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

On Mon, 2017-12-04 at 09:36 -0800, Andy Bierman wrote:
> 
> 
> On Mon, Dec 4, 2017 at 9:22 AM, Juergen Schoenwaelder <j.schoenwaelder@jacobs-
> university.de> wrote:
> > On Mon, Dec 04, 2017 at 06:05:58PM +0100, Ladislav Lhotka wrote:
> > > On Mon, 2017-12-04 at 17:34 +0100, Martin Bjorklund wrote:
> > > > Ladislav Lhotka <lhotka@nic.cz> 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>.
> > >
> > 
> > For me, its still the same schema - a when expression does not change
> > my notion of 'schema'.
> 
> 
> Agreed, but it changes the validation results against the schema.
> As Martin pointed out, we already have this separate validation issue in
> <candidate>.
> This is not a problem though because only <running> is required to pass
> validation tests.

This is not true because "when" constraints are among those that have to be
satisfied in *all* data trees (sec. 8.1 in RFC 7950).

I have argued many many times that it is conceptually wrong to make the schema
dependent on an instance data tree that the schema defines. If "when" was just a
special "must", everything would be fine.

Lada

> 
> I think NMDA works out fine because the YANG validation of config=true nodes
> is done in <intended>, not in <operational>.  Only the config=false nodes
> would be validated in <operational>, done by the client, not the server.
> (Server implementations do not validate their own output).
> 
> 
> > /js
> > 
> 
> 
> Andy
>  
> > --
> > Juergen Schoenwaelder           Jacobs University Bremen gGmbH
> > Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
> > Fax:   +49 421 200 3103         <http://www.jacobs-university.de/>
> > 
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> 
> 
-- 
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67