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

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/).

Note that although discussion takes place on the Atompub working group's mailing list, this is not a working group document.

XML versions, latest edits and the issues list for this document are available from http://greenbytes.de/tech/webdav/#draft-brown-versioning-link-relations.

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
Appendix A.  Relationship to Java Content Repository (JCR) and WebDAV
    A.1.  Example: Use of Link Relations in HTTP Link Header
Appendix 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
Appendix C.  Resolved issues (to be removed by RFC Editor before publication)
    C.1.  cmis
    C.2.  checked-out
Appendix D.  Open issues (to be removed by RFC Editor prior to publication)
    D.1.  edit
    D.2.  working-copy-of
§  Authors' Addresses

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.

These link relations are used in the AtomPub ([RFC5023] (Gregorio, J. and B. de hOra, “The Atom Publishing Protocol,” October 2007.)) bindings of the "Content Management Interoperability Services" (CMIS). See Section 3.4.3.1 of [CMIS] (Brown, A., Gur-Esh, E., McVeigh, R., and F. Muller, “Content Management Interoperability Services (CMIS) Version 1.0,” September 2009.) for further information.

2.  Terminology

Versioned Resource

When a resource is put under version control, it becomes a "versioned resource". 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 used to create a new version of a versioned resource.

Checkout

A "checkout" is an operation on a versioned resource that creates a working copy, or changes the versioned resource to be a working-copy as well ("in-place versioning").

Checkin

A "checkin" is an operation on a working copy that creates a new version of its corresponding versioned resource.

Note: the operations for putting a resource under version control, and for checking in and checking out depend on the protocol in use and are beyond the scope of this document; see [CMIS] (Brown, A., Gur-Esh, E., McVeigh, R., and F. Muller, “Content Management Interoperability Services (CMIS) Version 1.0,” September 2009.), [RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.) and [JSR‑283] (Day Software, Nuescheler, D., and P. Piegaze, “Content Repository API for Java(tm) Technology Specification,” August 2009.) for examples.

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.

3.4.  working-copy-of

When included on a working copy, this link points to the versioned resource from which this working copy was obtained.

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] (Nottingham, M. and R. Sayre, “The Atom Syndication Format,” December 2005.):

4.1.  'version-history' Link Relation Registration

Attribute Value:

version-history

Description:

See Section 3.1 (version-history).

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 (Security Considerations).

4.2.  'latest-version' Link Relation Registration

Attribute Value:

latest-version

Description:

See Section 3.2 (latest-version).

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 (Security Considerations).

4.3.  'working-copy' Link Relation Registration

Attribute Value:

working-copy

Description:

See Section 3.3 (working-copy).

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 (Security Considerations).

4.4.  'working-copy-of' Link Relation Registration

Attribute Value:

working-copy-of

Description:

See Section 3.4 (working-copy-of).

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 (Security Considerations).

4.5.  'predecessor-version' Link Relation Registration

Attribute Value:

predecessor-version

Description:

See Section 3.5 (predecessor-version).

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 (Security Considerations).

4.6.  'successor-version' Link Relation Registration

Attribute Value:

successor-version

Description:

See Section 3.6 (successor-version).

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 (Security Considerations).

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.

6.  Acknowledgments

Thanks to the members of Content Management Interoperability Services (CMIS) Technical Committee (TC) at OASIS for the initial proposal, and to Jan Algermissen for feedback during IETF review.

7.  References

7.1. Normative References

7.2. Informative References

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

The link relations defined in Section 3 (Link Relations) correspond to various properties used in WebDAV Versioning [RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.) and JCR [JSR‑283] (Day Software, Nuescheler, D., and P. Piegaze, “Content Repository API for Java(tm) Technology Specification,” August 2009.):

version-history

WebDAV: the resource identified by the DAV:version-history property ([RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.), Sections 5.2.1 and 5.3.1).

