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

[Andy Bierman <[e-mail]>]

On Wed, Oct 10, 2018 at 11:44 AM, Michael Rehder <[e-mail]>
wrote:

> Sure.
>
> I think the RFC is unclear since it seems that the semantics are
> consistent in the back-end checks.
> One can read the RFC and not notice by its absence that the when clause
> doesn't require anything to be present.
>
>      The "when" statement makes its parent data definition statement
> conditional.
> Should be
>     The "when" statement makes its parent data definition statement
> conditional and optional.
>
>
This is not correct.

Step 1) if-feature makes the schema node conditional

Step 2) when-stmt makes instances of the schema-node conditional

Step 3) YANG validation applies to instances of data nodes (or the YANG
default value if applicable)

Step 2 is only relevant if Step 1 is true or non-existent
Step 3 is only relevant if Step 2 is true or non-existent

Andy



> I think there should be a more definite statement about this overriding
> any mandatory status of the parent data definition statement.
> Like
>      "Any mandatory status of the parent data definition statement
> (mandatory statement, min-element statement etc.) is overridden by this
> statement and made non-mandatory."
>
> That would have made the side-effect of "when" on other declarations
> clearer.
>
> Thanks
> mike
> > -----Original Message-----
> > From: Juergen Schoenwaelder [mailto:[e-mail]
> ]
> > Sent: Wednesday, October 10, 2018 2:25 PM
> > To: Michael Rehder <[e-mail]>
> > Cc: Robert Wilton <[e-mail]>; Ladislav Lhotka <[e-mail]>;
> > [e-mail]; Walker, Jason ([e-mail])
> > <[e-mail]>
> > Subject: Re: [netmod] WHEN statement within mandatory objects doesn't
> > ensure presence of the mandatory object
> >
> > 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:[e-mail]]
> > > > Sent: Wednesday, October 10, 2018 11:33 AM
> > > > To: Michael Rehder <[e-mail]>; Ladislav Lhotka
> > > > <[e-mail]>; [e-mail]
> > > > Cc: Walker, Jason ([e-mail])
> > > > <[e-mail]>
> > > > 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:[e-mail]]
> > > > >> Sent: Wednesday, October 10, 2018 10:28 AM
> > > > >> To: Michael Rehder <[e-mail]>; [e-mail]
> > > > >> Subject: Re: [netmod] WHEN statement within mandatory objects
> > > > >> doesn't ensure presence of the mandatory object
> > > > >>
> > > > >> Michael Rehder <[e-mail]> 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.htm
> > > > >> l
> > > > >>
> > > > >> 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: [phone]         Campus Ring 1 | 28759 Bremen | Germany
> > Fax:   [phone]         <https://www.jacobs-university.de/>
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