v: 3 coding: utf-8
title: > Semantic Definition Format (SDF) for Data and Interactions of Things abbrev: SDF (Semantic Definition Format) docname: draft-ietf-asdf-sdf-latest category: std consensus: true submissiontype: IETF
area: Applications workgroup: ASDF keyword: Internet-Draft
venue: group: A Semantic Definition Format for Data and Interactions of Things (ASDF) mail: [email protected] github: ietf-wg-asdf/SDF
author:
- name: Michael Koster org: KTC Control AB street: 29415 Alderpoint Road city: Blocksburg, CA code: '95514' country: USA phone: "+1-707-502-5136" email: [email protected] role: editor
- name: Carsten Bormann org: Universität Bremen TZI orgascii: Universitaet Bremen TZI street: Postfach 330440 city: Bremen code: D-28359 country: Germany phone: +49-421-218-63921 email: [email protected] role: editor
- name: Ari Keränen org: Ericsson city: Jorvas code: '02420' country: Finland email: [email protected] contributor:
- name: Jan Romann org: Hochschule Emden/Leer email: [email protected] country: Germany
- name: Wouter | van der Beek
org: Cascoda Ltd.
street:
- Threefield House
- Threefield Lane city: Southampton country: United Kingdom email: [email protected]
normative: IANA.senml: units IANA.params: params RFC3339: dt RFC8428: senml RFC8798: senml-units-2 RFC3986: uri RFC4122: uuid RFC6901: pointer RFC7396: merge-patch RFC3629: utf8 RFC8259: json RFC8610: cddl RFC8949: cbor RFC9193: data-ct RFC8126: -: reg display: BCP26 W3C.NOTE-curie-20101216: curie RFC0020: ascii SPDX: title: SPDX License List target: https://spdx.org/licenses/ date: false RFC9165: control informative: JSO7: =: I-D.handrews-json-schema-01 -: jso ann: This is the base specification for json-schema.org "draft 7". JSO7V: =: I-D.handrews-json-schema-validation-01 -: jso7v ann: This is the validation specification for json-schema.org "draft 7". JSO4: =: I-D.draft-zyp-json-schema-04
-: jso4
ann: This is the base specification for json-schema.org "draft 4".
JSO4V: =: I-D.fge-json-schema-validation-00 -: jso4v ann: This is the validation specification for json-schema.org "draft 4".
I-D.irtf-t2trg-rest-iot: rest-iot ZCL: DOI.10.1016/B978-0-7506-8597-9.00006-9 OMA: title: OMA LightweightM2M (LwM2M) Object and Resource Registry date: false target: http://www.openmobilealliance.org/wp/omna/lwm2m/lwm2mregistry.html OCF: title: OCF Resource Type Specification date: false
target: https://openconnectivity.org/specs/OCF_Resource_Type_Specification.pdf
RFC8576: seccons ECMA-262: target: https://www.ecma-international.org/wp-content/uploads/ECMA-262.pdf title: ECMAScript 2020 Language Specification author: - org: Ecma International date: 2020-06 seriesinfo: ECMA: Standard ECMA-262, 11th Edition CamelCase: target: http://wiki.c2.com/?CamelCase title: Camel Case date: '2014-12-18' KebabCase: target: http://wiki.c2.com/?KebabCase title: Kebab Case date: '2014-08-29' I-D.bormann-asdf-sdftype-link: sdflink I-D.bormann-asdf-sdf-mapping: mapping I-D.bormann-t2trg-deref-id: deref RFC9485: iregexp I-D.ietf-jsonpath-base: jsonpath
entity: SELF: "[RFC-XXXX]"
--- abstract
--- middle
SDF is designed to be an extensible format.
The present document constitutes the base specification for SDF: we
speak of "base SDF" for short.
In addition, SDF extensions can be defined, some of which may make use
of extension points specifically defined for this in base SDF.
One area for such extensions would be refinements of SDF's abstract
interaction models into protocol bindings for specific ecosystems
(e.g., {{-mapping}}).
For the use of certain other extensions, it may be necessary to
indicate in the SDF
document using them that a specific extension is in effect; see
{{information-block}} for details of the features quality that can be
used for such indications.
With extension points and feature indications available,
base SDF does not define a "version" concept for the SDF format itself
(as opposed to version indications within SDF documents indicating
their own evolution, see {{information-block}}).
After introductory material and an overview ({{overview}}) over the
elements of the model and over the different kinds of names used,
{{sdf-structure}} introduces the main components of an SDF model.
{{names-and-namespaces}} revisits names and structures them into
namespaces.
{{kw-defgroups}} discusses the inner structure of the Objects defined by
SDF, the sdfObjects, in further detail.
{{high-level-composition}} discusses how SDF supports composition.
Conventional Sections ({{<<iana}}, {{<<seccons}},
{{<<sec-normative-references}}, and {{<<sec-informative-references}})
follow.
The normative {{syntax}} defines the syntax of SDF in
terms of its JSON structures, employing the Concise Data Definition
Language (CDDL) {{-cddl}}.
This is followed by the informative {{jso}}, a rendition of the SDF
syntax in a "JSON Schema" format as they are defined by
json-schema.org (collectively called JSO).
The normative {{jso-inspired}} defines certain terms ("data qualities")
used at the SDF data model level that were inspired by JSO.
The informative {{composition-examples}} provides a few
examples for the use of composition in SDF.
Finally, {{earlier}} provides some historical information that can be
useful in upgrading earlier, pre-standard SDF models and
implementations to SDF base.
{:unnumbered}
The terms introduced in this section are capitalized when used in this section; to maintain readability, this is not always done when they are used in the body of this document.
Thing: : A physical item that is also available for interaction over a network.
Element: : A part or an aspect of something abstract; i.e., the term is used here in its usual English definition. (Exceptionally, in {{jso-inspired}}, used explicitly in accordance with its meaning in the JSON ecosystem, i.e., the elements of JSON arrays.)
Affordance: : An element of an interface offered for interaction. Such an element becomes an Affordance when information is available (directly or indirectly) that indicates how it can or should be used. In the present document, the term is used for the digital (network-directed) interfaces of a Thing only; as it is a physical object as well, the Thing might also have physical affordances such as buttons, dials, and displays.
Property:
: An Affordance that can potentially be used to read, write, and/or
observe state (current/stored information) on a Grouping.\
(Note that other environments often use the term "property" for a
JSON concept that we call "entry" [see below].
In this document, the term Property is specifically reserved for
affordances, even if the map key "properties" might be imported with
the JSON-level semantics from a data definition language.)
Action: : An Affordance that can potentially be used to perform a named operation on a Grouping.
Event: : An Affordance that can potentially be used to obtain information about what happened to a Grouping.
{:unnumbered}
SDF Document: : Container for SDF Definitions, together with data about the SDF Document itself (information block). Represented as a JSON text representing a single JSON map, which is built from nested maps.
SDF Model: : Definitions and declarations that model the digital interaction opportunities offered by one or more kinds of Things, represented by Groupings (sdfObjects and sdfThings). An SDF Model can be fully contained in a single SDF Document, or it can be built from an SDF Document that references definitions and declarations from additional SDF documents.
Entry: : A key-value pair in a map. (In JSON maps, sometimes also called "member".)
Block: : One or more entries in a JSON map that is part of an SDF specification; these entries can be described as a Block to emphasize that they together serve a specific function.
Group: : An entry in the main JSON map that represents the SDF document, and in certain nested definitions. A group has a Class Name Keyword as its key and a map of named definition entries (Definition Group) as a value.
Class Name Keyword:
: One of sdfThing, sdfObject, sdfProperty, sdfAction,
sdfEvent, or sdfData; the Classes for these type keywords are
capitalized and prefixed with sdf.
Class: : Abstract term for the information that is contained in groups identified by a Class Name Keyword.
Quality: : A metadata item in a definition or declaration which says something about that definition or declaration. A quality is represented in SDF as an entry in a JSON map (JSON object) that serves as a definition or declaration. (The term "Quality" is used because another popular term, "Property", already has a different meaning.)
Definition: : An entry in a Definition Group. The entry creates a new semantic term for use in SDF models and associates it with a set of qualities. Unless the Class Name Keyword of the Group also makes it a Declaration (see {{definitions-block}}), a definition just defines a term, it does not create a component item within the enclosing definition.
Declaration: : A definition within an enclosing definition that is intended to create a component item within that enclosing definition. Every declaration can also be used as a definition for reference elsewhere.
Grouping: : An sdfThing or sdfObject, i.e., (directly or indirectly) a description for a combination of Affordances.
Object, sdfObject:
: A Grouping that contains Affordance declarations (Property, Action,
and Event declarations) only.
