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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 sync-enxtension {
…
prefix sync-ex;
…
   extension sync-flag{
     description
           “This is a sample extension statement description“
      argument "value";
   }
….
}


extended-stores model
module extended-stores {
    yang-version 1.1;
	...
    import sync-extension{
	    prefix sync-ex;
	}

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

    container bookstore {
       sync-ex:sync-flag "on";
    ...
	}
}


extension Statement

see example from Lines 5-9 on stores model with extension

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


  • Usage

see example from Line 15 on extended-stores model

<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 Line 6 on stores model

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



YIN-extended-stores model
<?xml version="1.0" encoding="UTF-8"?>
<module name="extended-stores"
        xmlns="urn:ietf:params:xml:ns:yang:yin:1"
        xmlns:ext-book-store="org:onap:ccsdk:sampleExtended"
        xmlns:book-store="org:onap:ccsdk:sample">
  <namespace uri="org:onap:ccsdk:sampleExtended"/>
  <prefix value="ext-book-store"/>
  <revision date="2020-09-15">
    <description>
      <text>Sample Extended Model</text>
    </description>
  </revision>
  <import module="stores">
    <prefix value="book-store"/>
  </import>
  <typedef name="year">
    <type name="uint16">
      <range value="1000..9999"/>
    </type>
  </typedef>
  <container name="bookstore">
    <book-store:sync-flag value="on"/>
    <leaf name="bookstore-name">
      <type name="string"/>
    </leaf>
	...
  </container>
</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


Notes

  1. Line 22 on YIN-extended-stores model
    1. result of using the argument without specifying the yin-element value
      1. yin-element is 'false'
      2. the argument 'value' is only an XML attribute
  2. if argument statement (Line 6 on stores model) contains yin-element substatement YIN-extend-stores model would result to the following:

    1. extension statement will produce a child node

      YIN-extended-stores model where yin-element is 'true'
      ...
  <container name="bookstore">
    <book-store:sync-flag>
      <book-store:value>on</book-store:value>
    </book-store:sync-flag>
...
  </container>
...



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

Existing YANG parser in CPS

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

OpenDayLight Yang tools recognize 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();
}      

Test cases and scenarios

The following test cases used and modified the standard stores model seen above.

Testing with groovy

The following cases were tested in YangTextSchemaSourceSetSpec.groovy using the test method 'Building a valid YangTextSchemaSourceSet using #filenameCase filename)'.

Key

**Green cell for case number indicates that tests have passed

**Red cell for case number indicates that tests have failed

  • extension defined as substatement in the module with argument statement as its substatement
Case #DescriptionJAVA objectNotes
1
  • extension used inside container statement before all other substatements
module stores {
    yang-version 1.1;
	...
    extension sync-flag{
        description "This extension allows to tag nodes with a sync flag";
        argument "value";
    }

    typedef year {...}

    container bookstore {
        book-store:sync-flag "on";
		...
        leaf bookstore-name {...}
		list categories {..}
...
}

  • extension declared after all other substatements of container still passed
2
  • extension used inside a leaf statement (child of a container) before all other substatements
module stores {
    yang-version 1.1;
	...
    extension sync-flag{
        description "This extension allows to tag nodes with a sync flag";
        argument "value";
    }

    typedef year {...}

    container bookstore {
		...
        leaf bookstore-name {
	      book-store:sync-flag "on"; 
		}
		list categories {..}
...
}

  • Extension declared after type-statement inside leaf statement (child of a container) still passed
3
  • extension used inside list statement (child of a container) before all other substatements
module stores {
    yang-version 1.1;
	...
    extension sync-flag{
        description "This extension allows to tag nodes with a sync flag";
        argument "value";
    }

    typedef year {...}

    container bookstore {
		...
        leaf bookstore-name {...}
		list categories {
		 	book-store:sync-flag "on";
			key "code";
			...  
		}
...
}

