[T2TRG] OCF Observe on special interfaces, design question, RFC requirements?

[Michael Koster <[e-mail]>]

Background:
OCF uses special URLs, selected by query parameters, to specialize views and functions on resources. Client applications use the CoRE defined "if" target attribute and use IANA registered values as OCF Core  Interface Types in the query to select these views.

We provide interfaces that report partial state representations, e.g. links list for just the links of a collection, batch for just the data items in a collection, etc., and one interface (called baseline) that does report the full state of the resource.

For example:
GET /machine/state/3?if=oic.if.s
selects the "sensor" interface which returns sensor values but maybe not things like units and range that may also be present in the resource.

The need:
In OCF we are in the process of adding more special URLs, as interface types, for creation and deletion of resources, and interface types for modifying link arrays.

We would like to use CoAP Observe on these new interface URLs to report only the updates to the resource, and not the entire state of the resource in each notification. You might say partial state from a temporal view; incremental state updates. 

The Problem:
When a client first Observes or tries to poll, for example using the create interface, there are no items to reply with (this issue is similar to the Pub/Sub empty topic issue). Later, when a resource is eventually created, then a notification is sent to all observers containing the representation that was used to create the resource.

Proposed Solution:
With OCF we have a default content format and can return an empty CBOR map or an empty CBOR array, as appropriate, for the initial GET + Observe response, and return an item in the payload on subsequent notifications whenever a new resource is created.

We would use 2.05 Content for the response code in all non-error notifications.

Question:
Does this violate any of the current requirements in RFC7252 or RFC7641, or other specifications? There is some text that suggests a full representation be transmitted on every response, but it seems to address the general case i.e. not special URLs, and also would not allow for over-the-wire protocol optimization.

Example:
client-1 ==> GET /some/collection?if=oic.if.create obs=0 // client-1 wants to be notified when items are created in the collection
server response ==> 2.05 Content obs {} 

(later)

client-2 ==> POST /some/collection?if=oic.if.create { item: foo } //item being created by client-2 in the collection
server response ==> 2.01 Created { item:foo, id:1234 }

(to client-1)
observe response ==> 2.05 Content { item:foo, id:1234 } // client-1 is notified of the new item