It serves as the main "atom" of reusable semantics for model
construction, representing the interaction model for a Thing that is
simple enough to not require nested structure.
sdfObjects are therefore similar to sdfThings but do not allow
nesting, i.e., they cannot contain other Groupings (sdfObjects or
sdfThings).\
(Note that
JSON maps are often called JSON objects due to JSON's JavaScript
heritage; in the context of SDF, the term Object as the colloquial shorthand for sdfObject, is specifically reserved for the
above Grouping, even if
the type name "object" is imported from a data definition
language with the other semantics.)
sdfThing: : A Grouping that can contain nested Groupings (sdfThings and sdfObjects). Like sdfObject, it can also contain Affordance declarations (Property, Action, and Event declarations). (Note that "Thing" has a different meaning from sdfThing and therefore is not available as a colloquial shorthand of sdfThing.)
Protocol Binding: : A companion document to an SDF Model that defines how to map the abstract concepts in the model into the protocols in use in a specific ecosystem. The Protocol Binding might supply URL components, numeric IDs, and similar details. Protocol Bindings are one case of an Augmentation Mechanism.
Augmentation Mechanism: : A companion document to a base SDF Model that provides additional information ("augments" the base specification). The information may be for use in a specific ecosystem or with a specific protocol ("Protocol Binding"). No specific Augmentation Mechanisms are defined in base SDF. A simple mechanism for such augmentations has been discussed as a "mapping file" {{-mapping}}.
{:unnumbered}
The term "byte" is used in its now-customary sense as a synonym for "octet".
Conventions:
- The singular form is chosen as the preferred one for the keywords defined here.
{::boilerplate bcp14-tagged}
We start with an example for the SDF definition of a simple sdfObject called "Switch" ({{example1}}).
{
"info": {
"title": "Example document for SDF (Semantic Definition Format)",
"version": "2019-04-24",
"copyright": "Copyright 2019 Example Corp. All rights reserved.",
"license": "https://example.com/license"
},
"namespace": {
"cap": "https://example.com/capability/cap"
},
"defaultNamespace": "cap",
"sdfObject": {
"Switch": {
"sdfProperty": {
"value": {
"description":
"The state of the switch; false for off and true for on.",
"type": "boolean"
}
},
"sdfAction": {
"on": {
"description":
"Turn the switch on; equivalent to setting value to true."
},
"off": {
"description":
"Turn the switch off; equivalent to setting value to false."
},
"toggle": {
"description":
"Toggle the switch; equivalent to setting value to its complement."
}
}
}
}
}{: #example1 title="A simple example of an SDF document"} {: sourcecode-name="example1.sdf.json"}
This is a model of a switch.
The state value declared in the sdfProperty group, represented by a Boolean, will be true for "on" and will be false for "off".
The actions on or off declared in the sdfAction group are redundant with setting the value and are in the example to illustrate that there are often different ways of achieving the same effect.
The action toggle will invert the value of the sdfProperty value, so that 2-way switches can be created; having such action will avoid the need for first retrieving the current value and then applying/setting the inverted value.
The sdfObject group lists the affordances of Things modeled by this sdfObject.
The sdfProperty group lists the property affordances described by the model; these represent various perspectives on the state of the sdfObject.
Properties can have additional qualities to describe the state more precisely.
Properties can be annotated to be read, write or read/write; how this is actually done by the underlying transfer protocols is not described in the SDF model but left to companion protocol bindings.
Properties are often used with RESTful paradigms {{-rest-iot}}, describing state.
The sdfAction group is the mechanism to describe other interactions in terms of their names, input, and output data (no data are used in the example), as in a POST method in REST or in a remote procedure call.
The example toggle is an Action that
changes the state based on the current state of the Property named value.
(The third type of affordance is Events, which are not described in this example.)
In the JSON representation, note how (with the exception of the info
group) maps that have keys taken from the SDF vocabulary (info,
namespace, sdfObject) alternate in nesting with maps that have keys
that are freely defined by the model writer (Switch, value, on,
etc.); the latter usually use the named<> production in the formal
syntax of SDF, while the former SDF-defined vocabulary items
are often, but not always, called qualities.
The SDF language uses six predefined Class Name Keywords for modeling connected Things which are illustrated in {{fig-class-2}}.
sdfThing --> "0+" sdfObject : hasObject
sdfThing --> "0+" sdfThing : hasThing
sdfThing --> "0+" sdfProperty : hasProperty
sdfThing --> "0+" sdfAction : hasAction
sdfThing --> "0+" sdfEvent : hasEvent
sdfObject --> "0+" sdfProperty : hasProperty
sdfObject --> "0+" sdfAction : hasAction
sdfObject --> "0+" sdfEvent : hasEvent
sdfAction --> "0+" sdfData : hasInputData
sdfAction --> "0+" sdfData : hasOutputData
sdfEvent --> "0+" sdfData : hasOutputData
sdfProperty --> "1" sdfData : isInstanceOf
class sdfThing {
}
class sdfObject {
}
class sdfProperty {
}
class sdfAction {
}
class sdfEvent {
}
class sdfData {
}
{: #fig-class-2 title="Main classes used in SDF models"}
The six main Class Name Keywords are discussed below.
sdfObjects, the items listed in an sdfObject definition group, are
the main "atom" of reusable semantics for model construction.
The concept aligns in scope with common definition items from many IoT modeling
systems, for example ZigBee Clusters {{ZCL}}, OMA SpecWorks LwM2M
Objects {{OMA}}, and
OCF Resource Types {{OCF}}.
An sdfObject definition contains a set of sdfProperty, sdfAction, and
sdfEvent definitions that describe the interaction affordances
associated with some scope of functionality.
For the granularity of definition, sdfObject definitions are meant to be kept narrow enough in scope to enable broad reuse and interoperability. For example, defining a light bulb using separate sdfObject definitions for on/off control, dimming, and color control affordances will enable interoperable functionality to be configured for diverse product types. An sdfObject definition for a common on/off control may be used to control many different kinds of Things that require on/off control.
The presence of one or both of the optional qualities "minItems" and
"maxItems" defines the sdfObject as an array, i.e., all the
affordances defined for the sdfObject exist a number of times, indexed
by a number constrained to be between minItems and maxItems,
inclusive, if given.
(Note: Setting "minItems" to zero and leaving out "maxItems" puts the
minimum constraints on that array.)
sdfProperty is used to model elements of state within Things modeled
by the enclosing grouping.
A named definition entry in an sdfProperty may be associated with some protocol
affordance to enable the application to obtain the state variable and,
optionally, modify the state variable.
Additionally, some protocols provide for in-time reporting of state
changes.
(These three aspects are described by the qualities readable,
writable, and observable defined for an sdfProperty.)
Definitions in sdfProperty groups look like the definitions in
sdfData groups.
However, they actually also declare that a Property
with the given qualities potentially is present in the containing sdfObject.
For definitions in sdfProperty and sdfData, SDF provides qualities
that can constrain the structure and values of data allowed in the
interactions modeled by them.
It also provides qualities that associate semantics to these
data, such as engineering units and unit scaling information.
For the data definition within sdfProperty or sdfData, SDF borrows
some vocabulary proposed for the drafts 4 {{-jso4}} {{-jso4v}} and 7
{{-jso}} {{-jso7v}} of the json-schema.org "JSON Schema" format
(collectively called JSO here), enhanced by qualities that are
specific to SDF.
Details about the JSO-inspired vocabulary are in {{jso-inspired}}.
For base SDF, data are constrained to be of
simple types (number, string, Boolean),
JSON maps composed of named data, and arrays of these types.
Syntax extension points are provided that can be used to provide
richer types in a future extension of this specification (possibly more
of which can be borrowed from json-schema.org).
Note that sdfProperty definitions (and sdfData definitions in general) are not intended to constrain the formats of data used for communication over network interfaces. Where needed, data definitions for payloads of protocol messages are expected to be part of the protocol binding.
The sdfAction group contains declarations of Actions, model affordances that, when triggered,
have more effect than just reading, updating, or observing Thing
state, often resulting in some outward physical effect (which, itself,
cannot be modeled in SDF). From a programmer's perspective, they
might be considered to be roughly analogous to method calls.
Actions may have data parameters: these are modeled as a single item of input
data and output data, each. Where multiple parameters need to be
modeled, an "object" type can be used to combine these parameters into one.
Actions may be long-running, that is to say that the effects may not take place immediately as would be expected for an update to an sdfProperty; the effects may play out over time and emit action results. Actions may also not always complete and may result in application errors, such as an item blocking the closing of an automatic door.
One idiom for giving an action initiator status and control about the ongoing action is to provide a URI for an ephemeral "action resource" in the sdfAction output data, allowing the action to deliver immediate feedback (including errors that prevent the action from starting) and the action initiator to use the action resource for further observation or modification of the ongoing action (including canceling it). Base SDF does not provide any tailored support for describing such action resources; an extension for modeling links in more detail (for instance, {{-sdflink}}) may be all that is needed to fully enable modeling them.
Actions may have (or lack) qualities of idempotence and side-effect safety.
Base SDF only provides data constraint modeling and semantics for the input and output data of definitions in sdfAction groups.
Again, data definitions for payloads of protocol messages, and
detailed protocol settings for invoking the action, are expected to be
part of the protocol binding.
