Link Relations for Simple Version Navigation
draft-brown-versioning-link-relations-04

Note: a later version of this document has been published as RFC5829.

Abstract

This specification defines Atom link relations for navigation between a resource and its versions.

Editorial Note (To be removed by RFC Editor before publication)

Please send comments to the Atom Syntax mailing list (<http://www.imc.org/atom-syntax/>).

Status of this Memo

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Copyright Notice

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Table of Contents

• 1.   Introduction
• 2.   Terminology
• 3.   Link Relations
  • 3.1   version-history
  • 3.2   latest-version
  • 3.3   working-copy
  • 3.4   working-copy-of
  • 3.5   predecessor-version
  • 3.6   successor-version
• 4.   IANA Considerations
  • 4.1   'version-history' Link Relation Registration
  • 4.2   'latest-version' Link Relation Registration
  • 4.3   'working-copy' Link Relation Registration
  • 4.4   'working-copy-of' Link Relation Registration
  • 4.5   'predecessor-version' Link Relation Registration
  • 4.6   'successor-version' Link Relation Registration
• 5.   Security Considerations
• 6.   Acknowledgments
• 7.   References
  • 7.1   Normative References
  • 7.2   Informative References
• Authors' Addresses
• A.   Relationship to Java Content Repository (JCR) and WebDAV
  • A.1   Example: Use of Link Relations in HTTP Link Header
• del-1Contributors
• B.   Change Log (to be removed by RFC Editor before publication)
  • B.1   Since draft-brown-link-relations-00
  • B.2   Since draft-brown-link-relations-01
  • B.3   Since draft-brown-link-relations-02
  • B.4   Since draft-brown-link-relations-03

Issues list

1. Introduction

This specification defines link relations that may be used on a resource that exists in a system that supports versioning to navigate among the different resources available, such as past versions.

2. Terminology

Versioned Resource

• When a resource is put under version control, it becomes a "versioned resource".  I ↓A versioned resource can be "checked out" to allow modification.Many servers protect versioned resources from modifications by considering them "checked in", and by requiring a "checkout" operation before modification, and a "checkin" operation to get back to the "checked-in" state. Other servers allow modification, in which case the checkout/checkin operation may happen implicitly.

Version History

• A "version history" resource is a resource that contains all the versions of a particular versioned resource.

Predecessor, Successor

• When a versioned resource is checked out and then subsequently checked in, the version that was checked out becomes a "predecessor" of the version created by the checkin. A client can specify multiple predecessors for a new version if the new version is logically a merge of those predecessors. The inverse of the predecessor relation is the "successor" relation. Therefore, if X is a predecessor of Y, then Y is a successor of X.

Working Copy

• A "working copy" is a resource at a server-defined URL that can be ↑ I ↓modifiedused to create a new version of a versioned resource.

3. Link Relations

The following link relations are defined:

3.1 version-history

When included on a versioned resource, this link points to a resource containing the version history for this resource.

3.2 latest-version

When included on a versioned resource, this link points to a resource containing the latest (e.g., current) version.

The latest version is defined by the system. For linear versioning systems, this is probably the latest version by timestamp. For systems that support branching, there will be multiple latest versions, one for each branch in the version history.

Some systems may allow multiple of these link relations.

3.3 working-copy

When included on a versioned resource, this link points to a working copy for this resource.

Some systems may allow multiple of these link relations.

3.5 predecessor-version

When included on a versioned resource, this link points to a resource containing the predecessor version in the version history.

Some systems may allow multiple of these link relations in the case of a multiple branches merging.

3.6 successor-version

When included on a versioned resource, this link points to a resource containing the successor version in the version history.

Some systems may allow multiple of these link relations in order to support branching.

4. IANA Considerations

The link relations below are to be registered by IANA per Section 7.1 of [RFC4287]:

4.1 'version-history' Link Relation Registration

Attribute Value:

version-history

Description:

See Section 3.1.

Expected display characteristics:

Undefined; this relation can be used for background processing or to provide extended functionality without displaying its value.

Security considerations:

See Section 5.

4.2 'latest-version' Link Relation Registration

Attribute Value:

latest-version

Description:

See Section 3.2.

Expected display characteristics:

Undefined; this relation can be used for background processing or to provide extended functionality without displaying its value.

Security considerations:

See Section 5.

4.3 'working-copy' Link Relation Registration

Attribute Value:

working-copy

Description:

See Section 3.3.

Expected display characteristics:

Undefined; this relation can be used for background processing or to provide extended functionality without displaying its value.

Security considerations:

See Section 5.

4.5 'predecessor-version' Link Relation Registration

Attribute Value:

predecessor-version

Description:

See Section 3.5.

