Composite operation support

[fdur]

In LwM2M 1.2 Core specification (OMA-TS-LightweightM2M_Core-V1_2_2-20240613-A), for Read-Composite, at §6.3.8:

The LwM2M Client MAY support the "Read-Composite" operation.
...
The LwM2M Client MUST retrieve all the requested Resources ...

Same for Write-Composite at next paragraph

FMPOV, the MUST depends on the MAY ...
Moreover, if the client does not support such operation (this is my understanding of the MAY support according to RFC 21119), what is the error code to be returned by the device (which can allow the server to understand that this operation is not supported by the device) ?
According to the spec, the error code which can be returned by the device for Read-Composite are:

• 401 Unauthorized or 403 Forbidden Access Right Permission Denied
• 404 Not Found URI of "Read" operation is not found
• 405 Method Not Allowed Target is not allowed for "Read" operation
• 406 Not Acceptable None of the preferred Content-Formats can be returned
• 415 Unsupported Media Type The specified format is not supported

[sbernard31]

(I'm not OMA authors)

FMPOV, the MUST depends on the MAY ...

I also understand that.

what is the error code to be returned by the device (which can allow the server to understand that this operation is not supported by the device) ?

The only 2 potention candidate are 404 Not Found or Method Not Allowed

Composite Operation target ressource /

404 Not Found : not the idea ?

Underlying Transport Error Code Meaning : ❌
Coap ask to refer to HTTP code.](https://datatracker.ietf.org/doc/html/rfc7252#section-5.9.2.5)
And Http code description are :

The 404 (Not Found) status code indicates that the origin server did not find a current representation for the target resource or is not willing to disclose that one exists. A 404 status code does not indicate whether this lack of representation is temporary or permanent; the 410 (Gone) status code is preferred over 404 if the origin server knows, presumably through some configurable means, that the condition is likely to be permanent.

Here the / resource is probably here because it is used for : Read-Composite, Write-Composite, Observe-Composite, Bootstrap Delete, Bootstrap Discover ... So could be not so much the spirit to answer Not Found if the resource exists but doesn't support the operation..

Generally, if a resource exists but doesn't support an operation on it you rather send a Method Not Allowed.

405 Method not allowed : almost perfect 😅

Underlying Transport Error Code Meaning : ✅
Coap ask to refer to HTTP code.
And Http code description are :

The 405 (Method Not Allowed) status code indicates that the method received in the request-line is known by the origin server but not supported by the target resource.

That sounds to match not so bad.

For Composite-Read : ✅
"Target is not allowed for "Read" operation" : this wording makes sense if the target change and if there is a kind of "readable" idea for the target... but in that case the target is always the same the root LWM2M tree / ... and the root is always readable... except if "Read-Composite" is not supported... So "Target is not allowed for "Read" operation" does not make too much sense to me in that case, so it's look like a kind of bad copy/paste would be clearer to say "Device doesn't support Read-Composite"

If 405 Method not allowed is not used for that purpose for composite operation, I don't know when it is supposed to be used ...
Especially Read-Composite is a kind of best effort operation, so even if some ressources requested doesn't support Read it seems you doesn't return a Method Not Allowed :

The LwM2M Client MUST retrieve all the requested Resources except the Resource(s) which does not support the "Read" operation and sends the retrieved Resource(s) information to the LwM2M Server.
(source : LWM2M-v1.2.2@core§6.3.8. Read-Composite Operation)

BUT When you look at Write-Composite this way doesn't fit well anymore : ❌ ? ✅ ?
As the spec says :

The LwM2M Client MUST perform the operation only if all the Resources conveyed in the operation are allowed to support the "Write" operation. If any Resource does not support the "Write" operation, the LwM2M Client MUST inform the LwM2M Server that the Object Instance cannot perform the requested operation by returning an error code indicating that the method is not allowed.
(source : LWM2M-v1.2.2@core§6.3.9. Write-Composite Operation

Method Not Allow seems to have already a clear meaning for Write-Composite...
So not sure this is good to have same error code for "some resource is not writable" and "Write-Composite" is not supported... 😬

So not ideal solution here ...

[mkgillmore]

I'll add to the OMA LwM2M discussion queue this week. Not sure i'll have a definitive answer before end of year. However, this will likely be added for clarification for the next version of LwM2M

[jdzier]

@fdur @sbernard31 Thank you for raising this issue and the detailed analysis.

Following discussion within the OMA LwM2M Working Group, the following recommendation and resolution were agreed:

Clarification of Optional Operation Semantics
The group agreed that optional composite operations (such as Read-Composite and Write-Composite) should explicitly define how a Client signals that it does not implement these optional features. This avoids ambiguity for Servers interpreting such responses.

Recommended Error Code for Unimplemented Composite Operations
When a Client chooses not to implement an optional composite operation, it should return a protocol-appropriate “Not Implemented” response:

• CoAP: 5.01 Not Implemented
• MQTT: 501 Not Implemented

This status code most accurately reflects a lack of capability rather than a resource-specific prohibition, and it aligns with how the LwM2M specification handles other unimplemented optional features.

The Working Group will incorporate this clarification into the next revision of the LwM2M Core Specification.

I hope above fully covers your initial question.