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Overview

The following study describes extending YANG language statements to allow customization of models.

References:

YANG Language

  • The YANG language provides us the ability to define and model configurations and state data by defining YANG modules
    • Modules contain a sequence of statements
      • Statement syntax is either of the following :
        • statement = keyword [argument] ;
        • statement = keyword [argument] { <substatement(s)..> } ;

* argument can be zero or one depending on the statement

* argument is a string

  • An XML-based equivalent version of YANG is called YIN
  • YANG uses a tree to define the hierarchy of data wherein each ‘node’ has a value or/and a set of child nodes
    • 4 types of nodes

      • container nodes

      • list nodes 
      • leaf nodes
      • leaf-list nodes

Basic YANG statements

Sample YANG

(stores.yang)

Statements and Description
stores model
module stores {
    yang-version 1.1;
    namespace "org:onap:ccsdk:sample";

    prefix book-store;

    revision "2020-09-15" {
        description
        "Sample Model";
    }

    typedef year {
        type uint16 {
            range "1000..9999";
        }
    }

    container bookstore { 

        leaf bookstore-name {
            type string;
        }
    list categories {

        key "code";

        leaf code {	
            type string;
        }

        leaf name {
            type string;
        }

        list books {
            key title;

            leaf title {
                type string;
            }
            leaf-list authors {
                type string; 
            }
        }
    }
    }
}

module Statement

see example from Line 1

  • YANG language defines models with modules and submodules
  • Takes one argument (module name) which is the identifier
  • Groups all statements that belong to the module
  • This module example contains the following statements for header information:

             see examples from Lines 2-16

      • yang-version statement
      • namespace statement prefix statement
      • revision statements


typedef Statement

see example from Line 12

  • a statement that allows a new type to be defined based on a base type which is a YANG built-in type


container Statement

see example from Line 18

  • defines interior (container node) in the schema tree
  • only contains child nodes, has no value
      • child nodes can be a leaf, lists, containers and leaf-lists


leaf Statement

see example from Line 27

  • defines a leaf node in the schema tree
  • its only one argument is the identifier
  • has no child nodes, has one value of a particular type
  • 'type statement' is mandatory
  • See optional substatements available in (Section 7.6 https://www.hjp.at/doc/rfc/rfc6020.html#sec_1)


list Statement

see example from Line 35

  • defines an interior data node (list node) in the schema tree
  • its only one argument is the identifier
  • follows a block of substatements:
  • mandatory substatements:
      • 'key statement'

leaf-list Statement

see example from Line 41

  • array of leaf nodes
  • one value of a particular type per leaf
  • its only one argument is the identifier



Figure 1.1 Schema tree of module 'stores'

Schema tree of module 'stores'
module: stores
  +--rw bookstore
     +--rw bookstore-name?   string
     +--rw categories* [code]
        +--rw code     string
        +--rw name?    string
        +--rw books* [title]
           +--rw title       string
           +--rw lang?       string
           +--rw authors*    string
           +--rw pub_year?   year
           +--rw price?      uint64


YANG extension Statement

Sample YANG

(stores.yang with extension)

Statements and Description


stores model with extension
module stores {
…
prefix book-store;
…
   extension sampleExtension{
     description
           “This is a sample extension statement description“
      argument name {
          yin-element true;
     }
   }
container bookstore { 
        leaf bookstore-name {
            type string;
        }
….
}


example-module2 model
module example-module2{
…
     import stores {
       prefix book-store;
     }

     book-store:sampleExtension locations {
       list address {
         key "state";
         leaf state {
           type string;
           description "State name"{
               book-store:sampleExtension-b "b-sample-name";
           }
         }
         leaf city {
           type string;
           description "City name"{
               book-store:sampleExtension-c "c-sample-name";
           }
….
   }


extension Statement

see example from Lines 5-11 on stores model with extension

  • Syntax
extension <keyword/identifier>{
     <extension substatements...>
}


  • Usage

see example from Lines 

<module prefix>:<extension keyword> "argument";


  • to be used to define new statements
  • available to be imported and used by other modules just like a normal YANG statement
      • by use of 'import statement' to import the module where the extension is defined
  • statements that do not have any substatements can have extensions defined if wanted
  • its only one argument is the identifier and keyword for the extension
  • Optional substatements:
      • argument Statement
      • description Statement
      • reference Statement
      • defined extension Statements







argument Statement

see examples from Lines on stores model

  • takes a string argument which is the name of the argument to the keyword
  • Optional substatement
      • yin-element Statement



example-model2 model (YIN version)
<?xml version="1.0" encoding="UTF-8"?>
<module name="example-module2"
        xmlns="urn:ietf:params:xml:ns:yang:yin:1"
        xmlns:em="org:onap:ccsdk:sample2"
        xmlns:book-store="org:onap:ccsdk:sample">
  <yang-version value="1.1"/>
  <namespace uri="org:onap:ccsdk:sample2"/>
  <prefix value="em"/>
  <import module="stores">
    <prefix value="book-store"/>
  </import>
  <book-store:sampleExtension>
    <book-store:name>locations</book-store:name>
    <list name="address">
      <key value="state"/>
      <leaf name="state">
        <type name="string"/>
        <description>
          <text>State name</text>
          <book-store:sampleExtension-b name="b-sample-name"/>
        </description>
      </leaf>
      <leaf name="city">
        <type name="string"/>
        <description>
          <text>City name</text>
          <book-store:sampleExtension-c>
            <book-store:name>c-sample-name</book-store:name>
          </book-store:sampleExtension-c>
        </description>
      </leaf>
    </list>
  </book-store:sampleExtension>
</module>

yin-element Statement

  • takes a string argument which is true or false
  • yin-element is 'false' by default
  • if the argument is 'true' it indicates that the argument is mapped to an XML element in YIN or to an XML attribute


  1. Line 

** the YIN version and Schema trees above are generated by YANG validator 'pyang'

Expected tree diagram for example-module based on RFC8340 (https://datatracker.ietf.org/doc/html/rfc8340):

Schema tree for sampleExtension
sampleExtension locations:
     +-- address* [state]
        +-- state string
        +-- city? string

Existing YANG parser in CPS

(Please see https://wiki.onap.org/display/DW/Existing+Yang+Parser)

OpenDayLight Yang tools recognises YANG extensions

  • Contains interface which has methods to access data of a YANG extension statement

    Yang tools ExtensionDefinition Interface
    package org.opendaylight.yangtools.yang.model.api;
import org.opendaylight.yangtools.yang.model.api.stmt.ExtensionEffectiveStatement;
public interface ExtensionDefinition extends SchemaNode, EffectiveStatementEquivalent<ExtensionEffectiveStatement> {
    String getArgument();
    boolean isYinElement();
}      
    • Implementing Class
        • Class






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