Semantic Definition Format (SDF) for Data and Interactions of Things: Compact Notation

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Table of Contents

1. Introduction

The Semantic Definition Format (SDF, [I-D.ietf-asdf-sdf]) is a format for domain experts to use in the creation and maintenance of data and interaction models that describe Things, i.e., physical objects that are available for interaction over a network. It was created as a common language for use in the development of the One Data Model liaison organization (OneDM) definitions. Tools convert this format to database formats and other serializations as needed.¶

The SDF format is mainly intended for interchange between machine generation and machine processing. However, there is often a need for humans to look at and edit SDF models.¶

Similar to the way Relax-NG as defined in [RELAXNG] has an XML-based format and a compact format (its Annex C), this specification defines a compact format to go along SDF's JSON-based format.¶

The present version of this document is mostly a proof of concept, but was deemed useful to obtain initial feedback on the approach taken.¶

The intention is to be able to bidirectionally translate between compact and JSON form, without appreciable semantic losses. This will allow viewing SDF in compact form, apply edits if needed, and then continuing processing it in JSON form. As a limitation of this approach, it will be difficult to always recreate the order of map entries (members of JSON objects) in the JSON form; this order is essentially arbitrary as maps (JSON objects) in JSON are unordered. (In the long run, it may be useful to define a canonical presentation order in the SDF specification or here.)¶

An initial prototype of a converter from compact form to JSON form was constructed during the ASDF/WISHI hackathon preceding IETF110. A more complete, bidirectionally operating version of this tool is planned for release soon.¶

2. Overview

The SDF compact format is a YAML file [YAML]; a good part of the work needed for a compact representation is already done by the increased user-friendliness of YAML over JSON.¶

In addition, all the sections defined with named<...> in the CDDL definition of SDF have been compacted into map entries with space-separated keys, giving the kind first and the name next. This saves the need for another level of hierarchy and reminds the reader of the kind of item being specified.¶

The map key description is replaced by :, which also is rendered in the most compact form possible in YAML.¶

The dataqualities readable, writable, and observable, together with optionality, are compressed into a four-character map key: The first three are translated into rwo when set (default in SDF) and into --- when not set. The fourth character is ? for optional and ! for required.¶

The value of this abbreviated key is a CDDL [RFC8610] rendition of the attributes defined in the jsonschema production in Appendix A of [I-D.ietf-asdf-sdf]. To further reduce noise, a top-level array production in the CDDL can be represented as an array in the YAML (i.e., does not require additional quotes).¶

info:
  copyright: Copyright 2018-2019 Open Connectivity Foundation, Inc.
    All rights reserved.
  version: '2019-06-11'
  title: Cadence
  license:
    https://github.com/one-data-model/oneDM/blob/master/LICENSE

object cadence:
  :: This Resource describes the cadence, which is the number of
     revolutions of crank per minute when cyclists pedal the
     pedals. The unit, which is the default unit, is rpm. The cadence
     Property is a read-only value that is provided by the
     server. When range (from "oic. r. baseresource") is omitted the
     default is 0 to +MAXFLOAT.
  property cadence:
    r-o!: integer .ge 0
    unit: 1/min
    :: This Property describes the rate at which a cyclist is
       pedaling/turning the pedals.
  property range:
    r-o?: [2*2 integer]
    :: The valid range for the Property in the Resource
       as an integer.
       The first value in the array is the minimum value,
       the second value in the array is the maximum value.
  property step:
    r-o?: integer
    :: Step value across the defined range when the range is an
       integer.  This is the increment for valid values across the
       range; so if range is 0..10 and step is 2 then valid values
       are 0,2,4,6,8,10.

3. IANA Considerations

This section is to be removed before publishing as an RFC.¶

This document does not make any requests of IANA.¶

4. Security Considerations

The security considerations of [I-D.ietf-asdf-sdf] apply.¶

Additional security considerations arise as in all alternative representation forms for a formal description technique. (Security considerations are given for RELAX-NG compact form in [RELAXNG], Annex C.7 (Media type registration template for the RELAX NG Compact Syntax); these actually apply to SDF in general. A more detailed discussion of the consequences of using dereferenceable identifiers can be found in the penultimate paragraph of Section 8 (Security Considerations) of [I-D.ietf-asdf-sdf].)¶

Appendix A. Example in SDF JSON format

This appendix shows the result of automatically converting Figure 1 into the JSON form of SDF. It was produced using the prototype sdfc tool.¶

Note that JSON was not designed to enable JSON texts to always fit into the confines of the RFC format; the presentation here employs [RFC8792] line wrapping, which is of course not visible in the actual JSON text for the example presented in Figure 2.¶

=============== NOTE: '\' line wrapping per RFC 8792 ================

{
  "info": {
    "title": "Cadence",
    "license": "https://github.com/one-data-model/oneDM/blob/master/\
                                                            LICENSE",
    "version": "2019-06-11",
    "copyright": "Copyright 2018-2019 Open Connectivity Foundation, \
                                           Inc. All rights reserved."
  },
  "sdfObject": {
    "cadence": {
      "description": "This Resource describes the cadence, which is \
the number of revolutions of crank per minute when cyclists pedal \
the pedals. The unit, which is the default unit, is rpm. The \
cadence Property is a read-only value that is provided by the server\
. When range (from \"oic. r. baseresource\") is omitted the default \
                                                 is 0 to +MAXFLOAT.",
      "sdfProperty": {
        "step": {
          "type": "integer",
          "writable": false,
          "description": "Step value across the defined range when \
the range is an integer.  This is the increment for valid values \
across the range; so if range is 0..10 and step is 2 then valid \
                                            values are 0,2,4,6,8,10."
        },
        "range": {
          "type": "array",
          "items": {
            "type": "integer"
          },
          "maxItems": 2,
          "minItems": 2,
          "writable": false,
          "description": "The valid range for the Property in the \
Resource as an integer. The first value in the array is the minimum \
          value, the second value in the array is the maximum value."
        },
        "cadence": {
          "type": "integer",
          "minimum": 0,
          "writable": false,
          "description": "This Property describes the rate at which \
                          a cyclist is pedalling/turning the pedals."
        }
      },
      "sdfRequired": [
        "#/sdfObject/cadence/sdfProperty/cadence"
      ]
    }
  }
}

Acknowledgements

The idea for this draft originated at the IETF110 ASDF/WISHI hackathon. The author would like to thank the attendees for initial feedback.¶

Author's Address