1 Overview
2 References:
3 YANG Language
- 3.1 Basic YANG statements
  - 3.1.1 stores model
  - 3.1.2 Schema tree of module 'stores'
4 YANG extension Statement
- 4.1 stores model with extension
- 4.2 extended-stores model
- 4.3 YIN-extended-stores model
5 Existing YANG parser in CPS
- 5.1 Test cases and scenarios
  - 5.1.1 Testing with groovy
  - 5.1.2 Parsing a original data instance (json) using the extended model
6 Conclusions

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

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

Sample YANG

(stores.yang with extension)

Statements and Description

stores model with extension

module sync-extension {
…
prefix sync-ext;
…
   extension sync-flag{
     description
           “This is a sample extension statement description“
      argument "value";
   }
}

The following model shows that it imported the model above where extensions is declared.

extended-stores model

extension Statement

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

Syntax

Usage

see example from Line 15 on extended-stores model

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

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

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
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'
  ** extension does not extend the data

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

Existing YANG parser in CPS

(Please see https://lf-onap.atlassian.net/wiki/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

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

the extension is defined as substatement in the module with argument statement as its substatement

Case #	Description	JAVA object	Notes
1	extension used inside container statement before all other substatements		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		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		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		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		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		extension declared after type-statement of leaf-list still passed
7	extension used inside module		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		same extension declaration/usage fails for: scenarios in all cases 2-7
9	no argument declared for extension		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)

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

	Description	Result

	Description	Result
1	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	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	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	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

Conclusions

The YANG language extension analysis above is only applicable for seeing that it is good for type configuration.
- The extension does not extend the actual data of the model instance
- Based on the test scenarios above, extensions can only be seen on the schema sets and not on the data
YANG extension can be used at every level of the tree model
The test results show that the model is being recognized by the current YANG tools used to parse a model
- It fails for scenarios such as parsing a model without an argument defined when it is expecting it
This analysis did not look further into the effect of setting yin-element to 'true' for the model instance
Further investigation is required to cover interpretation as the analysis above only covers parsing
- see CPS-866: SPIKE: Investigate YANG language extension interpretationOpen

ONAP Wiki

CPS-735 Spike: Support for Yang Language Extension