Expected display characteristics:

Undefined; this relation can be used for background processing or to provide extended functionality without displaying its value.

Security considerations:

See Section 5.

4.6 'successor-version' Link Relation Registration

Attribute Value:

successor-version

Description:

See Section 3.6.

Expected display characteristics:

Undefined; this relation can be used for background processing or to provide extended functionality without displaying its value.

Security considerations:

See Section 5.

5. Security Considerations

Automated agents should take care when these relation crosses administrative domains (e.g., the URI has a different authority than the current document). Such agents should also take care to detect circular references.

7. References

7.1 Normative References

7.2 Informative References

A. Relationship to Java Content Repository (JCR) and WebDAV

The link relations defined in Section 3 correspond to various properties used in WebDAV Versioning [RFC3253] and JCR [JSR-283]:

version-history

• WebDAV: the resource identified by the DAV:version-history property ([RFC3253], Sections 5.2.1 and 5.3.1).
• JCR: the node identified by jcr:versionHistory property ([JSR-283], Section 3.13.2.4) for versionable nodes, the parent folder for version nodes.

latest-version

• WebDAV: for version-controlled resources, DAV:checked-in ([RFC3253], Section 3.2.1) or DAV:checked-out ([RFC3253], Section 3.3.1), depending on checkin state. For version resources, a successor version that itself does not have any successors.
• JCR: the version node identified by the jcr:baseVersion property ([JSR-283], Section 3.13.2.5) for versionable nodes; for version nodes, a successor version that itself does not have any successors.

working-↑ I ↓resourcecopy

• WebDAV: for version-controlled resources that are checked-out in place: the resource itself. For version resources: each resource identified by a member of the DAV:checkout-set property (see [RFC3253], Section 3.4.3).
• JCR: for checked-out versionable nodes: the node itself.

predecessor-version

• WebDAV: each resource identified by a member of DAV:predecessor-set ([RFC3253], Sections 3.3.2 and 3.4.1).
• JCR: each node identified by a member of jcr:predecessors ([JSR-283], Section 3.13.3.3).

successor-version

• WebDAV: each resource identified by a member of DAV:successor-set ([RFC3253], Section 3.4.2).
• JCR: each node identified by a member of jcr:successors ([JSR-283], Section 3.13.3.4).

A.1 Example: Use of Link Relations in HTTP Link Header

The "Web Linking" specification ([draft-nottingham-http-link-header]) generalizes Atom link relations, and also re-introduces the HTTP "Link" header as a way to expose link relations in HTTP responses. This will make it possible to expose version links independently from a specific vocabulary, be it the Atom Feed Format ([RFC4287]) or WebDAV properties ([RFC3253]).

For instance, a response to an VERSION-CONTROL request ([RFC3253], Section 3.5) could expose newly created version-history and checked-in version as link relations:

>> Request:

VERSION-CONTROL /docs/test.txt HTTP/1.1
Host: example.net

>> Response:

HTTP/1.1 200 OK
Link: </system/v/84345634/1>; rel=latest-version;
      anchor=</docs/test.txt>
Link: </system/vh/84345634>; rel=version-history;
      anchor=</docs/test.txt>

(Note that in this case, the anchor parameter is used, as the response to a VERSION-CONTROL request is not a representation of the resource at the Request-URI)

A subsequent HEAD request on that resource could expose the version-history and latest-version relations as well:

>> Request:

HEAD /docs/test.txt HTTP/1.1
Host: example.net

>> Response:

HTTP/1.1 200 OK
Content-Type: text/plain; charset=UTF-8
Content-Length: 12345
Link: </system/v/84345634/1>; rel=latest-version
Link: </system/vh/84345634>; rel=version-history

After creating more versions, following the latest-version would then expose predecessors of a version:

>> Request:

HEAD /system/v/84345634/3 HTTP/1.1
Host: example.net

>> Response:

HTTP/1.1 200 OK
Content-Type: text/plain; charset=UTF-8
Content-Length: 12323
Link: </system/v/84345634/2>; rel=predecessor-version

B. Change Log (to be removed by RFC Editor before publication)

B.1 Since draft-brown-link-relations-00

Added Geoff Clemm as author.

Renamed link relation "all-versions" to "version-history". Fixed description of "working-resource" relation to state that it appears on a version resource.

B.2 Since draft-brown-link-relations-01

Rewrite terminology and link relations using simpler definitions that can reflect versioning approaches different from WebDAV.

Add JCR/WebDAV property table. And reference to Web Linking draft (for now informative) and examples showing use of the Link header.

B.3 Since draft-brown-link-relations-02

Add and resolve issue "iana".