Re: [yang-doctors] Question about absolute and relative paths in YANG

[Ladislav Lhotka <[e-mail]>]

On 15. 11. 21 17:55, Martin Björklund wrote:
> Hi,
> 
> Italo Busi <[e-mail]> wrote:
>> Any feedbacks/comments about this question?
>>
>> Thanks, Italo
>>
>> From: Italo Busi
>> Sent: giovedì 23 settembre 2021 18:56
>> To: [e-mail]
>> Cc: Zhenghaomian <[e-mail]>om>; 'Aihua Guo'
>> <[e-mail]>
>> Subject: Question about absolute and relative paths in YANG
>>
>> Hi YANG doctors,
>>
>> I am quite active within the IETF CCAMP and TEAS WGs on YANG models
>> for optical transport networks
>>
>> We are having some discussions about the use of absolute and relative
>> paths in some of the drafts we are developing for IETF
>>
>> I have been told that YANG doctors prefer absolute paths to relative
>> paths
> 
> I don't recognize this preference.  Also I don't know if you refer to
> XPath paths in must/when or something else.  I assume you mean XPath
> paths.

It's a mystery to me how such myths can ever arise. :-)

> 
> IMO, a short relative path is often easier to read and understand, and
> easier to get right.  In some cases it truly doesn't matter, and in
> some other cases you must use one and not the other.
> 
> In the examples below, I'd use a relative path.

+1

Lada

> 
> 
> /martin
> 
> 
> 
>> We think it makes a lot of sense to use absolute paths whenever
>> possible but there are some cases where we are not sure whether and
>> how we could use absolute paths
>>
>> We would appreciate your opinions and suggestions about how to address
>> these cases
>>
>> One case could be one of the augment statements defined in RFC8795 (TE
>> Topology model):
>>
>>    augment "/nw:networks/nw:network/nw:node/tet:te/"
>>          + "tet:te-node-attributes/tet:connectivity-matrices/"
>>          + "tet:label-restrictions/tet:label-restriction" {
>>      when "../../../../../../nw:network-types/tet:te-topology/"
>>         + "example-topology" {
>>
>> The intention is to say that this augmentation applies only to a
>> specific network-types of this network instance. Using an absolute
>> path the code could become:
>>
>>    augment "/nw:networks/nw:network/nw:node/tet:te/"
>>          + "tet:te-node-attributes/tet:connectivity-matrices/"
>>          + "tet:label-restrictions/tet:label-restriction" {
>> when
>> "/nw:networks/nw:network[nw:network-id=current()../../../../../../nw:network-id]/"
>>     + "nw:network-types/tet:te-topology/"
>>         + "example-topology" {
>>
>> It seems that using the absolute path is moving the issue on requiring
>> a relative path to get the value of the network-id of this network
>>
>> Is there an alternative solution to avoid using relative path in this
>> case?
>>
>> Another case we have considered is the definition of a grouping with a
>> leaf that references another node within the same network. In this
>> case, using a relative path or an absolution path embedding a relative
>> path required to get the network-id of this network, would make quite
>> difficult to re-use this grouping in multiple places within the YANG
>> tree
>>
>> One work-around could be to use the refine statement, such as:
>>
>> grouping node-reference {
>>    leaf network-ref {
>>      type leafref {
>>        path "/nw:networks/nw:network/nw:network-id"
>>      }
>>    }
>>
>>    leaf node-ref {
>>      type leafref {
>>        path "deref(../network-ref)/../nw:node/nw:nw:node-id"
>>      }
>>    }
>> }
>>
>> augment "/nw:networks/nw:network/nw:node/tet:te/"
>>        + "tet:te-node-attributes/tet:connectivity-matrices/"
>>        + "tet:label-restrictions/tet:label-restriction" {
>>    uses node-reference {
>>      refine network-ref {
>>        default current()/../../../../../../network-id;
>>        must current() = current()/../../../../../../network-id;
>>     }
>>    }
>>
>> This work-around seems a bit weird but at least it should the need to
>> use relative paths within the grouping but keep the requirement to use
>> the relative paths in the refine statements
>>
>> Is there a better solution to avoid using relative path within the
>> grouping?
>>
>> Thanks in advance for your help
>>
>> Italo
>>
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-- 
Ladislav Lhotka
Head, CZ.NIC Labs
PGP Key ID: 0xB8F92B08A9F76C67