  • Extension declared after key-statement inside list statement (child of a container) still passed
4
  • extension used inside leaf statement (child of list node from case #3) before all its substatements
module stores {
    yang-version 1.1;
 	...
    extension sync-flag{
        description "This extension allows to tag nodes with a sync flag";
        argument "value";
    }
    typedef year {...}

    container bookstore {
        leaf bookstore-name {...}

    list categories {
        key "code";
        leaf code {
            book-store:sync-flag "on";
            type string;
        }
        leaf name {...}
        list books {...}
...
}

  • Extension declared after type-statement inside leaf statement (child of list node from case #3) still passed
5
  • extension used inside list node (child of list node from case #4) before key-statement of list
module stores {
    yang-version 1.1;
 	...
    extension sync-flag{
        description "This extension allows to tag nodes with a sync flag";
        argument "value";
    }
    typedef year {...}

    container bookstore {
        leaf bookstore-name {...}

    list categories {
        key "code";
        leaf code {...}
        leaf name {...}
        list books {
		 book-store:sync-flag "on";
		...
		}
...
}

  • extension declared after key-statement of the list statement still passed
6
  • extension used inside leaf-list node (child of list node from case #5) before all other substatements
module stores {
    yang-version 1.1;
 	...
    extension sync-flag{
        description "This extension allows to tag nodes with a sync flag";
        argument "value";
    }
    typedef year {...}

    container bookstore {
        leaf bookstore-name {...}

    list categories {
        key "code";
        leaf code {...}
        leaf name {...}
        list books {
			leaf-list authors{
			  book-store:sync-flag "on"; 
			}
		}
...
}

  • extension declared after type-statement of leaf-list still passed
7
  • extension used inside module
module stores {
    yang-version 1.1;
	...
    extension sync-flag{
        description "This extension allows to tag nodes with a sync flag";
        argument "value";
    }

    typedef year {...}

    book-store:sync-flag "on"; 

    container bookstore {...}
}

  • data tree still only has one direct child (container node) as with the standard stores model
  • value of the argument is not seen?
8
  • scenario the same as case 1 but the extension was used in the container statement without an argument
module stores {
    yang-version 1.1;
	...
    extension sync-flag{
        description "This extension allows to tag nodes with a sync flag";
        argument "value";
    }

    typedef year {...}

    container bookstore {
        book-store:sync-flag;
		...
}

  • same extension declaration/usage fails for:
      • scenarios in all cases 2-7
9
  • no argument declared for extension
module stores {
    yang-version 1.1;
	...
    extension sync-flag;

    typedef year {...}

    container bookstore {
        book-store:sync-flag;
		...
        leaf bookstore-name {...}
		list categories {..}
...
}

  • use of the same definition and declaration of the extension on this case for cases #1-7 passes
10
  • extension used twice
    • inside container statement before all other substatements
    • inside leaf statement (child of container statement)
module stores {
    yang-version 1.1;
	...
    extension sync-flag;

    typedef year {...}

    container bookstore {
        book-store:sync-flag;
		...
        leaf bookstore-name {
			book-store:sync-flag;
		}
		list categories {..}
...
}



Extra Notes:

  • There seems to be no inheritance of the extension statement for the substatements wherein the extension was used
  • The date tree and schema tree does not change sizes with the addition of the extensions for all cases shown above

Parsing a original data instance (json) using the extended model

Case #DescriptionResult
1
  • Created schema set
    • the model contained only an extension definition
  • successfully stored yang resource
  • successfully created a node using the model
    • fragment table did not add the extension argument in attributes
2
  • Created schema set
    • the model contained an extension definition and the model was used inside a container node
  • successfully stored yang resource
  • successfully created a node using the model
    • fragment table did not add the extension argument in attributes


*the same result acquired for all passed cases on the groovy tests table above

3
  • Created schema set
    • the model contained only an extension definition
      • extension contains argument
          • yin-element set to true
  • creating the schema set failed
    • received 500 server error parsing schema set
4
  • Created schema set
    • the model contained an extension definition and was used inside the container node
      • extension contains argument
          • yin-element set to true
  • creating the schema set failed
    • received 500 server error parsing schema set



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