The sdfEvent group contains declarations of Events, which can model
affordances that inform about "happenings" associated with a Thing
modeled by the enclosing sdfObject; these may result in a signal being
stored or emitted as a result.
Note that there is a trivial overlap with sdfProperty state changes, which may also be defined as events but are not generally required to be defined as such. However, Events may exhibit certain ordering, consistency, and reliability requirements that are expected to be supported in various implementations of sdfEvent that do distinguish sdfEvent from sdfProperty. For instance, while a state change may simply be superseded by another state change, some events are "precious" and need to be preserved even if further events follow.
Base SDF only provides data constraint modeling and semantics for the output data of Event affordances. Again, data definitions for payloads of protocol messages, and detailed protocol settings for soliciting the event, are expected to be part of the protocol binding.
Definitions in sdfData groups do not themselves specify affordances.
These definitions
are provided separately from those in
sdfProperty groups to enable common
modeling patterns, data constraints, and semantic anchor concepts to
be factored out for data items that make up sdfProperty items and
serve as input and output data for sdfAction and sdfEvent items.
The data types defined in sdfData definitions only spring to life by being referenced in
one of these contexts (directly or indirectly via some other sdfData
definitions).
It is a common use case for such a data definition to be shared between an sdfProperty item and input or output parameters of an sdfAction or output data provided by an sdfEvent. sdfData definitions also enable factoring out extended application data types such as mode and machine state enumerations to be reused across multiple definitions that have similar basic characteristics and requirements.
Back at the top level, the sdfThing group enables definition of models for
complex devices that will use one or more sdfObject definitions.
Like sdfObject, sdfThing groups also allow for including interaction
affordances, sdfData, as well as "minItems" and "maxItems" qualities.
Therefore, they can be seen as a superset of sdfObject groups, additionally
allowing for composition.
As a result, an sdfThing directly or indirectly contains a set of sdfProperty, sdfAction, and sdfEvent definitions that describe the interaction affordances associated with some scope of functionality.
A definition in an sdfThing group can refine the metadata of the definitions it is composed of: other definitions in sdfThing groups or definitions in sdfObject groups.
SDF documents are JSON maps that mostly employ JSON maps as member values, which in turn mostly employ JSON maps as their member values, and so on. This nested structure of JSON maps creates a tree, where the edges are the member names (map keys) used in these JSON maps. (In certain cases, where member names are not needed, JSON arrays may be interspersed in this tree.)
For any particular JSON map in an SDF document, the set of member names that are used is either of:
-
A set of "Quality Names", where the entries in the map are Qualities. Quality Names are defined by the present specification and its extensions, together with specific semantics to be associated with the member value given with a certain Quality Name.
-
A set of "Given Names", where the entries in the map are separate entities (definitions, declarations, etc.) that each have names that are chosen by the SDF document author in order that these names can be employed by a user of that model.
In a path from the root of the tree to any leaf, Quality Names and Given Names roughly alternate (with the information block, {{information-block}}, as a prominent exception).
The meaning of the JSON map that is the member value associated
with a Given Name is derived from the Quality Name that was used as
the member name associated to the parent.
In the CDDL grammar given in {{syntax}}, JSON maps with member names that are
Given Names are defined using the CDDL generic rule reference named<membervalues>,
where membervalues is in turn the structure of the member values of the
JSON map, i.e., the value of the member named by the Given Name.
As quality-named maps and given-named maps roughly alternate in
a path down the tree, membervalues is usually a map built from
Quality Names as keys.
From the outside of a specification, Given Names are usually used as
part of a hierarchical name that looks like a JSON pointer {{-pointer}},
itself generally rooted in (used as the fragment identifier in) an
outer namespace that looks like an https:// URL (see {{names-and-namespaces}}).
As Quality Names and Given Names roughly alternate in a path into the model, the JSON pointer part of the hierarchical name also alternates between Quality Names and Given Names.
Note that the actual Given Names may need to be encoded when specified
via the JSON pointer fragment identifier syntax, and that there are
two layers of such encoding: tilde encoding of ~ and / as per
{{Section 3 of -pointer}}, and then percent encoding of the
tilde-encoded name into a valid URI fragment as per {{Section 6 of
-pointer}}.
For example, when a model is using the Given Name
warning/danger alarm
(with an embedded slash and a space) for an sdfObject, that sdfObject may need to be referenced as
#/sdfObject/warning~1danger%20alarm
To sidestep potential interoperability problems, it is probably wise to avoid characters in Given Names that need such encoding (Quality Names are already defined in such a way that they never do).
In SDF, both Quality Names and Given Names are extension points. This is more obvious for Quality Names: Extending SDF is mostly done by defining additional qualities. To enable non-conflicting third party extensions to SDF, qualified names (names with an embedded colon) can be used as Quality Names.
A nonqualified Quality Name is composed of ASCII letters, digits, and
$ signs, starting with a lower case letter or a $ sign (i.e.,
using a pattern of "[a-z$][A-Za-z$0-9]*").
Names with $ signs are intended to be used for functions separate
from most other names; for instance, in this specification $comment
is used for the comment quality (the presence or absence of a
$comment quality does not change the meaning of the SDF model).
Names that are composed of multiple English words can use the
"lowerCamelCase" convention {{CamelCase}} for indicating the word
boundaries; no other use is intended for upper case letters in quality
names.
A qualified Quality Name is composed of a Quality Name Prefix, a :
(colon) character, and a nonqualified Quality Name.
Quality Name Prefixes are registered in the "Quality Name Prefixes"
sub-registry in the "SDF Parameters" registry ({{qnp}}); they are
composed of lower case ASCII letters and digits, starting with a lower
case ASCII letter (i.e., using a pattern of "[a-z][a-z0-9]*").
Given Names are not restricted by the formal SDF syntax.
To enable non-surprising name translations in tools, combinations of
ASCII alphanumeric characters and - (ASCII hyphen/minus) are preferred,
typically employing kebab-case for names constructed out of multiple
words {{KebabCase}}. ASCII hyphen/minus can then unambiguously be
translated to an ASCII _ underscore character and back depending on
the programming environment.
Some styles also allow a dot . in given names.
Given Names are often sufficiently self-explanatory that they can be
used in place of the label quality if that is not given.
In turn, if a given name turns out too complicated, a more elaborate
label can be given and the given name kept simple.
Base SDF does not address internationalization of
given names.
Further, to enable Given Names to have a more powerful role in building global hierarchical names, an extension is planned that makes use of qualified names for Given Names. So, until that extension is defined, Given Names with (one or more) embedded colons are reserved and MUST NOT be used in an SDF document.
All names in SDF are case-sensitive.
SDF definitions are contained in SDF documents, together with data about the SDF document itself (information block). Definitions and declarations from additional SDF documents can be referenced; together with the definitions and declarations in the referencing SDF document they build the SDF model expressed by that SDF document.
Each SDF document is represented as a single JSON map. This map can be thought of as having three blocks: the information block, the namespaces block, and the definitions block. These blocks contain zero or more JSON name/value pairs, the names of which are quality names and the values of which mostly are (nested) maps (the exception defined in SDF base is the defaultNamespace quality, the value of which is a text string). An empty nested map of this kind is equivalent to not having the quality included at all.
The information block contains generic metadata for the SDF document itself and all included definitions. To enable tool integration, the information block is optional in the grammar of SDF; most processes for working with SDF documents will have policies that only SDF documents with an info block can be processed. It is therefore RECOMMENDED that SDF validator tools emit a warning when no information block is found.
The keyword (map key) that defines an information block is "info". Its value is a JSON map in turn, with a set of entries that represent qualities that apply to the included definition.
Qualities of this map are shown in {{infoblockqual}}. None of these qualities are required or have default values that are assumed if the quality is absent.
| Quality | Type | Description |
|---|---|---|
| title | string | A short summary to be displayed in search results, etc. |
| description | string | Long-form text description (no constraints) |
| version | string | The incremental version of the definition |
| modified | string | Time of the latest modification |
| copyright | string | Link to text or embedded text containing a copyright notice |
| license | string | Link to text or embedded text containing license terms |
| features | array of strings | List of extension features used |
| $comment | string | Source code comments only, no semantics |
| {: #infoblockqual title="Qualities of the Information Block"} |
The version quality is used to indicate version information about the
set of definitions in the SDF document.
The version is RECOMMENDED to be lexicographically increasing over the life of a model: a newer model always has a version string that string-compares higher than all previous versions.
This is easily achieved by following the convention to start the version with an {{RFC3339}} date-time or, if new versions are generated less frequently than once a day, just the full-date (i.e., YYYY-MM-DD); in many cases, that will be all that is needed (see {{example1}} for an example).
This specification does not give a strict definition for the format of the version string but each using system or organization should define internal structure and semantics to the level needed for their use.
If no further details are provided, a date-time or full-date in
this field can be assumed to indicate the latest update time of the
definitions in the SDF document.
The modified quality can be used with a value using {{RFC3339}} date-time (with Z for time-zone) or full-date format to express time of the latest revision of the definitions.
