Internet-Draft | SDF compact | October 2024 |
Bormann | Expires 24 April 2025 | [Page] |
The Semantic Definition Format (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 ISO/IEC 19757-2 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.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-bormann-asdf-sdf-compact/.¶
Discussion of this document takes place on the A Semantic Definition Format for Data and Interactions of Things (asdf) Working Group mailing list (mailto:asdf@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/asdf/. Subscribe at https://www.ietf.org/mailman/listinfo/asdf/.¶
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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.¶
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).¶
Figure 1 is an SDF-compact representation 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.)¶
The result of automatically converting this YAML file using the
prototype sdfc
tool back into the JSON form of SDF is given in
Appendix A.
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 the order does not carry
semantics, neither in the JSON form nor in the compact form.¶
This section is to be removed before publishing as an RFC.¶
This document does not make any requests of IANA.¶
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].)¶
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.¶
The idea for this draft originated at the IETF110 ASDF/WISHI hackathon. The author would like to thank the attendees for initial feedback.¶