LiveValue Feedback

[jwise-mfg]

Splitting out from #3

3.2.1 LiveValue
This is a nit pick, but 'Live' to me implies not cached/stale. I might change this to 'Latest'?

I'd love to remove the concept of 'quality' and just return data if we can vs. no data if it's bad with some way to get an error message back through the API on a failure. Quality to me is an OT legacy thing that isn't that useful outside of OT

The interpolation metadata seems odd? Remove it for LiveValue?

[jwise-mfg]

Copying in comments from Randy Armstrong on #3

The StatusCode in OPC UA is not the same as COM DA Quality. It is an error code indicating the reason for the Bad data. I think Quality should just be called 'Status' or 'StatusCode'

Returning metadata with a value is a convenience that comes with significant cost. Clients that know what they asked for do not need it so there should be an option available to only return values.

[jwise-mfg]

Copying in comments from Scott Wyant on #15

Callers may mistakenly make assumptions on QUALITY of the value when using an interface of "LiveValue".
Calling an interface of "LatestValue" leads you right into considering the "quality" of the data.

[matthewGEA]

I concur that "LiveValue" should be renamed as it's not clear whether the data is provided after a request or unsolicited through subscription.

I propose renaming the endpoint to "READ" to clarify that the intention of the endpoint is for the client to request a read of an endpoint and the server will respond with the latest value that it has.

[jwise-mfg]

Consensus is that this should be LastKnownValue (LKV). Will update doc.

[jwise-mfg]

We think TimeStamp should probably be required metadata for a LKV -- but the Q in QVT (Quality Value Time) is still being discussed here: #27