Universität Bremen TZI

Postfach 330440 Bremen D-28359 Germany +49-421-218-63921 cabo@tzi.org

Applications T2TRG Internet-Draft The Semantic Definition Format (SDF) is a format for domain experts to use in the creation and maintenance of data and interaction models in the Internet of Things. 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 often is a need for humans to look at and edit SDF models. Similar to the way Relax-NG as defined in ISO/IEC 19757-2 has an XML format and a compact format (Annex C), this specification defines a compact format to go along SDF's JSON 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. Contributing Recent versions of this document are available at its GitHub repository https://github.com/cabo/sdfc -- this also provides an issue tracker as well as a way to supply "pull requests". General discussion of this SDF Internet-Draft happens on the mailing list of the IETF ASDF Working Group, asdf@ietf.org (subscribe at https://www.ietf.org/mailman/listinfo/asdf). The IETF Note Well applies (https://www.ietf.org/about/note-well/).

Introduction (See Abstract for now.) 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 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 in week 12 of 2021.

Overview The SDF compact format is a YAML file ; 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 rendition of the attributes defined in the jsonschema production in . 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).

Example Definition This is an SDF-compact version of a slightly modified copy of the sdfobject-cadence.sdf.json model found at the time of writing in OneDM's SDF playground. (This example was chosen more or less randomly; better examples can probably be found. The modification is the addition of a unit quality.) 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 pedalling/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. The result of automatically converting this YAML file using the prototype sdfc tool back into the JSON form of SDF is given in . Except for the unit addition, it is semantically identical to the sdfobject-cadence.sdf.json. Differences are visible in the order of map entries (members in JSON objects); a future version of the sdfc tool could attempt to preserve more of this order, even though it does not carry semantics.

References Normative References This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. Dogtiger Labs Universität Bremen TZI The Semantic Definition Format (SDF) is a format for domain experts to use in the creation and maintenance of data and interaction models in the Internet of Things. 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. An SDF specification describes definitions of SDF Objects and their associated interactions (Events, Actions, Properties), as well as the Data types for the information exchanged in those interactions. A JSON format representation of SDF 1.0 was defined in the previous (-00) version of this document. SDF 1.1 is expected to be defined in a future version; the present document is a _candidate implementation draft_ for SDF 1.1. If the ASDF WG reaches consensus on this candidate, the next version will be SDF 1.1, an SDF implementation draft. Informative References ISO/IEC

Example in SDF JSON format { "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.