The license string is preferably either a URI that points to a web page with an unambiguous definition of the license, or an {{SPDX}} license identifier. (As an example, for models to be handled by the One Data Model liaison group, this license identifier will typically be "BSD-3-Clause".)
The features quality can be used to list names of critical (i.e., cannot be safely ignored) SDF extension features that need to be understood for the definitions to be properly processed.
Extension feature names will be specified in extension documents.
The namespaces block contains the namespace map and the
defaultNamespace setting; none of these qualities are required or
have default values that are assumed if the quality is absent.
The namespace map is a map from short names for URIs to the namespace URIs themselves.
The defaultNamespace setting selects one of the entries in the namespace map by giving its short name. The associated URI (value of this entry) becomes the default namespace for the SDF document.
| Quality | Type | Description |
|---|---|---|
| namespace | map | Defines short names mapped to namespace URIs, to be used as identifier prefixes |
| defaultNamespace | string | Identifies one of the prefixes in the namespace map to be used as a default in resolving identifiers |
| {: #nssec title="Namespaces Block"} |
The following example declares a set of namespaces and defines cap
as the default namespace.
By convention, the values in the namespace map contain full URIs
without a fragment identifier, and the fragment identifier is then
added, if needed, where the namespace entry is used.
"namespace": {
"cap": "https://example.com/capability/cap",
"zcl": "https://zcl.example.com/sdf"
},
"defaultNamespace": "cap"If no defaultNamespace setting is given, the SDF document does not contribute to a global namespace (all definitions remain local to the model and are not accessible for re-use by other models). As the defaultNamespace is set by giving a namespace short name, its presence requires a namespace map that contains a mapping for that namespace short name.
If no namespace map is given, no short names for namespace URIs are set up, and no defaultNamespace can be given.
The Definitions block contains one or more groups, each identified by a Class Name Keyword (there can only be one group per keyword; the actual grouping is just a shortcut and does not carry any specific semantics). The value of each group is a JSON map, the keys of which serve for naming the individual definitions in this group, and the corresponding values provide a set of qualities (name-value pairs) for the individual definition. (In short, we speak of the map entries as "named sets of qualities".)
Each group may contain zero or more definitions. Each identifier defined creates a new type and term in the target namespace. Declarations have a scope of the definition block they are directly contained in.
A definition may in turn contain other definitions. Each definition is a named set of qualities, i.e., it consists of the newly defined identifier and a set of key-value pairs that represent the defined qualities and contained definitions.
An example for an sdfObject definition is given in {{exobject}}:
"sdfObject": {
"foo": {
"sdfProperty": {
"bar": {
"type": "boolean"
}
}
}
}{: #exobject title="Example sdfObject definition"}
This example defines an sdfObject "foo" that is defined in the default namespace (full address: #/sdfObject/foo), containing a property that can be addressed as
#/sdfObject/foo/sdfProperty/bar, with data of type boolean.
Often, definitions are also declarations: the definition of the
entry "bar" in the property "foo" means that data corresponding to the
"foo" property in a property interaction offered by Thing can have zero or
one components modeled by "bar". Entries within sdfProperty,
sdfAction, and sdfEvent, in turn within sdfObject or sdfThing entries, are
declarations; entries within sdfData are not.
Similarly, sdfObject or sdfThing entries within an sdfThing
definition specify that the
interactions offered by a Thing modeled by this sdfThing include the
interactions modeled by the nested sdfObject or sdfThing.
Besides their placement within an sdfObject or sdfThing, affordances
(i.e., sdfProperty, sdfAction, and sdfEvent) as well as sdfData can
also be placed at the top level of an SDF document.
Since they are not associated with an sdfObject or sdfThing, these kinds of
definitions are intended to be re-used via the sdfRef mechanism
(see {{sdfref}}).
SDF documents may contribute to a global namespace, and may reference elements from that global namespace. (An SDF document that does not set a defaultNamespace does not contribute to a global namespace.)
Global names look exactly like https:// URIs with attached fragment identifiers.
There is no intention to require that these URIs can be dereferenced.
(However, as future extensions of SDF might find a use for dereferencing global names, the URI should be chosen in such a way that this may become possible in the future. See also {{-deref}} for a discussion of dereferenceable identifiers.)
The absolute URI of a global name should be a URI as per {{Section 3 of
-uri}}, with a scheme of "https" and a path (hier-part in {{-uri}}).
For base SDF, the query part should
not be used (it might be used in extensions).
The fragment identifier is constructed as per {{Section 6 of -pointer}}.
The fragment identifier part of a global name defined in an SDF document is constructed from a JSON pointer that selects the element defined for this name in the SDF document.
The absolute URI part is a copy of the default namespace, i.e., the default namespace is always the target namespace for a name for which a definition is contributed. When emphasizing that name definitions are contributed to the default namespace, we therefore also call it the "target namespace" of the SDF document.
For instance, in {{example1}}, definitions for the following global names are contributed:
- https://example.com/capability/cap#/sdfObject/Switch
- https://example.com/capability/cap#/sdfObject/Switch/sdfProperty/value
- https://example.com/capability/cap#/sdfObject/Switch/sdfAction/on
- https://example.com/capability/cap#/sdfObject/Switch/sdfAction/off
Note the #, which separates the absolute-URI part ({{Section 4.3 of
-uri}}) from the fragment identifier part.
A name reference takes the form of the production curie in
{{-curie}} (note that this excludes the production safe-curie),
but also limiting the IRIs involved in that production to URIs as per {{-uri}}
and the prefixes to ASCII characters {{-ascii}}.
A name that is contributed by the current SDF document can be
referenced by a Same-Document Reference as per {{Section 4.4 of
-uri}}.
As there is little point in referencing the entire SDF document, this will be a # followed by a JSON pointer.
This is the only kind of name reference to itself that is possible in an SDF
document that does not set a default namespace.
Name references that point outside the current SDF document need to contain curie prefixes. These then reference namespace declarations in the namespaces block.
For example, if a namespace prefix is defined:
"namespace": {
"foo": "https://example.com/"
}Then this reference to that namespace:
"sdfRef": "foo:#/sdfData/temperatureData"references the global name:
"https://example.com/#/sdfData/temperatureData"Note that there is no way to provide a URI scheme name in a curie, so all references to outside of the document need to go through the namespace map.
Name references occur only in specific elements of the syntax of SDF:
- copying elements via sdfRef values
- pointing to elements via sdfRequired value elements
In a JSON map establishing a definition, the keyword sdfRef is used
to copy all of the qualities and enclosed definitions of the referenced definition, indicated
by the included name reference, into the newly formed definition.
(This can be compared to the processing of the $ref keyword in {{-jso}}.)
For example, this reference:
"temperatureProperty": {
"sdfRef": "#/sdfData/temperatureData"
}creates a new definition "temperatureProperty" that contains all of the qualities defined in the definition at /sdfData/temperatureData.
The sdfRef member need not be the only member of a map. Additional members may be present with the intention to override parts of the referenced map or to add new qualities or definitions.
When processing sdfRef, if the target definition contains also sdfRef (i.e., is based on yet another definition), that MUST be processed as well.
More formally, for a JSON map that contains an sdfRef member, the semantics is defined to be as if the following steps were performed:
- The JSON map that contains the sdfRef member is copied into a variable named "patch".
- The sdfRef member of the copy in "patch" is removed.
- the JSON pointer that is the value of the sdfRef member is dereferenced and the result is copied into a variable named "original".
- The JSON Merge Patch algorithm {{-merge-patch}} is applied to patch the contents of "original" with the contents of "patch".
- The result of the Merge Patch is used in place of the value of the original JSON map.
Note that the formal syntaxes given in Appendices {{<syntax}} and {{<jso}} generally describe the result of applying a merge-patch; the notations are not powerful enough to describe, for instance, the effect of null values given with the sdfRef to remove members of JSON maps from the referenced target. Nonetheless, the syntaxes also give the syntax of the sdfRef itself, which vanishes during the resolution; in many cases therefore even merge-patch inputs will validate with these formal syntaxes.
Given the example ({{example1}}), and the following definition:
{
"info": {
"title": "Example light switch using sdfRef"
},
"namespace": {
"cap": "https://example.com/capability/cap"
},
"defaultNamespace": "cap",
"sdfObject": {
"BasicSwitch": {
"sdfRef": "cap:#/sdfObject/Switch",
"sdfAction": {
"toggle": null
}
}
}
}The resulting definition of the "BasicSwitch" sdfObject would be identical to the definition of the "Switch" sdfObject except it would not contain the "toggle" Action.
{
"info": {
"title": "Example light switch using sdfRef"
},
"namespace": {
"cap": "https://example.com/capability/cap"
},
"defaultNamespace": "cap",
"sdfObject": {
"BasicSwitch": {
"sdfProperty": {
"value": {
"description":
"The state of the switch; false for off and true for on.",
"type": "boolean"
}
},
"sdfAction": {
"on": {
"description":
"Turn the switch on; equivalent to setting value to true."
