JavaScript Object Notation (JSON) is a popular serialization format. Beside XML and RDF, it is an "official" serialization format for models of Asset Administration Shells (AAS). Since JSON is a very versatile format, there are many ways how we could map an AAS model to it. In this document, we explore our particular design of the serialization schema based on JSON schema 2019-09, and explain in detail the rules how we mapped the AAS meta-model to it.
The root of our serialization is a JSON object representing the instance of Environment. This environment contains three aggregations, corresponding to all Identifiable classes:
- AssetAdministrationShell's,
- Submodel's, and
- ConceptDescription's.
The JSON properties of the environment correspond to these three aggregations.
To simplify exploration of the JSON data, identifiable instances are only available at the level of the environment, and nowhere else.
For each class of the AAS meta-model, we provide a definition in the JSON schema. The instances of the classes, abstract and concrete alike, are modeled as JSON objects .
The class properties of the meta-model (attributes and aggregations) correspond directly to JSON properties.
Optional attributes, i.e., the attributes with the cardinality 0..1, are modeled
as non-required properties.
Aggregations, i.e., the properties with the cardinality 0..*, 1..* etc., are
modeled as JSON arrays.
We explicitly forbid empty JSON arrays to avoid confusion about properties which have
cardinality 0..*. Namely, an empty array is semantically equal to an omitted
attribute (according to the meta-model). Thus, the JSON property representing an
aggregation attribute must be omitted if the aggregation is empty.
In UML, it is the convention to name associations and aggregations in singular form. The
cardinality is to be taken into account to decide on whether there are none, a single or
several elements in the corresponding association or aggregation. In JSON it is best
practice to use plural form for array in class properties. The singular name is used for
its descriminator (see section on decriminators). Typically the plural name is derived
by just adding an "s" to the name. For example, submodelElements instead
of submodelElement in case of Submodel class.
If plural form made no sense for a property, we kept it as-is (e.g., isCaseOf). The
full list of exceptions is available as code in aas-core-meta.
The UML specification uses XSD types. For the mapping of XSD to JSON types please refer to Part 2 of the series of the Asset Adminsistration Shell.
There are the following exceptions:
Property/value and Range/min and Range/max are mapped to a JSON
string. The type it needs to be converted to by the data consumer is declared
in Property/valueType or Range/valueType, resp.
Primitive type BlobType (group of bytes) is mapped to a JSON string with base64
encoding.
Note: in valueOnly Format of Part 2 of the series of the Asset Adminsistration Shell
value has the JSON type as declared in Property/valueType taking
the mapping of XSD to JSON types into account.
Round-trip conversions XML to JSON to XML or RDF to JSON to RDF may not result in the original file.
The result of a model saved as XML is different to the model saved as JSON. For example,
if the user typed in 1 for a boolean UML attribute (e.g. for SubmodelElementList
/orderRelevant)
in the editor, saved the model as JSON and opened it again, she would suddenly
see true instead of 1
(since the JSON library would silently convert 1 to a JSON boolean true).
The inheritance relationships between the classes are modeled using allOf composition.
While JSON schema knows no inheritance, we enforce through allOf that all the
properties of the parent are defined in the descendants.
Many attributes in the meta-model refer to an abstract class. When we de-serialize such attributes, we need to know the concrete class at runtime, since otherwise the de-serialization algorithm would not know how to interpret the properties of the object.
For example, consider the attribute Submodel which contains instances
of SubmodelElement. When the de-serializer encounters the
property submodelElements as an array and starts de-serializing its items, how can
it know which constructor to call to parse the item? This necessary nugget of
information is commonly called "discriminator" (e.g.,
see OpenAPI 3 specification on polymorphism).
We define the discriminator for our serialization as an additional property, modelType
, which do not correspond to any attribute in the meta-model. Every class which has one
or more descendants will have the discriminator modelType in its definition.
When a deserializer needs to de-serialize an instance of an abstract class, it can do so
by dispatching the de-serialization to the appropriate de-serialization method based
on modelType.
The enumerations of the meta-model are directly mapped to enumerated values in JSON schema. Each enumeration is defined separately, and we do not in-line enumerations for readability.
Enumerations which are not directly used in the schema are omitted. For example, subsets of KeyTypes are omitted since only KeyTypes is used to define value of an attribute.
The meta-model defines data specifications in abstract (see Section 6 Predefined Data Specification Templates). However, the meta-model omits data specifications in an Environment, and data specifications are intentionally non-identifiable.
For practical applications, we need to access them somehow. Therefore, the meta-model mandates to embed them in serializations (see Section 7.2.5 Embedded Data Specifications).
We consequently embed the data specifications by adding embeddedDataSpecifications
property to the definition corresponding to HasDataSpecification, and deliberately
omit the attribute HasDataSpecification/dataSpecification in the schema.
Examples of the JSON serializations can be found in schemas/json/examples/ folder.
The serialization to and from JSON changed over the course of different versions of the meta-model. Originally, the schema had been manually designed, where a group of authors combed "the book" and manually transcribed it to JSON schema. This was obviously error-prone as it often caused mismatches between other serializations (e.g., XML and RDF), contained inconsistencies etc.
We eventually moved over to generate the serialization schemas based on a single-point-of-truth. The current source is aas-core-meta, a domain-specific Python representation of the meta-model. The schemas are generated using aas-core-codegen, a translating tool which transpiles aas-core-meta into different schemas and other targets such as SDKs.
While this approach reduced the rate of errors significantly, it also imposed certain limits on our schema design. For example, the classes and enumerations are now programmatically mapped to JSON definitions, allowing for no exceptions. Where we could in-line some of them for better readability, we are now forced to stick with the programmatic definitions.