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

Robert Wilton <rwilton@cisco.com> Wed, 10 October 2018 15:33 UTC

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To: Michael Rehder <Michael.Rehder@Amdocs.com>, Ladislav Lhotka <lhotka@nic.cz>, "netmod@ietf.org" <netmod@ietf.org>
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From: Robert Wilton <rwilton@cisco.com>
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Date: Wed, 10 Oct 2018 16:33:01 +0100
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Subject: Re: [netmod] WHEN statement within mandatory objects doesn't ensure presence of the mandatory object
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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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