JCR: the node identified by jcr:versionHistory property ([JSR‑283] (Day Software, Nuescheler, D., and P. Piegaze, “Content Repository API for Java(tm) Technology Specification,” August 2009.), 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] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.), Section 3.2.1) or DAV:checked-out ([RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.), 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] (Day Software, Nuescheler, D., and P. Piegaze, “Content Repository API for Java(tm) Technology Specification,” August 2009.), Section 3.13.2.5) for versionable nodes; for version nodes, a successor version that itself does not have any successors.

working-copy

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] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.), Section 3.4.3).

JCR: for checked-out versionable nodes: the node itself.

working-copy-of

WebDAV: the resource identified by the the DAV:checked-out property (see [RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.), Section 3.3.1).

JCR: for checked-out versionable nodes: the node identified by the jcr:baseVersion property ([JSR‑283] (Day Software, Nuescheler, D., and P. Piegaze, “Content Repository API for Java(tm) Technology Specification,” August 2009.), Section 3.13.12.5).

predecessor-version

WebDAV: each resource identified by a member of DAV:predecessor-set ([RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.), Sections 3.3.2 and 3.4.1).

JCR: each node identified by a member of jcr:predecessors ([JSR‑283] (Day Software, Nuescheler, D., and P. Piegaze, “Content Repository API for Java(tm) Technology Specification,” August 2009.), Section 3.13.3.3).

successor-version

WebDAV: each resource identified by a member of DAV:successor-set ([RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.), Section 3.4.2).

JCR: each node identified by a member of jcr:successors ([JSR‑283] (Day Software, Nuescheler, D., and P. Piegaze, “Content Repository API for Java(tm) Technology Specification,” August 2009.), 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] (Nottingham, M., “Web Linking,” July 2009.)) 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] (Nottingham, M. and R. Sayre, “The Atom Syndication Format,” December 2005.)) or WebDAV properties ([RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.)).

For instance, a response to an VERSION-CONTROL request ([RFC3253] (Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J. Whitehead, “Versioning Extensions to WebDAV (Web Distributed Authoring and Versioning),” March 2002.), 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

Appendix 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".

B.4.  Since draft-brown-link-relations-03

Fix typo ("working-resource" instead of "working-copy"). Add and resolve issues "checked-out", "cmis" and "working-copy-of".

Appendix C.  Resolved issues (to be removed by RFC Editor before publication)

Issues that were either rejected or resolved in this version of this document.

C.1.  cmis

Type: edit

julian.reschke@greenbytes.de (2009-12-01): Add a pointer to the CMIS spec, so AtomPub use cases become clearer.

Resolution (2009-12-01): Done.

C.2.  checked-out

In Section 2:

Type: change

<http://www.imc.org/atom-syntax/mail-archive/msg21321.html>

algermissen1971@mac.com (2009-11-24): It is not clear to me, what the meaning of 'check out' and 'check in'. Also, the text (IMO) creates the impression that versioning can only take place when 'check out' and 'check in' are applied. However, a resource could also be versioned by the server upon any modification made by a client regardless of any 'checking out' or 'checking in'. The link relations specified would still make sense.
Assuming that 'checking out' and 'checking in' are operations on resources, I think the draft should address how clients achieve these operations. This would at least involve another link relation and specification how to use the linked resource to perform a checkout.

Resolution (2009-12-04): Rephrased terminology; added explanations for checkin/checkout.

Appendix D.  Open issues (to be removed by RFC Editor prior to publication)

D.1.  edit

Type: edit

julian.reschke@greenbytes.de (2009-11-19): Umbrella issue for editorial fixes/enhancements.

D.2.  working-copy-of

Type: change

<http://www.imc.org/atom-syntax/mail-archive/msg21350.html>

algermissen1971@mac.com (2009-12-02): ...what is your opinion regarding the introduction of a link relation that is the opposite of working-copy in order to being able to find the versioned resource that the working copy I have is a working copy of?
I am undecided regarding the necessity, but without a working-copy-of relation it seems the client would need to maintain that information (the relationship or the fact that a given resource is a working copy) across requests.

Authors' Addresses