Extended Link Relationships
draft-montoya-xrel-02

Abstract

This document defines XREL, a data format for describing extended hypermedia relationships identified by Uniform Resource Locators.

Status of This Memo

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This Internet-Draft will expire on November 6, 2020.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Definitions and Terminology
• 1.1.1. Terminology and Conformance Language
• 1.1.2. General
• 1.2. Motivation
• 2. Specification
• 2.1. Format
• 2.2. Rich Text Formatting
• 2.3. Schema
• 2.3.1. Relationship Object
• 2.4. Document Types
• 2.4.1. XREL Document
• 2.4.2. XREL Collection Document
• 2.5. Identifying XREL Documents
• 2.6. Fragment identifiers
• 2.7. Identifying XREL Relationships
• 3. Security Considerations
• 4. IANA Considerations
• 4.1. application/xrel
• 4.2. application/xrel+json
• 5. Appendix A. Acknowledgments
• 6. Appendix B. Frequently Asked Questions
• 6.1. How can I submit comments or feedback to the editors?
• 6.2. Why not include target attributes as defined by RFC8288 ‘Web Linking’?
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

1. Introduction

This document defines XREL, a data format for describing extended hypermedia relationships identified by Uniform Resource Locators.

This document registers a media-type identifier with the IANA: application/xrel. This registration is for community review and will be submitted to the IESG for review, approval, and registration with IANA.

1.1. Definitions and Terminology

1.1.1. Terminology and Conformance Language

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.1.2. General

Representational State Transfer, or [REST], is an architectural style for distributed hypermedia systems. Introduced and first defined in 2000 in Chapter 5, REST, of the doctoral dissertation “Architectural Styles and the Design of Network-based Software Architecture” by Roy Fielding.

Hypermedia, or hypertext, is defined by the presence of application control information embedded within, or as a layer above, the presentation of information. Hypermedia allows for a virtually unbound network of resources while also guiding users through an application as they navigate said relationships.

A hypermedia relationship, also known as a link relation, describes the semantics behind a virtual uni-directional association between two resources.

A hypermedia relationship name is an identifier for a hypermedia relationship.

A resource is the intended conceptual target of a hypertext reference.

Representational state indicates the current state of the requested resource, the desired state for the requested resource, or the value of some other resource, such as a representation of the input data within a client’s query form, or a representation of some error condition for a response.

Application state is the state of the user’s application of computing to a given task, controlled and stored by the user agent and can be composed of representations from multiple servers.

1.2. Motivation

The Uniform Interface constraint of the REST architectural style dictates that hypermedia be the engine of application state. This means that the state of the application and its potential transitions are dictated by the presence of hypermedia relationships in-band and by the navigation of those relationships by an user (human or automated). In order for users to evaluate and select the appropriate relationships to navigate they must rely on an out-band understanding of relationships by their names.

While humans can derive meaning from relationship names in natural language, automated agents have relied on a central repository of standard names maintained by the Internet Assigned Numbers Authority (IANA). Instead of creating and registering entirely new link relations (i.e. patient, appointment, schedulingService, etc.) with a central repository, authors can choose to create an XREL document; one that explains the vital, perhaps domain-specific, semantics of the relationship and which is identified by an URL controlled by the author.

This decentralization allows for a much lower entry barrier, which is not inconsistent with the general concept of the web, and enables different use cases. For example, a private organization is fully capable of defining their own repository of XREL definitions outside of the open Internet, after all standards are a byproduct of authority. Conversely, public XREL definitions would allow for serendipitous reuse, where useful relationships backed by stable URLs might be discovered and possibly become de facto standard.

2. Specification

This document and the specification documented in it are heavily influenced by the OpenAPI 3.1 Spec.

2.1. Format

Following the OpenAPI specification, XREL documents may be represented either in JSON or YAML format.

All field names in the specification are case sensitive. This includes all fields that are used as keys in a map, except where explicitly noted that keys are case insensitive.

2.2. Rich Text Formatting

Throughout the specification description fields are noted as supporting CommonMark markdown formatting.

Where XREL tooling renders rich text it MUST support, at a minimum, markdown syntax as described by CommonMark 0.27. Tooling MAY choose to ignore some CommonMark features to address security concerns.

2.3. Schema

In the following description, if a field is not explicitly REQUIRED or described with a MUST or SHALL, it can be considered OPTIONAL.

2.3.1. Relationship Object

Explains the semantics of a hypermedia relationship.

2.3.1.1. Properties

2.4. Document Types

2.4.1. XREL Document

2.4.1.1. Examples

description: Refers to an event scheduling service resource related to
  the context resource.

{
  "description": "Refers to an event scheduling service resource
    related to the context resource."
}

2.4.2. XREL Collection Document

2.4.2.1. Examples

scheduling-service:
  description: Refers to an event scheduling service resource related
    to the context resource.
patient:
  description: Refers to a patient resource related to the context
    resource.

