如何使用 Leafref 在 YANG model 中实现递归？

Question

I have the following XML that I want to model with YANG. The XML contains a list of nodes and each node contains a list of neighbor nodes.

<nodes>
  <node>
    <id>1</id>
    <node>
      <id>2</id>
    </node>
  </node>
  <node>
    <id>3</id>
    <node>
      <id>4</id>
    </node>
  </node>
</nodes>

Please find below the YANG model I tried to create. Unfortunately, Yang does not support circular references in grouping.

grouping node {
  list node {
    leaf id {
      type int32;
    }
    uses node;
  }
}

container nodes {
   uses node;
}

I saw in draft-ietf-netmod-routing-cfg-16 and on ietf mail archive that a way to emulate recursion is to use leafref. How can the above xml be modeled with grouping and leafref?

Answer 1

As you said, recursion using groupings is not supported. The simplest approach would be to have a flat list of nodes, where each node has a parent node, referenced as a leafref. Something like:

container nodes {
  list node {
    key id;
    leaf id { type int32; }
    leaf parent-id {
      type leafref {
        path "../../node/id";
      }
    }
  }
}

The equivalent XML would be:

<nodes>
  <node>
    <id>1</id>
  <node>
  <node>
    <id>2</id>
    <parent-id>1</parent-id>
  </node>
  <node>
    <id>3</id>
  <node>
  <node>
    <id>4</id>
    <parent-id>3</parent-id>
  </node>
</nodes>

You could do the opposite, where a node refers its children (via a leafref leaf-list) instead of its parents.

---

Of course, using the same data node directly recursively does work:

container nodes {
  list node {
    key id;
    leaf id { type int32; }

    list node {
      key id;
      leaf id { type int32; }

      list node {
        key id;
        leaf id { type int32; }

        // and so on
      }
    }
  }
}

but this does not allow indefinitely deep hierarchies. I would avoid this pattern, though, not a clean model design.