},
"off": {
"description":
"Turn the switch off; equivalent to setting value to false."
}
}
}
}
}{: sourcecode-name="example1-without-toggle.sdf.json"}
A model where all sdfRef references are processed as described in {{sdfref}} is called a resolved model.
For example, given the following sdfData definitions:
"sdfData": {
"Coordinate" : {
"type": "number", "unit": "m"
},
"X-Coordinate" : {
"sdfRef" : "#/sdfData/Coordinate",
"description":
"Distance from the base of the Thing along the X axis."
},
"Non-neg-X-Coordinate" : {
"sdfRef": "#/sdfData/X-Coordinate",
"minimum": 0
}
}After resolving the definitions would look as follows:
"sdfData": {
"Coordinate" : {
"type": "number", "unit": "m"
},
"X-Coordinate" : {
"description":
"Distance from the base of the Thing along the X axis.",
"type": "number", "unit": "m"
},
"Non-neg-X-Coordinate" : {
"description":
"Distance from the base of the Thing along the X axis.",
"minimum": 0, "type": "number", "unit": "m"
}
}The keyword sdfRequired is provided to apply a constraint that
defines for which declarations the corresponding data are mandatory in a
Thing modeled by the current definition.
The value of sdfRequired is an array of references, each indicating
one or more declarations that are mandatory to be represented.
References in this array can be SDF names (JSON Pointers), or one of two abbreviated reference formats:
- a text string with a "referenceable-name", i.e., an affordance name or grouping name. All affordance declarations that are directly (i.e., not nested further in another grouping) in the same grouping and that carry this name (there can be multiple ones, one per affordance type) are declared to be mandatory to be represented. The same applies for groupings made mandatory within groupings containing them.
- the Boolean value
true. The affordance/grouping itself that carries thesdfRequiredkeyword is declared to be mandatory to be represented.
Note that referenceable-names are not
subject to the encoding JSON pointers require as discussed in {{hierarchical-names}}.
To ensure that referenceable-names are reliably distinguished from JSON pointers,
they are defined such that they cannot contain ":" or
"#" characters (see rule referenceable-name in {{syntax}}).
(If these characters are indeed contained in a Given Name, a JSON
pointer needs to be formed instead in order to reference it in "sdfRequired",
potentially requiring further path elements as well as JSON pointer
encoding. The need for this is best avoided by choosing Given Names
without these characters.)
The example in {{example-req}} shows two required elements in the sdfObject definition for "temperatureWithAlarm", the sdfProperty "currentTemperature", and the sdfEvent "overTemperatureEvent". The example also shows the use of JSON pointer with "sdfRef" to use a pre-existing definition in this definition, for the "alarmType" data (sdfOutputData) produced by the sdfEvent "overTemperatureEvent".
"sdfObject": {
"temperatureWithAlarm": {
"sdfRequired": [
"#/sdfObject/temperatureWithAlarm/sdfProperty/currentTemperature",
"#/sdfObject/temperatureWithAlarm/sdfEvent/overTemperatureEvent"
],
"sdfData":{
"temperatureData": {
"type": "number"
}
},
"sdfProperty": {
"currentTemperature": {
"sdfRef": "#/sdfObject/temperatureWithAlarm/sdfData/temperatureData"
}
},
"sdfEvent": {
"overTemperatureEvent": {
"sdfOutputData": {
"type": "object",
"properties": {
"alarmType": {
"sdfRef": "cap:#/sdfData/alarmTypes/quantityAlarms",
"const": "OverTemperatureAlarm"
},
"temperature": {
"sdfRef": "#/sdfObject/temperatureWithAlarm/sdfData/temperatureData"
}
}
}
}
}
}
}{: #example-req title="Using sdfRequired"}
In {{example-req}}, the same sdfRequired can also be represented in short form:
"sdfRequired": ["currentTemperature", "overTemperatureEvent"]Or, for instance "overTemperatureEvent" could carry
"overTemperatureEvent": {
"sdfRequired": [true],
"...": "..."
}Definitions in SDF share a number of qualities that provide metadata for
them. These are listed in {{tbl-common-qualities}}. None of these
qualities are required or have default values that are assumed if the
quality is absent.
If a short textual description is required for an application and no
label is given in the SDF model, in its place applications could use
the last part (the last reference-token, {{Section 3 of -pointer}}) of
the JSON pointer to the definition.
| Quality | Type | Description |
|---|---|---|
| description | string | long text (no constraints) |
| label | string | short text (no constraints) |
| $comment | string | source code comments only, no semantics |
| sdfRef | sdf-pointer | (see {{sdfref}}) |
| sdfRequired | pointer-list | (see {{sdfrequired}}, used in affordances or groupings) |
| {: #tbl-common-qualities title="Common Qualities"} |
Data qualities are used in sdfData and sdfProperty definitions,
which are named sets of data qualities (abbreviated as named-sdq).
These qualities include the common qualities, JSO-inspired qualities (see below), and data qualities defined specifically for the present specification; the latter are shown in {{sdfdataqual2}}. None of these qualities are required or have default values that are assumed if the quality is absent.
{{jso-inspired}} lists data qualities inspired by the various proposals at json-schema.org; the intention is that these (information model level) qualities are compatible with the (data model) semantics from the versions of the json-schema.org proposal they were imported from.
| Quality | Type | Description | Default | |---------------+---------------------------------------------+--------------------------------------------------------------------------+---------| | (common) | | {{common-qualities}} | | | unit | string | unit name (note 1) | N/A | | nullable | boolean | indicates a null value is available for this type | true | | contentFormat | string | content type (IANA media type string plus parameters), encoding (note 2) | N/A | | sdfType | string ({{sdftype}}) | sdfType enumeration (extensible) | N/A | | sdfChoice | named set of data qualities ({{sdfchoice}}) | named alternatives | N/A | | enum | array of strings | abbreviation for string-valued named alternatives | N/A | {: #sdfdataqual2 title="SDF-defined Qualities of sdfData and sdfProperty"}
-
The unit name SHOULD be as per the {{senml-units (SenML Units)<RFC8428}} Registry or the {{secondary-units (Secondary Units)<RFC8798}} Registry in {{-units}} as specified by {{Sections 4.5.1 and 12.1 of -senml}} and {{Section 3 of -senml-units-2}}, respectively.
Exceptionally, if a registration in these registries cannot be obtained or would be inappropriate, the unit name can also be a URI that is pointing to a definition of the unit. Note that SDF processors are not expected to (and normally SHOULD NOT) dereference these URIs (see also {{-deref}}); they may be useful to humans, though. A URI unit name is distinguished from a registered unit name by the presence of a colon; any registered unit names that contain a colon (at the time of writing, none) can therefore not be used in SDF.
For use by translators into ecosystems that require URIs for unit names, the URN sub-namespace "urn:ietf:params:unit" is provided ({{unit-urn}}); URNs from this sub-namespace MUST NOT be used in a
unitquality, in favor of simply notating the unit name (such askginstead ofurn:ietf:params:unit:kg). -
The
contentFormatquality follows the Content-Format-Spec as defined in {{Section 6 of RFC9193}}, allowing for expressing both numeric and string based Content-Formats.
SDF defines a number of basic types beyond those provided by JSON or
JSO. These types are identified by the sdfType quality, which
is a text string from a set of type names defined by the "sdfType
values" sub-registry in the "SDF Parameters" registry
({{sdftype-values}}).
The sdfType name is composed of lower case ASCII letters, digits,
and - (ASCII hyphen/minus) characters, starting
with a lower case ASCII letter (i.e., using a pattern of
"[a-z][-a-z0-9]*"), typically employing kebab-case for
names constructed out of multiple words {{KebabCase}}.
To aid interworking with JSO implementations, it is RECOMMENDED
that sdfType is always used in conjunction with the type quality
inherited from {{-jso7v}}, in such a way as to yield a common
representation of the type's values in JSON.
Values for sdfType that are defined in this specification are shown in
{{sdftype1}}.
This table also gives a description of the semantics of the sdfType,
the conventional value for type to be used with the sdfType value,
and a conventional JSON representation for values of the type.
| Name | Description | type | JSON Representation | Reference |
|---|---|---|---|---|
| byte-string | A sequence of zero or more bytes | string | base64url without padding | {{Section 3.4.5.2 of RFC8949}} |
| unix-time | A point in civil time (note 1) | number | POSIX time | {{Section 3.4.2 of RFC8949}} |
| {: #sdftype1 title="Values Defined in Base SDF for the sdfType Quality"} |
(1) Note that the definition of unix-time does not imply the
capability to represent points in time that fall on leap seconds.
More date/time-related sdfTypes are likely to be added in the sdfType
value registry.
Data can be a choice of named alternatives, called sdfChoice.
Each alternative is identified by a name (string, key in the outer JSON
map used to represent the overall choice) and a set of dataqualities
(each in an inner JSON map, the value used to represent the
individual alternative in the outer JSON map).
