Help understanding ../ or ../../ and Deref in YANG

[vikassri]

Hi Team ,

Hope you are doing well .  I am trying to wrap my head around the  ../  or ../../  use in the XPATH  with no success .  I have read the reference for the same in RFC but does not help much . I would appreciate if someone could explain this to me with an example.

For example on the youtube video on YANG training below , could someone please explain what does "/client[ip=current()/../v-ip]/port" expand to ?  What does ip=current() point to and then where does the ../ notation point to in the hierarchy .

Also appreciate if someone could explain the expansion of the path without deref() and with deref() .

Regards,

Vikas Srivastava

[rogaglia]

Hi,

current() means your current node. Imagine it as the linux directory system and current() = pwd().

So, with ../ you go back one level in the tree structure and with ../../ you do it twice.

Example: if video/v-ip="1.1.1.1"

     /client[ip=current()/../v-ip]/port

          would mean "/client[ip="1.1.1.1"]/port

Roque

[vikassri]

Thank you Rogue , That helps .

At some places its use of the ../  is clear , like the example you provided. What about the  ..   in the XML template below ( the one just after the /link[1]/var1_router_name ?

My understanding is that link[1]/var1_router_name expands to /ncs:devices/device[PE11]/config so why is there the two .. in this case ?

XML Template

              <address>
                {
                  deref(/link[1]/var1_router_name)/../config/interface/Loopback[name='0']/ip/address/primary/address |
                  deref(/link[1]/var1_router_name)/../config/interface/Loopback[id='0']/ipv4/address/ip
                }
              </address>

YANG

module learning_deref2 {
  namespace "http://com/example/learning_deref2";
  prefix learning_deref2;


  import ietf-inet-types { prefix inet; }
  import tailf-ncs { prefix ncs; }
  import tailf-common { prefix tailf; }
  import tailf-ned-cisco-ios { prefix ios;}




  augment "/ncs:services"  // AUGMENT is used to add to another data model , OR augment it. So in the example we are adding/augumenting the learning_deref to it.
  {


    list learning_deref2
    {
      key "name";
      uses ncs:service-data;
      ncs:servicepoint "learning_deref2";


      leaf name
      {
        mandatory true;
        type string;
      }


      list link
      {


        min-elements 2;
        max-elements 2;
        key "var1_router_name";


            leaf var1_router_name
            {
              mandatory true;
              type leafref
              {
                path "/ncs:devices/ncs:device/ncs:name";  // This path points to a list of routers , not a sepcific router
              }
            } //routername


            container ios
            {


              //  name=current()/../var1_router_name = PE11
              //  ncs:name=current()/../var1_router_name  EXPANDS to ncs:name=/ncs:services/learning_deref/link/var1_router_name
              when "/ncs:devices/ncs:device[ncs:name=current()/../var1_router_name]/ncs:device-type/ncs:cli/ncs:ned-id='ios-id:cisco-ios'"
              {


                //tailf:dependency "../device";
                //tailf:dependency "/ncs:devices/ncs:device/ncs:device-type";
              }

              leaf intf-number
              {
                mandatory true;
                type leafref
                {
                  path "deref(../../var1_router_name)/../ncs:config/ios:interface/ios:GigabitEthernet/ios:name";
                }
              } //intf-number


            } //ios

      }
    }
  }
}