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Aries RFC 0249: Aries Rich Schema Contexts

Summary

Every rich schema object may have an associated @context. Contexts are JSON or JSON-LD objects. They are the standard mechanism for defining shared semantic meaning among rich schema objects.

Context objects are processed in a generic way defined in Rich Schema Objects Common.

Motivation

@context is JSON-LD’s namespacing mechanism. Contexts allow rich schema objects to use a common vocabulary when referring to common attributes, i.e. they provide an explicit shared semantic meaning.

Tutorial

Intro to @context

@context is a JSON-LD construct that allows for namespacing and the establishment of a common vocabulary.

Context object is immutable, so it's not possible to update existing Context, If the Context needs to be evolved, a new Context with a new version or name needs to be created.

Context object may be stored in either JSON or JSON-LD format.

Example Context

An example of the content field of a Context object: 

{
    "@context": [
        "did:sov:UVj5w8DRzcmPVDpUMr4AZhJ",
        "did:sov:JjmmTqGfgvCBnnPJRas6f8xT",
        "did:sov:3FtTB4kzSyApkyJ6hEEtxNH4",
        {
            "dct": "http://purl.org/dc/terms/",
            "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
            "rdfs": "http://www.w3.org/2000/01/rdf-schema#",
            "Driver": "did:sov:2mCyzXhmGANoVg5TnsEyfV8",
            "DriverLicense": "did:sov:36PCT3Vj576gfSXmesDdAasc",
            "CategoryOfVehicles": "DriverLicense:CategoryOfVehicles"
        }
    ]
}

Data Registry Storage

Aries will provide a means for writing contexts to and reading contexts from a verifiable data registry (such as a distributed ledger).

@context will be written to the ledger in a generic way defined in Rich Schema Objects Common.

Aries Data Registry Interface

Aries Data Registry Interface methods for adding and retrieving @context from the ledger comply with the generic approach described in Rich Schema Objects Common.

This means the following methods can be used: - write_rich_schema_object - read_rich_schema_object_by_id - read_rich_schema_object_by_metadata

Reference

More information on the Verifiable Credential data model use of @context may be found here.

More information on @context from the JSON-LD specification may be found here and here.

Drawbacks

Requiring a @context for each rich schema object introduces more complexity.

Rationale and alternatives

  • A context object is required by the W3C Verifiable Credentials Data Model specification.
  • It also supports rich schema capabilities for credentials.

Unresolved questions

  • Is the proposed interface generic enough to support other data registries?
  • We are not defining Rich Schema objects as DID DOCs for now. We may re-consider this in future once DID DOC format is finalized.
  • It may make sense to extend DID specification to include using DID for referencing Rich Schema objects.
  • The proposed canonicalization form of a content to be used for DID's id-string generation is in a Draft version, so we may find a better way to do it.
  • We don't check if the specified @context is valid by resolving all external links.

Implementations

The following lists the implementations (if any) of this RFC. Please do a pull request to add your implementation. If the implementation is open source, include a link to the repo or to the implementation within the repo. Please be consistent in the "Name" field so that a mechanical processing of the RFCs can generate a list of all RFCs supported by an Aries implementation.

Name / Link Implementation Notes