Dataqualities that are specified at the same level as the sdfChoice
apply to all choices in the sdfChoice, except those specific choices
where the dataquality is overridden at the choice level.
sdfChoice merges the functions of two constructs found in {{-jso7v}}:
-
enumWhat could be expressed as
"enum": ["foo", "bar", "baz"]
in JSO, is often best represented as:
"sdfChoice": { "foo": { "description": "This is a foonly"}, "bar": { "description": "As defined in the second world congress"}, "baz": { "description": "From bigzee foobaz"} }
This allows the placement of other dataqualities such as
descriptionin the example.If an enum needs to use a data type different from text string, what would for instance have been:
"type": "number", "enum": [1, 2, 3]
in JSO, is represented as:
"type": "number", "sdfChoice": { "a-better-name-for-alternative-1": { "const": 1 }, "alternative-2": { "const": 2 }, "the-third-alternative": { "const": 3 } }
where the string names obviously would be chosen in a way that is descriptive for what these numbers actually stand for; sdfChoice also makes it easy to add number ranges into the mix.
(Note that
constcan also be used for strings as in the previous example, for instance, if the actual string value is indeed a crucial element for the data model.) -
anyOf
JSO provides a type union called
anyOf, which provides a choice between anonymous alternatives.What could have been in JSO:
"anyOf": [ {"type": "array", "minItems": 3, "maxItems": "3", "items": {"$ref": "#/sdfData/rgbVal"}}, {"type": "array", "minItems": 4, "maxItems": "4", "items": {"$ref": "#/sdfData/cmykVal"}} ]
can be more descriptively notated in SDF as:
"sdfChoice": { "rgb": {"type": "array", "minItems": 3, "maxItems": "3", "items": {"sdfRef": "#/sdfData/rgbVal"}}, "cmyk": {"type": "array", "minItems": 4, "maxItems": "4", "items": {"sdfRef": "#/sdfData/cmykVal"}} }
Note that there is no need in SDF for the type intersection construct
allOf or the peculiar type-xor construct oneOf found in {{-jso7v}}.
As a simplification for users of SDF models who are accustomed to the JSO enum keyword, this is retained, but limited to a choice of text string values, such that
"enum": ["foo", "bar", "baz"]is syntactic sugar for
"sdfChoice": {
"foo": { "const": "foo"},
"bar": { "const": "bar"},
"baz": { "const": "baz"}
}In a single definition, the keyword enum cannot be used at the same
time as the keyword sdfChoice, as the former is just syntactic
sugar for the latter.
The following SDF keywords are used to create definition groups in the target namespace. All these definitions share some common qualities as discussed in {{common-qualities}}.
The sdfObject keyword denotes a group of zero or more sdfObject definitions.
sdfObject definitions may contain or include definitions of named Properties, Actions, Events declared for the sdfObject, as well as named data types (sdfData group) to be used in this or other sdfObjects.
The qualities of an sdfObject include the common qualities; additional qualities are shown in {{sdfobjqual}}. None of these qualities are required or have default values that are assumed if the quality is absent.
| Quality | Type | Description | |-------------+-----------+--------------------------------------------------------------------------| | (common) | | {{common-qualities}} | | sdfProperty | property | zero or more named property definitions for this sdfObject | | sdfAction | action | zero or more named action definitions for this sdfObject | | sdfEvent | event | zero or more named event definitions for this sdfObject | | sdfData | named-sdq | zero or more named data type definitions that might be used in the above | | minItems | number | (array) Minimum number of multiplied affordances in array | | maxItems | number | (array) Maximum number of multiplied affordances in array | {: #sdfobjqual title="Qualities of sdfObject"}
The sdfProperty keyword denotes a group of zero or more Property definitions.
Properties are used to model elements of state.
The qualities of a Property definition include the data qualities (and thus the common qualities), see {{data-qualities}}, additional qualities are shown in {{sdfpropqual}}.
| Quality | Type | Description | Default | |---------------+-----------+---------------------------------------------------------|---------| | (data) | | {{data-qualities}} | | | readable | boolean | Reads are allowed | true | | writable | boolean | Writes are allowed | true | | observable | boolean | flag to indicate asynchronous notification is available | true | {: #sdfpropqual title="Qualities of sdfProperty"}
The sdfAction keyword denotes a group of zero or more Action definitions.
Actions are used to model commands and methods which are invoked. Actions have parameter data that are supplied upon invocation.
The qualities of an Action definition include the common qualities, additional qualities are shown in {{sdfactqual}}.
| Quality | Type | Description | |---------------+-----------+--------------------------------------------------------------------------| | (common) | | {{common-qualities}} | | sdfInputData | map | data qualities of the input data for an Action | | sdfOutputData | map | data qualities of the output data for an Action | | sdfData | named-sdq | zero or more named data type definitions that might be used in the above | {: #sdfactqual title="Qualities of sdfAction"}
sdfInputData defines the input data of the action. sdfOutputData
defines the output data of the action.
As discussed in {{sdfaction-overview}}, a set of data qualities with
type "object" can be used to substructure either data item, with
optionality indicated by the data quality required.
The sdfEvent keyword denotes zero or more Event definitions.
Events are used to model asynchronous occurrences that may be communicated proactively. Events have data elements which are communicated upon the occurrence of the event.
The qualities of sdfEvent include the common qualities, additional qualities are shown in {{sdfevqual}}.
| Quality | Type | Description | |---------------+-----------+--------------------------------------------------------------------------| | (common) | | {{common-qualities}} | | sdfOutputData | map | data qualities of the output data for an Event | | sdfData | named-sdq | zero or more named data type definitions that might be used in the above | {: #sdfevqual title="Qualities of sdfEvent"}
sdfOutputData defines the output data of the action.
As discussed in {{sdfevent-overview}}, a set of data qualities with
type "object" can be used to substructure the output data item, with
optionality indicated by the data quality required.
The sdfData keyword denotes a group of zero or more named data type
definitions (named-sdq).
An sdfData definition provides a reusable semantic identifier for a type of data item and describes the constraints on the defined type. It is not itself a declaration, i.e., it does not cause any of these data items to be included in an affordance definition.
The qualities of sdfData include the data qualities (and thus the common qualities), see {{data-qualities}}.
The requirements for high level composition include the following:
-
The ability to represent products, standardized product types, and modular products while maintaining the atomicity of sdfObjects.
-
The ability to compose a reusable definition block from sdfObjects, for example a single plug unit of an outlet strip with on/off control, energy monitor, and optional dimmer sdfObjects, while retaining the atomicity of the individual sdfObjects.
-
The ability to compose sdfObjects and other definition blocks into a higher level sdfThing that represents a product, while retaining the atomicity of sdfObjects.
-
The ability to enrich and refine a base definition to have product-specific qualities and quality values, such as unit, range, and scale settings.
-
The ability to reference items in one part of a complex definition from another part of the same definition, for example to summarize the energy readings from all plugs in an outlet strip.
The model namespace is organized according to terms that are defined in the SDF documents that contribute to the namespace. For example, definitions that originate from an organization or vendor are expected to be in a namespace that is specific to that organization or vendor.
The structure of a path in a namespace is defined by the JSON Pointers
to the definitions in the SDF documents in that namespace.
For example, if there is an SDF document defining an sdfObject "Switch"
with an action "on", then the reference to the action would be
"ns:/sdfObject/Switch/sdfAction/on" where ns is the namespace prefix
(short name for the namespace).
Modular composition of definitions enables an existing definition (could be in the same or another SDF document) to become part of a new definition by including a reference to the existing definition within the model namespace.
An existing definition may be used as a template for a new definition, that is, a new definition is created in the target namespace which uses the defined qualities of some existing definition. This pattern will use the keyword sdfRef as a quality of a new definition with a value consisting of a reference to the existing definition that is to be used as a template.
In the definition that uses sdfRef, new qualities may be added
and existing qualities from the referenced definition may be
overridden. (Note that JSON maps do not have a defined
order, so the SDF processor may see these overrides before seeing the
sdfRef.)
Note that if the referenced definition contains qualities or definitions that are not valid in the context where the sdfRef is used (for instance, if an sdfThing definition would be added in an sdfObject definition), the resulting model, when resolved, may be invalid.
As a convention, overrides are intended to be used only for further restricting
the set of data values, as shown in {{exa-sdfref}}: any value for a
cable-length also is a valid value for a length, with the
additional restriction that the length cannot be smaller than 5 cm.
(This is labeled as a convention as it cannot be checked in the
general case; a quality of implementation consideration for a tool
might be to provide at least some form of checking.)
Note that a description is provided that overrides the description of
the referenced definition; as this quality is intended for human
consumption there is no conflict with the intended goal.
"sdfData":
"length" : {
"type": "number",
"minimum": 0,
"unit": "m"
"description": "There can be no negative lengths."
}
...
"cable-length" : {
"sdfRef": "#/sdfData/length"
"minimum": 5e-2,
"description": "Cables must be at least 5 cm."
