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

Robert Wilton <rwilton@cisco.com> Tue, 06 March 2018 12:41 UTC

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To: "Bogaert, Bart (Nokia - BE/Antwerp)" <bart.bogaert@nokia.com>
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From: Robert Wilton <rwilton@cisco.com>
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Date: Tue, 6 Mar 2018 12:41:27 +0000
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Subject: Re: [netmod] Guideline on modeling including features and phased support by a device
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On 06/03/2018 12:04, Bogaert, Bart (Nokia - BE/Antwerp) wrote:
>
> Rob,
>
> Just to clarify: in this case we are not changing the model, it is the 
> same model but the device SW in release X is not supporting the 
> feature and in release Y it is.  Supporting the feature results in 
> adding that part of the tree to the configuration that is related to 
> the feature and the data leafs related to that feature are simply not 
> there in the data configured when SW release X was active.
>
Yes, but an equivalent way of writing this without a feature, and 
ignoring namespaces, would be put the feature nodes into a separate YANG 
module (F) which augments the base module (B).

In release X, the device only supports B.
In release Y, the device supports B and F.

But YANG would not allow F to augment B with a mandatory node for the 
reasons described previously.  Hence, I think that it is probably an 
oversight that YANG allows this.

Probably it should go on the YANG 2.0 issue tracker to consider and 
potentially disallow this.

Thanks,
Rob


> Regards, Bart
>
> *From:*Robert Wilton [mailto:rwilton@cisco.com]
> *Sent:* Tuesday, March 6, 2018 12:59 PM
> *To:* Bogaert, Bart (Nokia - BE/Antwerp) <bart.bogaert@nokia.com>
> *Cc:* netmod@ietf.org
> *Subject:* Re: [netmod] Guideline on modeling including features and 
> phased support by a device
>
> 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:rwilton@cisco.com]
>     *Sent:* Tuesday, March 6, 2018 10:38 AM
>     *To:* Bogaert, Bart (Nokia - BE/Antwerp) <bart.bogaert@nokia.com>
>     <mailto:bart.bogaert@nokia.com>; netmod@ietf.org
>     <mailto:netmod@ietf.org>
>     *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-featureleaf
>     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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