ContentType seems to confuse OPC UA

[danielpeintner]

The following code tries to read an OPC UA property

const read1 = await thing.readProperty("1:Speed");
console.log("speed value is: ", await read1.value());

with the following TD

              "properties": {
                  "1:Speed": {
                      "title": "Speed",
                      "type": "number",
                      "forms": [
                          {
                              "href": "/?id=ns=1;s=MotorSpeed"
                          }
                      ],
                      "observable": true
                  }
             }

everything works fine while adding a "contentType": "application/octet-stream" which looks like this breaks it.

              "properties": {
                  "1:Speed": {
                      "title": "Speed",
                      "type": "number",
                      "forms": [
                          {
                              "href": "/?id=ns=1;s=MotorSpeed",
                              "contentType": "application/octet-stream"
                          }
                      ],
                      "observable": true
                  }
             }

fails with

TypeError: dataType.toLowerCase is not a function
at OctetstreamCodec.valueToBytes 

This seems to indicate that OPC UA data is processed by OctetstreamCodec also (again?)

[danielpeintner]

Changing it to something unknown like "contentType": "application/octet-stream-x" fails, with
NotSupportedError: Content type application/octet-stream-x not supported
which is fine.

Changing it to "contentType": "application/opcua+octet-stream" creates a different error.

NotSupportedError: Invalid value according to DataSchema
    at InteractionOutput.value (...\opcua-test\node_modules\@node-wot\core\dist\interaction-output.js:129:19)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async ...\opcua-test\test.js:54:47 {
  value: {
    Value: { Type: 11, Body: 1500 },
    SourceTimestamp: 2025-06-24T15:00:32.167Z
  }
}

[danielpeintner]

I think the problem is, that the node-opcua library we are using converts the data on the wire directly to a DataValue class by calling read(...).

node-wot/packages/binding-opcua/src/opcua-protocol-client.ts

Lines 305 to 313 in 3325e12

	public async readResource(form: OPCUAForm): Promise<Content> {
	debug(`readResource: reading ${form}`);
	const content = await this._withSession(form, async (session) => {
	const nodeId = await this._resolveNodeId(form);
	const dataValue = await session.read({
	nodeId,
	attributeId: AttributeIds.Value,
	});

Maybe it is possible to get the raw data, but I am not sure.
Hence, I wonder whether a simplified fix is also reasonable. What do I mean by that, we remove the contentType for any OPC UA form so that no other ContentSerdes codec can kick in?

@sebastiankb @egekorkan do you think that is okay?
@erossignon do you have another proposal?

[danielpeintner]

In #1402 I show what I intend to do...

[egekorkan]

What do I mean by that, we remove the contentType for any OPC UA form so that no other ContentSerdes codec can kick in?

The developer should not need to do that but if node-wot does it internally until a better solution is available, I would be ok with it.

[danielpeintner]

I agree. We should keep this issue open in any case until we find a better solution.

[danielpeintner]

The developer should not need to do that

BTW, the developer doesn't need to do anything. We take care internally...