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What is the connector for OPC UA?

OPC UA (OPC Unified Architecture) is a standard developed by the OPC Foundation to enable the exchange of data between industrial components at the edge and with the cloud. It can route messages from OPC UA servers to the MQTT broker and send control messages to OPC UA servers. OPC UA provides a consistent, secure, documented standard based on widely used data formats. Industrial components can implement the OPC UA standard to enable universal data exchange.

The OPC UA write capability (preview) lets industrial developers and operations engineers use the connector for OPC UA to perform real-time control at the edge by writing values directly to OPC UA nodes. This capability enables immediate updates to configurations, triggers for automation, and dynamic process adjustments without relying on round-trips to the cloud.

The write capability useful in scenarios where latency, autonomy, or local decision making is critical such as in manufacturing lines, predictive maintenance, or in AI-driven control loops.

The connector for OPC UA is a part of Azure IoT Operations. The connector for OPC UA connects to OPC UA servers to retrieve data that it publishes to topics in the MQTT broker and write data based in values from an MQTT broker topic subscription. The connector for OPC UA enables your industrial OPC UA environment to ingress data into your local workloads running on a Kubernetes cluster, and into your cloud workloads.

The connector for OPC UA is a client application that runs as a middleware service in Azure IoT Operations. The connector for OPC UA connects to OPC UA servers, lets you browse the server address space, monitor data changes and events in connected assets, and write data to nodes in the server address space. Operations teams and developers use the connector for OPC UA to streamline the task of connecting OPC UA assets to their industrial solution at the edge.

Capabilities

As part of Azure IoT Operations, the connector for OPC UA is a native Kubernetes application that:

  • Connects existing OPC UA servers and assets to a native Kubernetes cluster at the edge.
  • Publishes JSON-encoded messages from OPC UA servers in OPC UA PubSub format, using a JSON payload. By using this standard format for data exchange, you can reduce the risk of future compatibility issues.
  • Writes values directly to nodes in a connected OPC UA server based on MQTT subscriptions.
  • Connects to Azure Arc-enabled services in the cloud.

Other features

The connector for OPC UA supports the following features as part of Azure IoT Operations:

  • Simultaneous connections to multiple OPC UA servers configured by using Kubernetes AssetEndpointProfile custom resources (CRs).
  • Publish OPC UA data value changes in OPC UA PubSub format with JSON encoding.
  • Publish message headers as user properties in the MQTT message. The headers in the messages published by the connector for OPC UA are based on the CloudEvents specification for OPC UA.
  • Publish OPC UA events with predefined event fields.
  • Asset definition by using Kubernetes Asset CRs
  • Payload compression including gzip and brotli.
  • Automatic reconnection to OPC UA servers.
  • Integrated OpenTelemetry compatible observability.
  • OPC UA transport encryption.
  • Anonymous authentication and authorization based on username and password.
  • AssetEndpointProfile and Asset CRs configurable by using Azure REST API and the operations experience web UI.

How it works

To read data from a connected OPC UA server, the connector for OPC UA application:

  1. Reads the asset's associated device configuration to determine the OPC UA server endpoint and the security settings to use for the connection.
  2. Reads the asset's configured publishing interval to determine how frequently the connector publishes data to an MQTT broker topic.
  3. Reads the asset's configured data points and events to determine which values from the OPC UA server to publish to the MQTT broker.
  4. Creates a session to the OPC UA server for each configured asset.
  5. Creates a separate subscription in the session for each 1,000 data points.
  6. Creates a separate subscription for each event defined in the asset.
  7. Publishes messages to the MQTT broker based on the publishing interval. The connector implements retry logic to identify connections to endpoints that don't respond after a specified number of keep-alive requests. For example, there could be a nonresponsive endpoint in your environment when an OPC UA server stops responding because of a power outage.

To write values to a node in a connected OPC UA server, the connector for OPC UA:

  1. Reads the asset's associated device configuration to determine the OPC UA server endpoint and the security settings to use for the connection.

  2. Reads the asset configuration to determine which nodes to write to on the OPC UA server.

  3. Subscribes to an MQTT topic that contains write requests for the asset. The topic name is in the format {Namespace}/asset-operations/{AssetId}/{DatasetName}/, where {Namespace} is the namespace for Azure IoT Operations instance, {AssetId} is the unique identifier for the asset, and {DatasetName} is the name of the dataset that contains the nodes to write to.

