Re: [netmod] WHEN statement within mandatory objects doesn't ensure presence of the mandatory object

Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de> Wed, 10 October 2018 18:25 UTC

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Date: Wed, 10 Oct 2018 20:25:29 +0200
From: Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de>
To: Michael Rehder <Michael.Rehder@Amdocs.com>
CC: Robert Wilton <rwilton@cisco.com>, Ladislav Lhotka <lhotka@nic.cz>, "netmod@ietf.org" <netmod@ietf.org>, "Walker, Jason (Jason_Walker2@comcast.com)" <Jason_Walker2@comcast.com>
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Subject: Re: [netmod] WHEN statement within mandatory objects doesn't ensure presence of the mandatory object
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Michael,

what matters here is what the YANG specification (RFC 7950) says. Is
there a reason to believe the IPAddresses list in your example can be
absent or have no elements based on what RFC 7950 says? Or do we talk
about a shortcoming of RFC 6110?

/js

On Wed, Oct 10, 2018 at 06:17:26PM +0000, Michael Rehder wrote:
> If the list has a "when" clause the RNG file actually produces a "OneOrMore" which has a choice of <empty> or the list so it actually doesn't enforce the presence at least one row of the list (unless I'm mistaken in my reading).
>               <oneOrMore>
>                 <choice>
>                   <empty/>
>                   <element name="IPAddresses">
>                     <element name="Address">
>                       <ref name="types__IPv4Address"/>
>                     </element>
>                     <empty/>
>                   </element>
>                 </choice>
>               </oneOrMore>
> 
> A leaf/container would be a simpler example but would result in the same lack of enforcement of the mandatory status of an element with a "when" clause.
> 
> This RNG seems consistent with the Schematron rules that "when" makes something optional.
> 
> 
> I think a workaround would be choice with mandatory true and a when clause on the cases. This would ensure that at least one case is present since the mandatory clause implements a Schematron existence constraint.
> 
> Thanks
> Mike
> > -----Original Message-----
> > From: Robert Wilton [mailto:rwilton@cisco.com]
> > Sent: Wednesday, October 10, 2018 11:33 AM
> > To: Michael Rehder <Michael.Rehder@Amdocs.com>om>; Ladislav Lhotka
> > <lhotka@nic.cz>cz>; netmod@ietf.org
> > Cc: Walker, Jason (Jason_Walker2@comcast.com)
> > <Jason_Walker2@comcast.com>
> > Subject: Re: [netmod] WHEN statement within mandatory objects doesn't
> > ensure presence of the mandatory object
> > 
> > Hi Mike,
> > 
> > I think that the YANG below already enforces what you want, or otherwise I
> > don't follow your issue.
> > 
> > The YANG below is valid in two cases:
> > 
> > (1) AssignmentMechanism = DHCP, and IPAddresses is not present in the config
> > (due to the when statement).
> > (2) AssignmentMechanism = Static, IPAddresses exists and has at least one
> > element (due to min-elements 1).
> > 
> > Thanks,
> > Rob
> > 
> > 
> > On 10/10/2018 16:23, Michael Rehder wrote:
> > > Container "foo" would be mandatory if not for the "when" child element.
> > > With the "when" child element, the logic becomes "inverted" and the
> > constraint is a negative one of "disallowed under certain condition".
> > >
> > > The UC is for enforcement in REST API payloads.
> > > For a practical example:
> > >
> > >           leaf AssignmentMechanism {
> > >              type enumeration {
> > >                enum "DHCP";
> > >                enum "Static";
> > >              }
> > >              mandatory true;
> > >              description "The address assignment mechanism.";
> > >            }
> > >            list IPAddresses {
> > >              when "../AssignmentMechanism = 'Static'";
> > >              key Address;
> > >              min-elements 1;
> > >
> > >              leaf Address {
> > >                type capit:IPv4Address;
> > >                description "An ipv4 address.";
> > >              }
> > >             }
> > >
> > > There is no way in the IPAddresses list to enforce that there is at least one IP
> > Address when the assignment method is "Static".
