Re: [netmod] datastore-specific constraints

[Martin Bjorklund <[e-mail]>]

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 [...]

   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 would be useful with a YANG statement that indicates when the
modeller's intention is that a constraint doesn't apply to
<operational>.  For now, this can be indicated in the description
statement, for example:

  leaf foo {
    when "../auto-foo = 'false'" {
      description
        "This when expression does not apply to <operational>.";
    }
    ...
  }



/martin



> 
> Anyway, the logic of my example could then be implemented using "must". I agree
> though that ignoring the configured value if auto-foo is true may be preferable.
> 
> Lada
> 
> 
> > Thanks,
> > Rob
> > 
> > On 12/12/2018 14:52, Jan Lindblad wrote:
> > > Lada,
> > > 
> > > Maybe you could just skip the when and explain the behavior in the
> > > description? E.g.
> > > 
> > > leaf foo {
> > >  ...
> > >  description "Foo controls bla, bla. 
> > >   Any configured value will be ignored when auto-foo is true.";
> > > }
> > > 
> > > Best Regards,
> > > /jan
> > > 
> > > > On 12 Dec 2018, at 15:33, Ladislav Lhotka <[e-mail]> wrote:
> > > > 
> > > > Hi,
> > > > 
> > > > in some cases, constraints expressed with "when" or "must" may only be
> > > > intended for configuration datastores. A typical example is an
> > > > auto-negotiable parameter:
> > > > 
> > > > leaf auto-foo {
> > > >  type boolean;
> > > >  default true;
> > > >  description "If true, parameter 'foo' will be auto-negotiated.";
> > > > }
> > > > leaf foo {
> > > >  when "../auto-foo = 'false'";
> > > >  ...
> > > > }
> > > > 
> > > > This means that if auto-foo is true, it is impossible to configure the
> > > > foo parameter. However, even with auto-foo = true, it is desirable to
> > > > see the auto-negotiated value in <operational>, so, ideally, the "when"
> > > > constraint should not apply in <operational>.
> > > > 
> > > > How can this logic be modelled under NMDA? Is an extra leaf
> > > > "foo-operational" needed?
> > > > 
> > > > Thanks, Lada
> > > > 
> > > > -- 
> > > > Ladislav Lhotka
> > > > Head, CZ.NIC Labs
> > > > PGP Key ID: 0xB8F92B08A9F76C67
> > > > 
> > > > _______________________________________________
> > > > netmod mailing list
> > > > [e-mail]
> > > > https://www.ietf.org/mailman/listinfo/netmod
> > > > 
> > > 
> > > 
> > > 
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> -- 
> Ladislav Lhotka
> Head, CZ.NIC Labs
> PGP Key ID: 0xB8F92B08A9F76C67
> 
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