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

Michael Rehder <Michael.Rehder@Amdocs.com> Wed, 10 October 2018 18:17 UTC

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From: Michael Rehder <Michael.Rehder@Amdocs.com>
To: Robert Wilton <rwilton@cisco.com>, Ladislav Lhotka <lhotka@nic.cz>, "netmod@ietf.org" <netmod@ietf.org>
CC: "Walker, Jason (Jason_Walker2@comcast.com)" <Jason_Walker2@comcast.com>
Thread-Topic: [netmod] WHEN statement within mandatory objects doesn't ensure presence of the mandatory object
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Archived-At: <https://mailarchive.ietf.org/arch/msg/netmod/O9rqTDhMAWVx78EtwtqfXY4b5BE>
Subject: Re: [netmod] WHEN statement within mandatory objects doesn't ensure presence of the mandatory object
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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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