}
{: #exa-sdfref title="Using an Override to Further Restrict the Set of Data Values"}
An sdfThing is a set of declarations and qualities that may be part of a more complex model. For example, the sdfObject declarations that make up the definition of a single socket of an outlet strip could be encapsulated in an sdfThing, which itself could be used in a declaration in the sdfThing definition for the outlet strip (see {{exa-sdfthing-outlet-strip}} in {{outlet-strip-example}} for an example SDF model).
sdfThing definitions carry semantic meaning, such as a defined refrigerator compartment and a defined freezer compartment, making up a combination refrigerator-freezer product. An sdfThing may be composed of sdfObjects and other sdfThings. It can also contain sdfData definitions, as well as declarations of interaction affordances itself, such as a status (on/off) for the refrigerator-freezer as a whole (see {{exa-sdfthing-fridge-freezer}} in {{fridge-freezer-example}} for an example SDF model illustrating these aspects).
The qualities of sdfThing are shown in {{sdfthingqual}}.
Analogous to sdfObject, the presence of one or both of the optional
qualities "minItems" and "maxItems" defines the sdfThing as an
array.
| Quality | Type | Description |
|---|---|---|
| (common) | {{common-qualities}} | |
| sdfThing | thing | |
| sdfObject | object | |
| sdfProperty | property | zero or more named property definitions for this thing |
| sdfAction | action | zero or more named action definitions for this thing |
| sdfEvent | event | zero or more named event definitions for this thing |
| sdfData | named-sdq | zero or more named data type definitions that might be used in the above |
| minItems | number | (array) Minimum number of multiplied affordances in array |
| maxItems | number | (array) Maximum number of multiplied affordances in array |
| {: #sdfthingqual title="Qualities of sdfThing"} |
IANA is requested to add the following Media-Type to the "Media Types" registry.
| Name | Template | Reference | | sdf+json | application/sdf+json | RFC XXXX, {{media-type}} | {: align="left" title="Media Type Registration for SDF"}
{:compact} Type name: : application
Subtype name: : sdf+json
Required parameters: : none
Optional parameters: : none
Encoding considerations: : binary (JSON is UTF-8-encoded text)
Security considerations: : {{seccons}} of RFC XXXX
Interoperability considerations: : none
Published specification: : {{media-type}} of RFC XXXX
Applications that use this media type: : Tools for data and interaction modeling in the Internet of Things and related environments
Fragment identifier considerations: : A JSON Pointer fragment identifier may be used, as defined in {{Section 6 of RFC6901}}.
Additional information: : Magic number(s): : n/a
File extension(s): : .sdf.json
Windows Clipboard Name: : "Semantic Definition Format (SDF) for Data and Interactions of Things"
Macintosh file type code(s): : n/a
Macintosh Universal Type Identifier code:
: org.ietf.sdf-json
conforms to public.text
Person & email address to contact for further information: : ASDF WG mailing list ([email protected]), or IETF Applications and Real-Time Area ([email protected])
Intended usage: : COMMON
Restrictions on usage: : none
Author/Change controller: : IETF
Provisional registration: : no
This document adds the following Content-Format to the "CoAP Content-Formats", within the "Constrained RESTful Environments (CoRE) Parameters" registry, where TBD1 comes from the "IETF Review" 256-999 range.
| Content Type | Content Coding | ID | Reference | | application/sdf+json | - | TBD1 | RFC XXXX | {: align="left" title="SDF Content-format Registration"}
// RFC Ed.: please replace TBD1 with the assigned ID, remove the requested range, and remove this note.\ // RFC Ed.: please replace RFC XXXX with this RFC number and remove this note.
IANA is requested to register the following value in the "{{params-1 (IETF URN Sub-namespace for Registered Protocol Parameter Identifiers)<IANA.params}}" registry in {{IANA.params}}, following the template in {{!RFC3553}}:
Registry name: : unit
Specification: : RFC XXXX
Repository: : combining the symbol values from the {{senml-units (SenML Units)<IANA.senml}} Registry and the {{secondary-units (Secondary Units)<IANA.senml}} Registry in {{-units}} as specified by {{Sections 4.5.1 and 12.1 of -senml}} and {{Section 3 of -senml-units-2}}, respectively (which by the registration policy are guaranteed to be non-overlapping).
Index value: : Percent-encoding ({{Section 2.1 of -uri}}) is required of any characters in unit names as required by ABNF rule "pchar" in {{Section 3.3 of -uri}}, specifically at the time of writing for the unit names "%" (deprecated in favor of "/"), "%RH", "%EL".
IANA is requested to create an "SDF Parameters" registry, with the sub-registries defined in this Section.
IANA is requested to create a "Quality Name Prefixes" sub-registry in the "SDF Parameters" registry, with the following template:
Prefix:
: A name composed of lower case ASCII letters and digits, starting
with a lower case ASCII letter (i.e., using a pattern of "[a-z][a-z0-9]*").
Contact: : A contact point for the organization that assigns quality names with this prefix.
Reference: : A pointer to additional information, if available.
Quality Name Prefixes are intended to be registered by organizations that plan to define quality names constructed with an organization-specifix prefix ({{gnqn}}).
The registration policy is Expert Review as per {{Section 4.5 of -reg}}. The instructions to the Expert are to ascertain that the organization will handle quality names constructed using their prefix in a way that roughly achieves the objectives for an IANA registry that support interoperability of SDF models employing these quality names, including:
- Stability, "stable and permanent";
- Transparency, "readily available", "in sufficient detail" ({{Section 4.6 of -reg}}).
The Expert will take into account that other organizations operate in different ways than the IETF, and that as a result some of these overall objectives will be achieved in a different way and to a different level of comfort.
The "Quality Name Prefixes" sub-registry starts out empty.
IANA is requested to create a "sdfType values" sub-registry in the "SDF Parameters" registry, with the following template:
Name:
: A name composed of lower case ASCII letters, digits and - (ASCII
hyphen/minus) characters, starting with a lower case ASCII letter
(i.e., using a pattern of "[a-z][-a-z0-9]*").
Description: : A short description of the information model level structure and semantics
type: : The value of the quality "type" to be used with this sdfType
JSON Representation : A short description of a JSON representation that can be used for this sdfType. This MUST be consistent with the type.
Reference: : A more detailed specification of meaning and use of sdfType.
sdfType values are intended to be registered to enable modeling additional SDF-specific types (see {{sdftype}}).
The registration policy is Specification Required as per {{Section 4.6 of -reg}}. The instructions to the Expert are to ascertain that the specification provides enough detail to enable interoperability between implementations of the sdfType being registered, and that names are chosen with enough specificity that ecosystem-specific sdfTypes will not be confused with more generally applicable ones.
The initial set of registrations is described in {{sdftype1}}.
Some wider security considerations applicable to Things are discussed in {{-seccons}}. {{Section 5 of -cddl}} gives an overview over security considerations that arise when formal description techniques are used to govern interoperability; analogs of these security considerations can apply to SDF.
The security considerations of underlying building blocks such as those detailed in {{Section 10 of -utf8}} apply. SDF uses JSON as a representation language; for a number of cases {{-json}} indicates that implementation behavior for certain constructs allowed by the JSON grammar is unpredictable. Implementations need to be robust against invalid or unpredictable cases on input, preferably by rejecting input that is invalid or that would lead to unpredictable behavior, and need to avoid generating these cases on output.
Implementations of model languages may also exhibit performance-related availability issues when the attacker can control the input, see {{Section 4.1 of -jsonpath}} for a brief discussion.
SDF may be used in two processes that are often security relevant: model-based validation of data that is intended to be described by SDF models, and model-based augmentation of these data with information obtained from the SDF models that apply.
Implementations need to ascertain the provenance (and thus authenticity and integrity) and applicability of the SDF models they employ operationally in such security relevant ways. Implementations that make use of the composition mechanisms defined in this document need to do this for each of the components they combine into the SDF models they employ. Essentially, this process needs to undergo the same care and scrutiny as any other introduction of source code into a build environment; the possibility of supply-chain attacks on the modules imported needs to be considered.
Specifically, implementations might rely on model-based input validation for enforcing certain properties of the data structure ingested (which, if not validated, could lead to malfunctions such as crashes and remote code execution). These implementations need to be particularly careful about the data models they apply, including their provenance and potential changes of these properties that upgrades to the referenced modules may (inadvertently or as part of an attack) cause. More generally speaking, implementations should strive to be robust against expected and unexpected limitations of the model-based input validation mechanisms and their implementations.
Similarly, implementations that rely on model-based augmentation may generate false data from their inputs; this is an integrity violation in any case but also can possibly be exploited for further attacks.
In applications that dynamically acquire models and obtain modules from module references in these, the security considerations of dereferenceable identifiers apply (see {{-deref}} for a more extensive discussion of dereferenceable identifiers).
There may be confidentiality requirements on SDF models, both on their content and on the fact that a specific model is used in a particular Thing or environment. The present specification does not discuss model discovery or define an access control model for SDF models, nor does it define a way to obtain selective disclosure where that may be required. It is likely that these definitions require additional context from underlying ecosystems and the characteristics of the protocols employed there; this is therefore left as future work (e.g., for documents such as {{-mapping}}).
