Modeling JSON Text with YANG
draft-lhotka-netmod-yang-json-01

Abstract

This document defines rules for mapping data models expressed in YANG to configuration and operational state data encoded as JSON text. It does so by specifying a procedure for translating the subset of YANG-compatible XML documents to JSON text, and vice versa.

Status of This Memo

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This Internet-Draft will expire on October 13, 2013.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Terminology and Notation
• 3. Specification of the Translation Procedure
• 3.1. Names and Namespaces
• 3.2. Mapping XML Elements to JSON Objects
• 3.3. Mapping YANG Datatypes to JSON Values
• 3.3.1. Numeric Types
• 3.3.2. The "string" Type
• 3.3.3. The "boolean" Type
• 3.3.4. The "enumeration" Type
• 3.3.5. The "bits" Type
• 3.3.6. The "binary" Type
• 3.3.7. The "leafref" Type
• 3.3.8. The "identityref" Type
• 3.3.9. The "empty" Type
• 3.3.10. The "union" Type
• 3.3.11. The "instance-identifier" Type
• 3.4. Example
• 3.5. IANA Considerations
• 3.6. Security Considerations
• 3.7. Acknowledgments
• 4. References
• 4.1. Normative References
• 4.2. Informative References
• Author's Address

1. Introduction

The aim of this document is define rules for mapping data models expressed in the YANG data modeling language [RFC6020] to configuration and operational state data encoded as JavaScript Object Notation (JSON) text [RFC4627]. The result can be potentially applied in two different ways:

1. JSON may be used instead of the standard XML [XML] encoding in the context of the NETCONF protocol [RFC6241] and/or with existing data models expressed in YANG. An example application is the YANG-API Protocol [YANG-API].
2. Other documents that choose JSON to represent structured data can use YANG for defining the data model, i.e., both syntactic and semantic constraints that the data have to satisfy.

JSON mapping rules could be specified in a similar way as the XML mapping rules in [RFC6020]. This would however require solving several problems. To begin with, YANG uses XPath [XPath] quite extensively, but XPath is not defined for JSON and such a definition would be far from straightforward.

In order to avoid these technical difficulties, this document employs an alternative approach: it defines a relatively simple procedure which allows for translating the subset of XML that can be modeled using YANG to JSON, and vice versa. Consequently, validation of a JSON text against a data model can done by translating the JSON text to XML, which is then validated according to the rules stated in [RFC6020].

The translation procedure is adapted to YANG specifics and requirements, namely:

1. The translation is driven by a concrete YANG data model and uses information about data types to achieve better results than generic XML-JSON translation procedures.
2. Various document types are supported, namely configuration data, configuration + state data, RPC input and output parameters, and notifications.
3. XML namespaces specified in the data model are mapped to namespaces of JSON objects. However, explicit namespace identifiers are rarely needed in JSON text.
4. Translation of XML attributes, mixed content, comments and processing instructions is not supported.

2. Terminology and Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

The following terms are defined in [RFC6020]:

• anyxml
• augment
• container
• data node
• data tree
• datatype
• feature
• identity
• instance identifier
• leaf
• leaf-list
• list
• module
• submodule

The following terms are defined in [XMLNS]:

• local name
• prefixed name
• qualified name

3. Specification of the Translation Procedure

The translation procedure defines a 1-1 correspondence between the subset of YANG-compatible XML documents and JSON text. This means that the translation can be applied in both directions and is always invertible.

Any YANG-compatible XML document can be translated, except documents with mixed content. This is only a minor limitation since mixed content is marginal in YANG - it is allowed only in "anyxml" nodes.

An implementation of the translation procedure MAY translate "anyxml" nodes and their contents from XML to JSON or vice versa, but the specific details of this translation are outside the scope of this document. Note that the contents of "anyxml" nodes are not relevant for validity in terms of a YANG data model.

The following subsections specify rules mainly for translating XML documents to JSON text. Rules for the inverse translation are stated only where necessary, otherwise they can be easily inferred.

REQUIRED parameters of the translation procedure are:

• YANG data model,
• type of the input XML document,
• optional features (defined via the "feature" statement) that are considered active.

The permissible types of XML documents are listed in Table 1 together with the corresponding part of the data model that is used for the translation.

        <module name>:<local name>

    <foo>42</foo>
        

    "foo": 42
        

    "foo": ["42"]
        

    leaf-list bar {
        type union {
            type uint16;
            type string;
        }
    }
          

    <bar>6378</bar>
    <bar>14.5</bar>
    <bar>infinity</bar>
          

    "bar": [6378, "14.5", "infinity"]
          

module ex-json {

  namespace "http://example.com/ex-json";

  prefix "ej";

  import ietf-inet-types {
    prefix "inet";
  }

  container top {
    list address {
      key "seqno";
      leaf seqno {
        type uint8;
      }
      leaf ip {
        type inet:ip-address;
        mandatory "true";
      }
    }
    container phases {
      typedef angle {
        type decimal64 {
          fraction-digits "2";
        }
        units "radians";
      }
      leaf max-phase {
        default "6.28";
        type angle;
      }
      leaf-list phase {
        type angle;
        must ". <= ../max-phase";
        min-elements "1";
      }
    }
  }
}

{
  "top": {
    "address": [
      {
        "seqno": 1,
        "ip": "192.0.2.1"
      },
      {
        "seqno": 2,
        "ip": "2001:db8:0:1::1"
      }
    ],
    "phases": {
      "phase": [
        0.79,
        1.04,
        3.14
      ]
    }
  }
}