{
  "scheduling-service": {
    "description": "Refers to an event scheduling service resource
      related to the context resource."
  },
  "patient": {
    "description": "Refers to a patient resource related to the context
      resource."
  }
}

2.5. Identifying XREL Documents

XREL documents are identified by unique URLs, these URL SHOULD be dereferenceable.

In order to reduce load on servers responding to XREL document requests, it is RECOMMENDED that servers use cache control directives that instruct client apps to locally cache the results. Clients making these XREL document requests SHOULD honor the server’s caching directives.

2.6. Fragment identifiers

When applied to an XREL document, a URI fragment identifier MUST be a JSON Pointer and be computed as such.

2.7. Identifying XREL Relationships

In the case of XREL Documents as specified in Section 2.3, the URL that identifies that document also identifies the hypermedia relationship described in that document. For example, if the document example in Section 2.3.1 is served at http://docs.example.org/xrels/sheduling-service then this URL is the identifier for the relationship described in that document.

In the case of XREL Collection Documents as specified in Section 2.4, fragment identifiers MUST be used for the relationships objects described in that document. For example, if the document example in Section 2.4.1 is served at http://docs.example.org/xrels/clinical then http://docs.example.org/xrels/clinical#/sheduling-service and http://docs.example.org/xrels/clinical#/patient identify the first and second Relationship Objects, respectively.

3. Security Considerations

This document registers two media types in the following section, IANA Considerations. See the Security Considerations for each type in that section.

4. IANA Considerations

This specification establishes the media type application/xrel and application/xrel+json for community review and will be submitted to the IESG for review, approval, and registration with IANA.

4.1. application/xrel

Type name: application

Subtype name: xrel

Required parameters: none

Optional parameters:

• type: The “type” parameter has a value of “collection” or “single”.

• Neither the parameter name nor its value are case sensitive.

• The value “single” indicates that the media type identifies an XREL Document.

• The value “collection” indicates that the media type identifies an XREL Collection Document.

• If not specified, the type is assumed to be “single”.

Encoding considerations:

• binary: Because of YAML’s relation to JSON the same encoding considerations of JSON, as specified in [RFC8259], apply.

Security considerations:

• Because of YAML’s relation to JSON this format shares security issues common to all JSON content types. The security issues of [RFC8259], section 6, should be considered.

Interoperability considerations: none

Fragment identifier considerations:

• Fragment identifiers MUST be computed as defined by the [RFC6901] JSON Pointer specification.

Published specification: This Document

Applications that use this media type: Various

Additional information:

• magic number(s): none

• file extensions: .yaml

• macintosh type file code: TEXT

• object idenfiers: none

Person to contact for further information:

• Name: Jose Montoya

• Email: jmontoya@ms3-inc.com

Intended usage: Common

Author/change controller: Jose Montoya

4.2. application/xrel+json

Type name: application

Subtype name: xrel+json

Required parameters: none

Optional parameters:

• type: The “type” parameter has a value of “collection” or “single”.

• Neither the parameter name nor its value are case sensitive.

• The value “single” indicates that the media type identifies an XREL Document.

• The value “collection” indicates that the media type identifies an XREL Collection Document.

• If not specified, the type is assumed to be “single”.

Encoding considerations:

• binary: The same encoding considerations of JSON, as specified in [RFC8259], apply.

Security considerations:

• This media type shares security issues common to all JSON content types. The security issues of [RFC8259], section 6, should be considered.

Interoperability considerations: none

Fragment identifier considerations:

• Fragment identifiers MUST be computed as defined by the [RFC6901] JSON Pointer specification.

Published specification: This Document

Applications that use this media type: Various

Additional information:

• magic number(s): none

• file extensions: .json

• macintosh type file code: TEXT

• object idenfiers: none

Person to contact for further information:

• Name: Jose Montoya

• Email: jmontoya@ms3-inc.com

Intended usage: Common

Author/change controller: Jose Montoya

5. Appendix A. Acknowledgments

Many thanks to Mike Amundsen, Jeff Michaud, Stu Charlton, Eric Wilde and Darrel Miller for their contributions in this space, even if not directly related to XREL documents.

6. Appendix B. Frequently Asked Questions

6.1. How can I submit comments or feedback to the editors?

The issues list for this draft can be found at https://github.com/phtal-org/internet-draft-xrel/issues. For additional information, see https://phtal-org.github.io/internet-draft-xrel/.

To provide feedback, use this issue tracker, the communication methods listed on the homepage, or email the document editors.

6.2. Why not include target attributes as defined by RFC8288 ‘Web Linking’?

Link relations are universal, they describe an association to a conceptual target and not the targets themselves nor their representations. It is the responsibility of the application authors to communicate to their clients what data types are necessary to navigate a relationship and/or the data types that might be expected as a result.

This level of abstraction has value because it’s easier to standardize representations (HTML Microformats, RAML data types, etc.) and link relations than it is to standardize objects and object-specific interfaces. Application servers are free to combine representations and link relations in any way they wish and to provide them in any order, all while remaining understandable to the client.

7. References

7.1. Normative References

7.2. Informative References

Author's Address