> > > One could put a "must" on "AssignmentMechanism" to ensure at least one
> > element of the IPAddresses list when "Static", but I don't see this as a good
> > schema design, to have the controlling attribute check controlled attributes.
> > >
> > > I appreciate that this semantic can't be changed in YANG at this point.
> > > Could the "when" statement have a modifying child element to state that the
> > mandatory status of the element is to be enforced?
> > > Like
> > >      container foo {
> > >        when "condition" {
> > >            enforce-mandatory-status;
> > >        }
> > >
> > > There is already back-end for existential checks for mandatory choice so this
> > seems reasonably consistent to me.
> > > I appreciate there are existing issues for "when" but I don't see why this
> > would make things any worse.
> > > In fact by promoting a better dependency "direction" between schema
> > elements,  think it could simplify things (so I naively think :) ).
> > >
> > > Thanks
> > > Mike
> > >> -----Original Message-----
> > >> From: Ladislav Lhotka [mailto:lhotka@nic.cz]
> > >> Sent: Wednesday, October 10, 2018 10:28 AM
> > >> To: Michael Rehder <Michael.Rehder@Amdocs.com>om>; netmod@ietf.org
> > >> Subject: Re: [netmod] WHEN statement within mandatory objects doesn't
> > >> ensure presence of the mandatory object
> > >>
> > >> Michael Rehder <Michael.Rehder@Amdocs.com> writes:
> > >>
> > >>> I have a question about “when” and mandatory objects.
> > >>>
> > >>> It seems to me that the implemented semantics of “when” are really
> > >> “optional when”, in that the enclosing object can be absent even
> > >> though it is mandatory and the “when” clause holds true.
> > >>> The RFC could be clearer about this.
> > >>>
> > >>> Example
> > >>>
> > >>>     leaf color {
> > >>>       enumeration  {
> > >>>          enum “blue”;
> > >>>          enum “black”;
> > >>>       }
> > >>>       mandatory true;
> > >>>     }
> > >>>     container foo {
> > >>>        when ../color = ‘blue’;
> > >>>        etc.
> > >>>     }
> > >>>
> > >>> “foo” is optional due to the presence of the “when” statement even
> > >>> though the object is mandatory (same is true for mandatory leaf,
> > >>> min-elements=1 list etc.).
> > >> Maybe you intended to have, e.g., a "mandatory true" leaf inside
> > >> "container foo"?
> > >>
> > >>> This is considered valid XML for the above
> > >>>      <color>blue</color>
> > >> Yes, it is, under current YANG rules, no matter what "etc." stands
> > >> for. Note that evaluation of the XPath expression in this case (with
> > >> "foo" missing) requires the peculiar procedure of sec. 7.21.5 in RFC 7950.
> > >>
> > >>> In my view this makes conditionally variant schemas “loose” in their
> > >>> enforcement (some scenarios can use choice but it doesn’t cover
> > >>> everything).
> > >>>
> > >>> I think that mandatory should be respected for the enclosing objects
> > >>> of a “when” statement.  That is, a mandatory object must be present
> > >>> when its “when” clause holds true and a Schematron statement should
> > >>> enforce that.
> > >> In fact, this is one case where the DSDL mapping (RFC 6110) deviates
> > >> from YANG 1.0. Nodes that mandatory aren't enclosed in the RELAX NG
> > >> <optional> pattern, and are then required no matter what any "when"
> > >> statements say (because RELAX NG validation comes before Schematron).
> > >>
> > >>> What is the rationale behind the current YANG rules behavior, that
> > >>> the “when” Schematron mapping doesn’t check for presence of the
> > >>> enclosing mandatory object?
> > >> FWIW, I have been repeatedly protesting against this behaviour but
> > >> without much luck. See for example
> > >>
> > >> https://www.ietf.org/mail-archive/web/netmod/current/msg14012.html
> > >>
> > >> As a result, "when" is the trickiest feature in YANG by far.
> > >>
> > >> Lada
> > >>
> > >>> thanks
> > >>> Mike Rehder
> > >> --
> > >> Ladislav Lhotka
> > >> Head, CZ.NIC Labs
> > >> PGP Key ID: 0xB8F92B08A9F76C67
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-- 
Juergen Schoenwaelder           Jacobs University Bremen gGmbH
Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
Fax:   +49 421 200 3103         <https://www.jacobs-university.de/>