Re: [netmod] [yang-doctors] Question on yang?

[Andy Bierman <[e-mail]>]

On Mon, Jan 12, 2015 at 6:22 AM, Ladislav Lhotka <[e-mail]> wrote:
>
>> On 12 Jan 2015, at 15:05, Andy Bierman <[e-mail]> wrote:
>>
>> On Mon, Jan 12, 2015 at 5:34 AM, Ladislav Lhotka <[e-mail]> wrote:
>>>
>>>> On 12 Jan 2015, at 13:42, Andy Bierman <[e-mail]> wrote:
>>>>
>>>> On Mon, Jan 12, 2015 at 3:59 AM, Ladislav Lhotka <[e-mail]> wrote:
>>>>> Hi Christian,
>>>>>
>>>>> Christian Hopps <[e-mail]> writes:
>>>>>
>>>>>>
>>>>>> Sorry I trimmed it down as the site def is much larger, the list contains a key:
>>>>>>
>>>>>>   list site {
>>>>>>     key "site-id";
>>>>>>     leaf site-id {
>>>>>>       type site-id;
>>>>>>     }
>>>>>>     leaf site-type {
>>>>>>       mandatory true;
>>>>>>       type site-type;
>>>>>>     }
>>>>>>     // [..cut..]
>>>>>>     leaf-list router {
>>>>>>       // The combination of when and min-elms appears to allow 0 elements
>>>>>>       // for the when matching site types
>>>>>>       min-elements 1;
>>>>>>       when "../site-type = 'ots' or ../site-type = 'odws'";
>>>>>>       type leafref {
>>>>>>         path "../../../router/router-id";
>>>>>>       }
>>>>>>     }
>>>>>>
>>>>>> I’m using yang2dsdl with (both -t data and -t config) to
>>>>>> validate. Both ways (config or data) accept XML with no routers
>>>>>> specified in ‘ots’ or ‘odws’ sites. Both ways correctly reject a
>>>>>> router specified in a non-ots/odws site.
>>>>>
>>>>> This has to do with YANG 1.1 ussue Y18:
>>>>>
>>>>> https://svn.tools.ietf.org/svn/wg/netmod/yang-1.1/issues.html#sec-19
>>>>>
>>>>
>>>> This does not appear to be related to Y18.
>>>
>>> It is exactly the substance of Y18: If no <router> element is present in the instance document, the XPath expression cannot be evaluated (if we abide by XPath rules) to decide whether the “router” node is in the schema or not. That’s where Y18-01 proposes to insert a tentative <router> element and use it as the context node.
>>>
>>
>>
>> OK - you are right -- this is Y18.
>> Our code creates a dummy context node in this case.
>> If the when-stmt is true, the min-elements violation
>> would be generated.
>
> Right, but this is really difficult to do via DSDL mapping because the RELAX NG schema generated from the YANG data model doesn’t depend on a particular instance document, and RELAX NG doesn’t do XPath evaluation.
>

You would have to generate a new config document with the context node in it.
This is actually one of the easier aspects of implementing when-stmt
correctly within a real server.  (For some definition of "correct", depending on
the evaluation order and other details).


> Lada
>
>>
>> Andy
>


Andy