--- back
This normative appendix describes the syntax of SDF using CDDL {{-cddl}}.
This appendix shows the framework syntax only, i.e., a syntax with liberal extension points.
Since this syntax is nearly useless in finding typos in an SDF
specification, a second syntax, the validation syntax, is defined that
does not include the extension points.
The validation syntax can be generated from the framework syntax by
leaving out all lines containing the string EXTENSION-POINT; as this
is trivial, the result is not shown here.
This appendix makes use of CDDL "features" as defined in {{Section 4 of -control}}. Features whose names end in "-ext" indicate extension points for further evolution.
{::include sdf-framework.cddl}
This informative appendix describes the syntax of SDF defined in {{syntax}}, but using a version of the description techniques advertised on json-schema.org {{-jso}} {{-jso7v}}.
The appendix shows both the validation and the framework syntax.
Since most of the lines are the same between these two files, those lines are shown only once, with a leading space, in the form of a unified diff.
Lines leading with a - are part of the validation syntax, and lines leading with a + are part of the framework syntax.
{::include sdf.jso.json-unidiff}This appendix is normative.
Data qualities define data used in SDF affordances at an information model level. A popular way to describe JSON data at a data model level is proposed by a number of drafts on json-schema.org (which collectively are abbreviated JSO here); for reference to a popular version we will point here to {{-jso}} and {{-jso7v}}. As the vocabulary used by JSO is familiar to many JSON modelers, the present specification borrows some of the terms and ports their semantics to the information model level needed for SDF.
The main data quality imported is the "type".
In SDF, this can take one of six (text string) values, which are
discussed in the following subsections (note that the JSO type
"null" is not supported as a value of this data quality in SDF).
The additional quality "const" restricts the data to one specific
value (given as the value of the const quality).
Similarly, the additional quality "default" provides data that can
be used in the absence of the data (given as the value of the const
quality); this is mainly documentary and not very well-defined for SDF
as no process is defined that would add default values to an instance
of some interaction data.
The types "number" and "integer" are associated with floating point
and integer numbers, as they are available in JSON.
A type value of integer means that only integer values of JSON
numbers can be used (note that 10.0 is an integer value, even if it
is in a notation that would also allow non-zero decimal fractions).
The additional data qualities "minimum", "maximum",
"exclusiveMinimum", "exclusiveMaximum" provide number values that
serve as inclusive/exclusive lower/upper bounds for the number.
(Note that the Boolean form of
"exclusiveMinimum"/"exclusiveMaximum" found in earlier JSO drafts {{-jso4v}}
is not used.)
The data quality "multipleOf" gives a positive number that
constrains the data value to be an integer multiple of the number
given.
(Type "integer" can also be expressed as a "multipleOf" quality of
value 1, unless another "multipleOf" quality is present.)
The type "string" is associated with Unicode text string values as
they can be represented in JSON.
The length (as measured in characters) can be constrained by the
additional data qualities "minLength" and "maxLength", which are
inclusive bounds.
(More specifically, Unicode text strings as defined in this specification are sequences of Unicode scalar values, the number of which is taken as the length of such a text string.
The data quality "pattern" takes a string value that is interpreted
as an [ECMA-262] regular expression in Unicode mode that constrains the
string (note that these are not anchored by default, so unless ^ and
$ anchors are employed, ECMA-262 regular expressions match any string that contains a match).
The JSO proposals acknowledge that regular expression support is
rather diverse in various platforms, so the suggestion is to limit
them to:
{:compact}
- characters;
- character classes in square brackets, including ranges; their complements;
- simple quantifiers
*,+,?, and range quantifiers{n},{n,m}, and{n,}; - grouping parentheses;
- the choice operator
|; - and anchors (beginning-of-input
^and end-of-input$).
Note that this subset is somewhat similar to the subset introduced by I-Regexps {{-iregexp}}, which however are anchored regular expressions, and which include certain backslash escapes for characters and character classes.
The additional data quality "format" can take one of the following
values. Note that, at an information model level, the presence of
this data quality changes the type from being a simple text string to
the abstract meaning of the format given (i.e., the format "date-time"
is less about the specific syntax employed in {{RFC3339}} than about the usage
as an absolute point in civil time).
{:compact}
- "
date-time", "date", "time": An {{RFC3339}}date-time,full-date, orfull-time, respectively. - "
uri", "uri-reference": An {{RFC3986}} URI or URI Reference, respectively. - "
uuid": An {{RFC4122}} UUID.
The type "boolean" can take the values "true" or "false".
The type "array" is associated with arrays as they are available in
JSON.
The additional quality "items" gives the type that each of the
elements of the array must match.
The number of elements in the array can be constrained by the additional
data qualities "minItems" and "maxItems", which are inclusive
bounds.
The additional data quality "uniqueItems" gives a Boolean value
that, if true, requires the elements to be all different.
The type "object" is associated with maps, from strings to values, as
they are available in JSON.
The additional quality "properties" is a map the entries of which
describe entries in the specified JSON map: The key gives an
allowable map key for the specified JSON map, and the value is a
map with a named set of data qualities giving the type for the
corresponding value in the specified JSON map.
All entries specified this way are optional, unless they are listed in
the value of the additional quality "required", which is an array of
string values that give the key names of required entries.
Note that the term "properties" as an additional quality for defining map entries is unrelated to sdfProperty.
JSO-based keywords are also used in the specification techniques of a number of ecosystems, but some adjustments may be required.
For instance, {{OCF}} is based on Swagger 2.0 which appears to be based on
"draft-4" {{-jso4}}{{-jso4v}} (also called draft-5, but semantically intended to
be equivalent to draft-4).
The "exclusiveMinimum" and "exclusiveMaximum" keywords use the
Boolean form there, so on import to SDF their values have to be
replaced by the values of the respective "minimum"/"maximum"
keyword, which are themselves then removed; the reverse transformation
applies on export.
This informative appendix contains two examples illustrating different composition approaches
using the sdfThing quality.
{
"sdfThing": {
"outlet-strip": {
"label": "Outlet strip",
"description": "Contains a number of Sockets",
"sdfObject": {
"socket": {
"description": "An array of sockets in the outlet strip",
"minItems": 2,
"maxItems": 10
}
}
}
}
}{: sourcecode-name="example-sdfthing-outlet-strip.sdf.json"} {: title="Outlet Strip Example"} {: #exa-sdfthing-outlet-strip}
{
"sdfThing": {
"refrigerator-freezer": {
"description": "A refrigerator combined with a freezer",
"sdfProperty": {
"status": {
"type": "boolean",
"description":
"Indicates if the refrigerator-freezer is powered"
}
},
"sdfObject": {
"refrigerator": {
"description": "A refrigerator compartment",
"sdfProperty": {
"temperature": {
"sdfRef": "#/sdfProproperty/temperature",
"maximum": 8
}
}
},
"freezer": {
"label": "A freezer compartment",
"sdfProperty": {
"temperature": {
"sdfRef": "#/sdfProproperty/temperature",
"maximum": -6
}
}
}
}
}
},
"sdfProperty": {
"temperature": {
"description": "The temperature for this compartment",
"type": "number",
"unit": "Cel"
}
}
}{: sourcecode-name="example-sdfthing-refrigerator-freezer.sdf.json"} {: title="Refrigerator-Freezer Example"} {: #exa-sdfthing-fridge-freezer}
This appendix is informative.
The present document provides the SDF base definition. Previous revisions of SDF have been in use for several years, and both significant collections of older SDF models and older SDF conversion tools are available today. This appendix provides a brief checklist that can aid in upgrading these to the standard.
- The quality
unitwas previously calledunits. sdfTypewas developed out of a concept previously calledsubtype.sdfChoiceis the preferred way to represent JSOenum(only a limited form of which is retained), and also the way to represent JSOanyOf.- the length of text strings (as used with
minLength/maxLengthconstraints) was previously defined in bytes. It now is defined as the number of characters (Unicode scalar values, to be exact); a length in bytes is not meaningful unless bound to a specific encoding, which might differ from UTF-8 in some ecosystem mappings and protocol bindings.
{:unnumbered}
This specification is based on work by the One Data Model group.
Footnotes
-
The Semantic Definition Format (SDF) is concerned with Things, namely physical objects that are available for interaction over a network. SDF is a format for domain experts to use in the creation and maintenance of data and interaction models that describe Things. An SDF specification describes definitions of SDF Objects/SDF Things and their associated interactions (Events, Actions, Properties), as well as the Data types for the information exchanged in those interactions. Tools convert this format to database formats and other serializations as needed. ↩ ↩2
-
(This note will be removed by the RFC editor:) The present revision (–19) addresses comments from the AD review and from the IESG directorate reviews. ↩ ↩2
-
RFC Ed.: throughout this section, please replace RFC XXXX with this RFC number, and remove this note. ↩