[question] How to describe observeOnly properties?

[egekorkan]

In the current specification, we cannot describe if a property is only observable and not readable and writeable. In the case of MQTT, if one publishes a message without a retain flag or if the broker doesn't support retain, only the current subscribers of the topic will get the message, hence they will be observing. A readproperty via MQTT would be possible only if the retain flag is settable to true. I think this is a real use case for MQTT Things. Some solutions:

• Saying that this is an event (needs no spec change)
• A new observeOnly for properties. This would be quite confusing since readOnly or writeOnly equal to false would mean that it is observable as well. This needs spec change and would be possible only for next version. Even then, it is not a perfect solution since it makes it impossible to have readable and writeable but not observable properties.
• Bringing back writeable, readable. This would be perfect solution to fix this little thing but would open all the issues of having writeable discussed in writable vs. readOnly #209

[zolkis]

In that case modeling with an event makes most sense IMHO. One could argue about the difference between observing (changes reported) and events (all occurences reported), but in this case the meaning is to report all changes, which is fine.

observeOnly brings in too much complexity vs other protocols, like OCF/CoAP where we can't observe a property if not readable.

The [observable, readable, writeable] tuple was a pretty good solution IMHO, we changed it for mostly syntactic and tradition-esthetic reasons.

[sebastiankb]

This is actually something what I expect in a TD design for MQTT binding:

• properties have to be observable AND readable since the broker supports the retain flag and the MQTT Client defines flag= true
• actions only supports inputs and can be only applied for publishing
• events supports observation only. This should be always used when the broker does not supports the retain flag feature or the MQTT client defines retain flag= false

[egekorkan]

In this case, this will be explained in the protocol bindings document. I am copying this issue over there as well.

[egekorkan]

Closing since it is at protocol bindings