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 15:23 UTC

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From: Michael Rehder <Michael.Rehder@Amdocs.com>
To: 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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Date: Wed, 10 Oct 2018 15:23:27 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/netmod/8Bne_bVYCMCjvmmdW0f3-NuCxW8>
Subject: Re: [netmod] WHEN statement within mandatory objects doesn't ensure presence of the mandatory object
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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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