Re: [netmod] evaluation of "when" under NMDA

[Robert Wilton <[e-mail]>]

Hi Lada,


On 04/12/2017 17:05, Ladislav Lhotka wrote:
> On Mon, 2017-12-04 at 17:34 +0100, Martin Bjorklund wrote:
>> Ladislav Lhotka <[e-mail]> wrote:
>>> Hi,
>>>
>>> if we have
>>>
>>> augment "/target/node" {
>>>    when "...";
>>>    ...
>>> }
>>>
>>> is the "when" expression supposed to be evaluated separately in each
>> datastore,
>>> and the augment applied only in those datastores where the result is true?
>> Yes.
> But then it cannot be guaranteed that the schema for <operational> is a superset
> of the schema of configuration datastores - the when expression can evaluate to
> false in <operational> but true in <intended>.
I think that comes down to terminology, RFC 7950 section 7.21.5 on when 
statements just talks about making data definition statements 
conditional.  I don't really think that when statements (or choice 
statements) change the schema associated with a datastore, they just 
make parts of it inactive based on the current data.  I.e. you wouldn't 
expect different information to be returned via YANG library depending 
on how particular when statements in the implemented modules get evaluated.

Is seems to me that the when processing falls out quite naturally. Is 
there a scenario where you think that this could cause a problem?

Thanks,
Rob

>
> Lada
>
>>> RFC 7950 says in sec. 7.21.5 that the context node for XPath evaluation is
>> "the
>>> augment's target node in the data tree", but with NMDA we have multiple data
>>> trees, hence multiple target nodes.
>> We had multiple datastores even before NMDA.  The when expression
>> could be true in candidate but false in running.
>>
>> /martin