Clarify read-only/write-only behavior

[mlagally]

Current language for property affordance is a bit ambiguous about read-only/write only behavior:
https://w3c.github.io/wot-thing-description/#propertyaffordance

An Interaction Affordance that exposes state of the Thing. This state can then be retrieved (read) and optionally updated (write).

The intention of the text is to indicate that the state can always be retrieved with a read operation and can be optionally updated with a write operation.

The paragraph should be changed to make this more clear, e.g.:

An Interaction Affordance that exposes state of the Thing. The state MUST be retrievable with a read operation and MAY be optionally updated with a write operation.

[takuki]

A Property affordance has a member "writeOnly".
When writeOnly is true, the property cannot be read.
I think the text in question is a bit obsolete.

[takuki]

In TD call on 2/14, we discussed and suggest to say explicitly something like "devices can provide both read and write operations, or read operation only, or write operation only, and both read only and write only combination is prohibited."

[zolkis]

Another way to look at it is based on capabilities, i.e. a Property can be readable, writable and observable.

For instance, it is not clear if "writeOnly" always also means not observable in a TD?
(at least in OCF, observability requires readability).

[JKRhb]

For instance, it is not clear if "writeOnly" always also means not observable in a TD?
(at least in OCF, observability requires readability).

I was just asking myself this very question – have there been any updates on this matter in the meantime? As, at least in CoAP, the (de)registration of observers is part of a GET request (and therefore a read operation) I also assumed that writeOnly implies that a property is not observable.

[egekorkan]

Call of 15.06:

• We do agree that there are only writeable properties.
• There are multiple people rooting for bringing back writeable and readable. readOnly and writeOnly are from JSON Schema but they are not actual validation terms and are not used much.