Re: [netmod] datastore-specific constraints

[Ladislav Lhotka <[e-mail]>]

On Thu, 2018-12-13 at 11:24 +0100, Martin Bjorklund wrote:
> Ladislav Lhotka <[e-mail]> wrote:
> > On Thu, 2018-12-13 at 09:51 +0100, Martin Bjorklund wrote:
> > > Ladislav Lhotka <[e-mail]> wrote:
> > > > On Wed, 2018-12-12 at 15:23 +0000, Robert Wilton wrote:
> > > > > Hi Lada,
> > > > > I basically agree with Jan's suggestion.
> > > > > I don't think that I would use a when statement for this scenario
> since
> > > you
> > > > > want a leaf to report the operational value that is in effect, and one
> of
> > > the
> > > > > aims of NMDA is to try and get the configured and operational value
> onto
> > > the
> > > > > same path wherever possible.
> > > > > But, I think that the question about validity of must statements is
> more
> > > > > interesting.  RFC 8342 states that these can be violated in
> operational,
> > > but
> > > > > the intention is that the constraints in <operational> should
> generally
> > > apply
> > > > > (but never be actually checked by the server).  In this case, it might
> be
> > > > > useful to be able to annotate a must statement to indicate that it
> only
> > > > > applies to configuration data and not operational data.
> > > > 
> > > > Another option could be that "must" constraints on config data do not
> apply
> > > in
> > > > <operational>, whereas constraints on "config false" data serve as
> binding
> > > > guidelines for implementors.
> > > 
> > > Not sure what "binding guideline" means, but note that RFC 8342 says
> > > for <operational>:
> > > 
> > >    <operational> SHOULD conform to any constraints specified in the data
> > >    model, but given the principal aim of returning "in use" values, it
> > >    is possible that constraints MAY be violated [...]
> > 
> > According to the definition of SHOULD and MAY in RFC 2119, this sentence
> > contradicts itself.
> 
> It should probably have been "may" then...?

With a sound definition of syntactic and semantic constraints, we could state
that data in <operational>

- MUST satisfy syntactic constraints
- SHOULD satisfy semantic constraints that apply to config=false data

(semantic constraints on config=true data are checked before the data get into
<running>)

> 
> > >    Only semantic constraints MAY be violated.  These are the YANG
> > >    "when", "must", "mandatory", "unique", "min-elements", and
> > >    "max-elements" statements; and the uniqueness of key values.
> > 
> > It is nice to see "when" listed among semantic constraints.
> 
> Yeah, I was a bit surprised that this sentence classifies "when" as a
> semantic constraint...

Every truth will eventually be revealed. :-) 

> 
> > Note, however, that
> > in sec. 8.1 of RFC 7950, "when" ended up among the constraints that
> > are true in 
> > all data trees (despite my hefty protests in the past).
> 
> Note that also uniqueness of keys is listed in 8.1 as true all data
> trees, but relaxed by 8342.

Well, section 8.1 only says this about keys:

   o  All key leafs MUST be present for all list entries.

Their uniqueness is not mentioned (an omission?). Anyway, it is IMO a typical
semantic constraint that shouldn't be enforced in <candidate>, but due to the
limitations of the NETCONF protocol it has to be.

Lada

> 
> 
> /martin
-- 
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67