Network Working Group J. Montoya
Internet-Draft May 05, 2020
Intended status: Informational
Expires: November 6, 2020

Extended Link Relationships
draft-montoya-xrel-latest

Abstract

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

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Table of Contents

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

Name Type Description
description string REQUIRED Describes the semantics of a hypermedia relationship. The semantics SHOULD describe the relationship of the target resource to the context resource, and not any particular representation formats. Markdown MAY be used for rich text representation.

2.4. Document Types

2.4.1. XREL Document

Type Description
Relationship Object A single Relationship Object.

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

Type Description
Map[string, Relationship Object] A map where the keys are the document scoped relationship names and the values are Relationship Objects. XREL Collections can be used to group any number of Relationship Objects.

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:

Encoding considerations:

Security considerations:

Interoperability considerations: none

Fragment identifier considerations:

Published specification: This Document

Applications that use this media type: Various

Additional information:

Person to contact for further information:

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:

Encoding considerations:

Security considerations:

Interoperability considerations: none

Fragment identifier considerations:

Published specification: This Document

Applications that use this media type: Various

Additional information:

Person to contact for further information:

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

[CommonMark] McFarlane, J., "CommonMark Spec", n.d..
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6901] Bryan, P., Zyp, K. and M. Nottingham, "JavaScript Object Notation (JSON) Pointer", RFC 6901, DOI 10.17487/RFC6901, April 2013.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8259] Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017.
[W3C.yaml] Ben Kiki, O, ., Evans, C, . and I. Net, "YAML Aint Markup Language", 2009.

7.2. Informative References

[OAS] OpenAPI Initiative, a Linux Foundation Collaborative Project, "OpenAPI Specification", n.d..
[REST] Fielding, R., "Architectural Styles and the Design of Network-based Software Architectures", Ph.D. Dissertation, University of California, Irvine, 2000.
[untangled] Fielding, R., "Untangled, musings of Roy T. Fielding", n.d..
[yahoo.rest] "The REST Architectural Style List", n.d..

Author's Address

Jose Montoya EMail: jam01@protonmail.com