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

["Bogaert, Bart (Nokia - BE/Antwerp)" <[e-mail]>]

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