d0, q0, r0, m0, 'azure-iot-operations/data/thermostat', ... (152 bytes))
{"temperature":{"SourceTimestamp":"2024-07-29T15:02:21.1856798Z","Value":4562},"Tag 10":{"SourceTimestamp":"2024-07-29T15:02:21.1857211Z","Value":4562}}
User properties
The headers in the messages published by the connector for OPC UA are based on the CloudEvents specification for OPC UA. The headers from an OPC UA message become user properties in a message published to the MQTT broker. The following example shows the user properties of a message from the sample thermostat asset used in the quickstarts. Use the following command to subscribe to messages in the azure-iot-operations/data topic:
mosquitto_sub --host aio-broker --port 18883 --topic "azure-iot-operations/data/#" -V mqttv5 -F %P --cafile /var/run/certs/ca.crt -D CONNECT authentication-method 'K8S-SAT' -D CONNECT authentication-data $(cat /var/run/secrets/tokens/broker-sat)
The output from the previous command looks like the following example:
uuid:813740c3-48f3-4332-a7cf-bf5424918b8d externalAssetId:813740c3-48f3-4332-a7cf-bf5424918b8d serverToConnectorMilliseconds:0.3153 id:908fb134-2619-42a5-9aa2-4826bac5b3e0 specversion:1.0 type:ua-keyframe source:urn:OpcPlc:opcplc-000000 time:2024-08-05T14:19:08.8738457Z datacontenttype:application/json subject:813740c3-48f3-4332-a7cf-bf5424918b8d sequence:9768 traceparent:00-4eb4313536bc006c918e936686921cfc-4ee795f6fdd5fae7-01 recordedtime:2024-08-05 14:19:08.874 +00:00
uuid:813740c3-48f3-4332-a7cf-bf5424918b8d externalAssetId:813740c3-48f3-4332-a7cf-bf5424918b8d serverToConnectorMilliseconds:0.3561 id:a4acba23-ff15-4b1b-9544-e07deb4e941b specversion:1.0 type:ua-keyframe source:urn:OpcPlc:opcplc-000000 time:2024-08-05T14:19:09.8746396Z datacontenttype:application/json subject:813740c3-48f3-4332-a7cf-bf5424918b8d sequence:9769 traceparent:00-388697f77c2dcb5e9b30589c0a4cef6e-de9351186ff5833e-01 recordedtime:2024-08-05 14:19:09.875 +00:00
uuid:813740c3-48f3-4332-a7cf-bf5424918b8d externalAssetId:813740c3-48f3-4332-a7cf-bf5424918b8d serverToConnectorMilliseconds:0.3423 id:875739f2-c8f0-4a17-bd4a-d40343ff2a50 specversion:1.0 type:ua-keyframe source:urn:OpcPlc:opcplc-000000 time:2024-08-05T14:19:10.8754860Z datacontenttype:application/json subject:813740c3-48f3-4332-a7cf-bf5424918b8d sequence:9770 traceparent:00-7c65a93fa7668bbe0cdfd051168c88ac-ab86b83fb1b7944f-01 recordedtime:2024-08-05 14:19:10.875 +00:00
uuid:813740c3-48f3-4332-a7cf-bf5424918b8d externalAssetId:813740c3-48f3-4332-a7cf-bf5424918b8d serverToConnectorMilliseconds:0.3277 id:255ea77e-b313-43be-9ec0-44b98bb1c632 specversion:1.0 type:ua-keyframe source:urn:OpcPlc:opcplc-000000 time:2024-08-05T14:19:11.8765569Z datacontenttype:application/json subject:813740c3-48f3-4332-a7cf-bf5424918b8d sequence:9771 traceparent:00-5851e56a6f358ab5e1af1d798f7580a1-bf6dfbda8196cba0-01 recordedtime:2024-08-05 14:19:11.877 +00:00
The subject field contains the name of the asset that the message is related to. The sequence field contains the sequence number of the message.
Note
There's a known issue for assets created using the operations experience web UI where the subject property for any messages sent by the asset is set to the externalAssetId value. In this case, the subject is a GUID rather than a friendly asset name.