  4. Creates a temporary session with the OPC UA server using the device configuration.

  5. Checks the payload to ensure all data points exist in the target dataset.

  6. Writes the values to the OPC UA server, and publishes a success or failure response to the MQTT broker. The changed value is published to the standard telemetry topic associated with the dataset.

To generate a write request, publish a JSON message to the MQTT topic using MQTT v5 request/response semantics. Specify the dataset name and the values to be written in the payload. Each MQTT message includes metadata that defines system-level and user-defined properties such as SourceId, ProtocolVersion, and CorrelationData to ensure traceability and conformance.

Note

During preview, write operations are limited to data points that are explicitly defined in a dataset.

Connector for OPC UA message format

The connector for OPC UA publishes messages from OPC UA servers to the MQTT broker as JSON. 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.

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}}

Example write payload

Here's a minimal example for writing a simple float value to a node:

{ 
   "SetPoint": 50
}

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 the 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:0000aaaa-11bb-cccc-dd22-eeeeee333333 externalAssetId:0000aaaa-11bb-cccc-dd22-eeeeee333333 serverToConnectorMilliseconds:0.3153 id:1111bbbb-22cc-dddd-ee33-ffffff444444 specversion:1.0 type:ua-keyframe source:urn:OpcPlc:opcplc-000000 time:2024-08-05T14:19:08.8738457Z datacontenttype:application/json subject:0000aaaa-11bb-cccc-dd22-eeeeee333333 sequence:9768 traceparent:00-4eb4313536bc006c918e936686921cfc-4ee795f6fdd5fae7-01 recordedtime:2024-08-05 14:19:08.874 +00:00
uuid:0000aaaa-11bb-cccc-dd22-eeeeee333333 externalAssetId:0000aaaa-11bb-cccc-dd22-eeeeee333333 serverToConnectorMilliseconds:0.3561 id:1111bbbb-22cc-dddd-ee33-ffffff444444 specversion:1.0 type:ua-keyframe source:urn:OpcPlc:opcplc-000000 time:2024-08-05T14:19:09.8746396Z datacontenttype:application/json subject:0000aaaa-11bb-cccc-dd22-eeeeee333333 sequence:9769 traceparent:00-388697f77c2dcb5e9b30589c0a4cef6e-de9351186ff5833e-01 recordedtime:2024-08-05 14:19:09.875 +00:00
uuid:0000aaaa-11bb-cccc-dd22-eeeeee333333 externalAssetId:0000aaaa-11bb-cccc-dd22-eeeeee333333 serverToConnectorMilliseconds:0.3423 id:1111bbbb-22cc-dddd-ee33-ffffff444444 specversion:1.0 type:ua-keyframe source:urn:OpcPlc:opcplc-000000 time:2024-08-05T14:19:10.8754860Z datacontenttype:application/json subject:0000aaaa-11bb-cccc-dd22-eeeeee333333 sequence:9770 traceparent:00-7c65a93fa7668bbe0cdfd051168c88ac-ab86b83fb1b7944f-01 recordedtime:2024-08-05 14:19:10.875 +00:00
uuid:0000aaaa-11bb-cccc-dd22-eeeeee333333 externalAssetId:0000aaaa-11bb-cccc-dd22-eeeeee333333 serverToConnectorMilliseconds:0.3277 id:1111bbbb-22cc-dddd-ee33-ffffff444444 specversion:1.0 type:ua-keyframe source:urn:OpcPlc:opcplc-000000 time:2024-08-05T14:19:11.8765569Z datacontenttype:application/json subject:0000aaaa-11bb-cccc-dd22-eeeeee333333 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

For assets created in the operations experience web UI, the subject property for any messages sent by the asset is set to the externalAssetId value. In this case, the subject property contains a GUID rather than a friendly asset name.

How does it relate to Azure IoT Operations?

The connector for ONVIF is part of Azure IoT Operations. The connector deploys to an Arc-enabled Kubernetes cluster on the edge as part of an Azure IoT Operations deployment. The connector interacts with other Azure IoT Operations elements, such as:

Next step

How to use the connector for OPC UA