Re: [netmod] Guideline on modeling including features and phased support by a device

[Robert Wilton <[e-mail]>]

Arguably the guidelines, or YANG, should say "don't allow this" ;-)

I think that what you are describing is just another instance of "don't 
augment with a mandatory node rule", or "only backwards compatible 
changes revisions should be made to a published YANG module".

The key reasoning behind these rules is that the a client should be able 
to work unchanged after the server has been upgraded, as long as they 
are not making use of any new functionality.

Thanks,
Rob


On 06/03/2018 11:34, Bogaert, Bart (Nokia - BE/Antwerp) wrote:
>
> Hi Rob,
>
> I agree but the fact is that some of the BBF models have constructions 
> like that and we were wondering whether this should not be mentioned 
> in the guildelines document. Normally a server can’t set config true 
> leafs if there is no default available in the model.  That is the 
> reason we reached out to NETMOD.  Your suggestions can work but 
> require adaptation of the current model.
>
> Regards, Bart
>
> *From:*Robert Wilton [mailto:[e-mail]]
> *Sent:* Tuesday, March 6, 2018 10:38 AM
> *To:* Bogaert, Bart (Nokia - BE/Antwerp) <[e-mail]>om>; 
> [e-mail]
> *Subject:* Re: [netmod] Guideline on modeling including features and 
> phased support by a device
>
> Hi Bart,
>
> I think that the best solution to problem is perhaps to avoid it 
> altogether.  I.e. I don't think that the only-if-feature leaf should 
> be marked mandatory.  Instead, it would be better to define a sensible 
> default value/behaviour if the leaf is absent even when the feature is 
> supported.
>
> Alternatively, you can simulate something similar to an if-feature 
> statement by using a when or must expression instead that is 
> predicated on a leaf that the client must explicitly set to enable the 
> feature, giving control back to the client.
>
> E.g. something along the lines of ...
>
> leaf enable-super-feature {
>   if-feature test-feature;
>   type boolean;
>   default "false";
> }
>
> ...
>
>       leaf only-if-feature {
>
>         when '/enable-super-feature = "true"';
>
>         type string;
>
>         mandatory true;
>
>       }
>
> It would be interesting if you have a concrete example where neither 
> of the above suggestions would work or be appropriate.
>
> Thanks,
> Rob
>
> On 05/03/2018 09:25, Bogaert, Bart (Nokia - BE/Antwerp) wrote:
>
>     Hi,
>
>     We have a question with respect to YANG models using features. 
>     Assume that a part of the model is defined under a feature and
>     that this feature-dependent part defines a leaf as mandatory.
>
>     module servers {
>
>       namespace "http://www.example.com/servers";
>
>       prefix servers;
>
>       import ietf-inet-types {
>
>         prefix inet;
>
>       }
>
>       revision 2018-03-01 {
>
>         description
>
>            "Initial version.";
>
>       }
>
>       feature test-feature {
>
>         description "testing feature";
>
>       }
>
>       container servers {
>
>         list server {
>
>           key name;
>
>           max-elements 64;
>
>           leaf name {
>
>             type string;
>
>           }
>
>           leaf ip {
>
>             type inet:ip-address;
>
>             mandatory true;
>
>           }
>
>           leaf port {
>
>             type inet:port-number;
>
>             mandatory true;
>
>           }
>
>           leaf only-if-feature {
>
>             if-feature test-feature;
>
>             type string;
>
>             mandatory true;
>
>           }
>
>         }
>
>       }
>
>     }
>
>     Now assume that we have a device that implements the model
>     step-wise by first not supporting this feature and in a
>     sub-sequent release by supporting this feature (and uses a
>     persistent running datastore).  The question arising now is how to
>     deal with this mandatory leaf?  Normally this can only be
>     configured by a client, meaning that without any “help”, the NC
>     server will not be able to startup with the data contained in the
>     device’s persistent datastore unless a value is set for the
>     mandatory leaf that now becomes available as a result of
>     supporting the feature.
>
>     When modeling as follows it seems the NC server can start with the
>     model supporting the feature that was not supported before:
>
>     module servers {
>
>       namespace "http://www.example.com/servers";
>
>       prefix servers;
>
>       import ietf-inet-types {
>
>         prefix inet;
>
>       }
>
>       revision 2018-03-01 {
>
>         description
>
>            "Initial version.";
>
>       }
>
>       feature test-feature {
>
>         description "testing feature";
>
>       }
>
>       container servers {
>
>         list server {
>
>           key name;
>
>           max-elements 64;
>
>           leaf name {
>
>             type string;
>
>           }
>
>           leaf ip {
>
>             type inet:ip-address;
>
>             mandatory true;
>
>           }
>
>           leaf port {
>
>             type inet:port-number;
>
>             mandatory true;
>
>           }
>
>           container only-if-feature {
>
>             presence "see if this helps";
>
>             if-feature test-feature;
>
>             leaf only-if-feature {
>
>               type string;
>
>               mandatory true;
>
>             }
>
>           }
>
>         }
>
>       }
>
>     }
>
>     Are recommendations or guidelines in place to deal with this?
>
>     Regards, Bart
>
>
>
>
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