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:[email protected]]
> Sent: Wednesday, October 10, 2018 10:28 AM
> To: Michael Rehder <[email protected]>; [email protected]
> Subject: Re: [netmod] WHEN statement within mandatory objects doesn't
> ensure presence of the mandatory object
>
> Michael Rehder <[email protected]> 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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