Lada,
Thanks for drawing my attention to this section. I hadn't noticed this advice.
Perhaps corresponding things need to be said about the XML?
Always use a prefix on identity values, even if the default namespace (YANG
module) is the module that defined the identity.
When importing a namespace (YANG module) that contains identities that are
referenced in the XML, always use the same prefix that was used within the
imported module.
The need for the first of the above XML-related statements wasn't illustrated
by the YANG and XML in my first message but it's illustrated by the fragments
shown below.
I guess the point about derived-from() and derived-from-or-self() is that,
although they still specify the identities as strings, the strings are known to
be identities, so prefixes can be processed (applying defaults etc)?
Finally, I think perhaps this is a separate point (because not related to
string values), but seems to be necessary to use the same prefixes for imported
modules as were used within those modules. For example, if (in my example) I
change the ietf-interfaces prefix from if to ifx, validation fails with an
"undefined namespace prefix" error.
Cheers,
William
YANG fragment:
identity sub-type;
identity sub-type-a {
base sub-type;
}
augment "/if:interfaces/if:interface" {
when "if:type = 'mymod:some-new-iftype'";
container some-new-container {
leaf sub-type {
type identityref {
base sub-type;
}
}
container sub-container {
when "../mymod:sub-type = 'mymod:sub-type-a'";
}
}
}
XML fragment (mymod prefix is necessary in order for XML to validate):
<some-new-container xmlns="http://example.com/augment";>
<sub-type>mymod:sub-type-a</sub-type>
<sub-container/>
</some-new-container>
> On 22 Jan 2016, at 09:32, Ladislav Lhotka <lho...@nic.cz> wrote:
>
> Hi William,
>
> YANG uses XPath 1.0, and so the equality tests mean *string
> equality*. That's why 6087bis recommends in sec. 5.6.4:
>
> XPath expressions that contain a literal value representing a YANG
> identity SHOULD always include the declared prefix of the module
> where the identity is defined.
>
> In YANG 1.1, the new XPath functions derived-from() and
> derived-from-or-self() will alleviate this problem.
>
> Lada
>
> William Lupton <wlup...@broadband-forum.org> writes:
>
>> All,
>>
>> We have been experimenting with validating XML instances along the lines
>> explained in the tutorials at
>> http://www.yang-central.org/twiki/bin/view/Main/DSDLMappingTutorial
>> <http://www.yang-central.org/twiki/bin/view/Main/DSDLMappingTutorial> and
>> https://github.com/mbj4668/pyang/wiki/InstanceValidation
>> <https://github.com/mbj4668/pyang/wiki/InstanceValidation> and we have hit a
>> problem with identities in XPath expressions.
>>
>> This is illustrated by the YANG module shown below, which is inspired by an
>> example in RFC 6020bis, and the XML instance shown below the YANG. Note that:
>> The second "when" statement doesn't use a prefix on the interface type.
>> Shouldn't be a problem because the interface type is defined in the current
>> module and so doesn't need a prefix?
>> The XML uses a prefix name of "exaug" rather than "mymod" which is used in
>> the YANG.
>> Shouldn't be a problem because prefixes should be local?
>>
>> Validating this instance gives the following two errors (which go away if
>> the prefix "mymod" is used in all "when" statements and in the XML).
>>
>> == Validating semantic constraints ...
>> --- Validity error at "/nc:data/if:interfaces/if:interface":
>> Found nodes that are valid only when "if:type = 'mymod:some-new-iftype'"
>> --- Validity error at "/nc:data/if:interfaces-state/if:interface":
>> Found nodes that are valid only when "if:type = 'some-new-iftype'"
>>
>> Are we doing something wrong here, or is there a problem with how identities
>> in XPath expressions are translated (per RFC 6110)? On the face of it it
>> seems that identities are being treated as literal strings (but we haven't
>> investigated this assertion).
>>
>> Thanks,
>> William Lupton
>>
>> --------
>>
>> YANG:
>> module example-augment {
>> namespace "http://example.com/augment";;
>> prefix mymod;
>>
>> import ietf-interfaces {
>> prefix if;
>> }
>>
>> identity some-new-iftype {
>> base if:interface-type;
>> }
>>
>> augment "/if:interfaces/if:interface" {
>> when "if:type = 'mymod:some-new-iftype'";
>>
>> container some-new-container {
>> }
>> }
>>
>> augment "/if:interfaces-state/if:interface" {
>> when "if:type = 'some-new-iftype'";
>>
>> container some-new-container {
>> }
>> }
>> }
>>
>> XML:
>> <?xml version="1.0" encoding="UTF-8"?>
>> <data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
>> xmlns:exaug="http://example.com/augment";>
>> <interfaces xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
>> <interface>
>> <name/>
>> <description/>
>> <type>exaug:some-new-iftype</type>
>> <enabled>true</enabled>
>> <link-up-down-trap-enable>enabled</link-up-down-trap-enable>
>> <some-new-container xmlns="http://example.com/augment";>
>> </some-new-container>
>> </interface>
>> </interfaces>
>> <interfaces-state xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
>> <interface>
>> <name/>
>> <type>exaug:some-new-iftype</type>
>> <admin-status>up</admin-status>
>> <oper-status>up</oper-status>
>> <if-index>1</if-index>
>> <statistics>
>> <discontinuity-time>2016-01-15T12:55:00Z</discontinuity-time>
>> </statistics>
>> <some-new-container xmlns="http://example.com/augment";>
>> </some-new-container>
>> </interface>
>> </interfaces-state>
>> </data>
>>
>>
>> _______________________________________________
>> netmod mailing list
>> netmod@ietf.org
>> https://www.ietf.org/mailman/listinfo/netmod
>
> --
> Ladislav Lhotka, CZ.NIC Labs
> PGP Key ID: E74E8C0C
>
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