W3C

[Editorial Draft] Abstract Component References

Proposed TAG Finding 16 Oct 2003

This version:
http://www.w3.org/2001/tag/doc/versioning (HTML, XML)
Editor:
David Orchard, BEA Systems, Inc. <David.Orchard@BEA.com>

Abstract

This finding describes the TAG's response to the question of the suitability of using URIs with fragment identifiers for identifying abstract components, as exemplified by Jonathan Marsh's raising of the issue for the WSD working group at [IssueRaise]. This finding contains the TAG response and a summary of the possible solution space to the problem. Requirements, a sample use case, a short description of each solution, and the pros and cons of each solution is provided. Arthur Ryman's posting of the current WSDL solution [2] provides excellent background material.

Status of this Document

This document has been developed for discussion by the W3C Technical Architecture Group. It does represent a draft of the consensus opinion of the TAG.

Publication of this finding does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time.

Additional TAG findings, both approved and in draft state, may also be available. The TAG expects to incorporate this and other findings into a Web Architecture Document that will be published according to the process of the W3C Recommendation Track.

Please send comments on this finding to the publicly archived TAG mailing list www-tag@w3.org (archive).

Table of Contents

1 TAG Recommendation
2 Problem analysis
2.1 Requirements
2.2 Sample problem
3 Solution space
3.1 Namespace name and new fragment identifier syntax.
3.2 Use ID attribute and XML fragment identifier syntax
3.3 Unique Names
3.4 Full XPointer
3.5 XPointer framework and element()
3.6 WSD specific Xpointer scheme
3.7 XML Schema Component designators
3.8 URN
3.9 RDDL fragment identifier syntax
3.10 A URI convention that slashes separate namespace URI and component identifier
3.11 URI query strings

Appendix

A References


1 TAG Recommendation

The TAG consensus is that description languages - such as WSDL, XML Schema, OWL - should define fragment identifiers for identifying a language's abstract components. The TAG supports the use of fragment identifiers by the WSD working group.

Good Practice

Description languages that identify abstract components should define a fragment identifier syntax for these components.

The TAG does believe that the issue of identifying abstract components and the use of namespace names has architectural implications for the Web. The TAG has consensus that a vocabulary can recommend that an instance of the vocabulary is available upon de-referencing the namespace name.

Good Practice

Description languages that identify abstract components should make available an instance of the language at the URI used for identifying the abstract component.

A typical base URI is the namespace name for the instance of the language.

The TAG is working on a human-centric namespace name format. This format and the related fragment identifier syntax are not sufficiently advanced in the W3C process for the TAG to recommend that a working group use them. The abstract component identifiers as defined in a particular language, and the relationship to the description language syntax, the fragment identifier syntax, the use of a namespace name document, and the namespace name document fragment identifier syntax is not clear at this time. The TAG expects to continue work on this area.

As for the particulars of the syntax, the TAG does not wish to delve deeply into syntax design of the WSD fragment identifier syntax, believing that the WSD WG is better qualified for such activities. A number of TAG members did have some particular comments on URI syntax construction. The use of parenthesis should be avoided, and the use of periods "." as a separator seems useful. Effectively, the TAG offers a syntax that is very close to the last syntax produced by the Jonathan Marsh but using fragment identifier separators. To be clear, it is some TAG members that offer this opinion on the specific syntax rather than TAG consensus. The option that some TAG members support is option #1A.

The TAG is planning on being available for liaison meetings in the Tech plenary 2004 week. Further discussion of this issue, and others, with the WSD WG is possible should the WSD WG desire.

2 Problem analysis

3 Solution space

3.7 XML Schema Component designators

XML Schema component designators description at [scuds]. The sample URI is

"http://airline.wsdl/ticketagent/#xmlns(ta=http://www.airline.wsdl/ticketagent/)wsdl(/portType(ta:TicketAgent)/operation(listFlights)/input(listFlightsRequest))"

Pros:

  • Type is apparent

  • extensions are identifiable

Cons:

  • complex syntax

  • Schema component designators still under development

  • Uses balanced parens

Notes:

Noah provides some rationale for Schema's decisions on Component Designators at [noahonscuds]

3.8 URN

First proposal by Arthur Ryman[useURN].

The URI sample would be

"urn:wsdl:airline.wsdl:ticketagent:listFlights:listFlightsRequest"

Pros:

  • Independence from potential dereferencing operations

Cons:

  • embeds URIs into URNs

  • URI not dereferenceable

  • Unsure how extensions are identified

  • No advantage over http: URIs wrt longevity as a new URN needs to be minted any time the interface changes.

3.9 RDDL fragment identifier syntax

Tim Bray's first posting on this is [rddl-fragid]. Dave Orchard first raised this in [tagf2f]

Sample URI is

"http://airline.wsdl/ticketagent/#wsdl/input.TicketAgent.listFlights.listFlightsRequest"

Pros:

  • Type is apparent

  • incorporates RDDL into Web services

  • Could unify schema, wsdl, other description languages with namespaces.

Cons:

  • Idea is a barely beyond a twinkle in our eyes. It requires specification of RDDL frag identifier syntax, and typical time to develop issues.

  • Unknown whether it will actually solve the problem.

  • Unsure how extensions are identified.

  • roy:we don't need another universal media type.

3.10 A URI convention that slashes separate namespace URI and component identifier

Suggested by Roy at [mar24telcon]

He says "In the CMS world, a compound hierarchical document is no different from a hierarchical directory system -- all names are hierarchies and the names are separated by "/" all the way down to the smallest atom of content. WSDL defines a compound document namespace rooted at its namespace URI. So, add a slash and define the hierarchy below the namespace URI according to WSDL.". Roy disagrees with using frag-ids in names in [royonfragsasnames]

Sample URI is

"http://airline.wsdl/ticketagent/TicketAgent/listFlights/listFlightsRequest"

roy:preferred approach

Option #2 URI is

"http://airline.wsdl/ticketagent/input/TicketAgent/listFlights/listFlightsRequest"

Option #3 URI is

"http://airline.wsdl/ticketagent/TicketAgent/listFlights/listFlightsRequest/input"

Option #4 URI is

"http://airline.wsdl/ticketagent/TicketAgent/listFlights/listFlightsRequest;input"

Roy:preferred approach if input is required

Option #5 URI is

"http://airline.wsdl/ticketagent/input(TicketAgent/listFlights/listFlightsRequest)"

Pros:

  • Relative URIs can be used (except with option #5)

  • Option #2,3,4,5 has type apparent

  • Does not dependend upon RDDL, XPointer, etc.

Cons:

  • Can't tell where the namespace name ends and the name begins Roy:So?

  • Type is not apparent

  • #5 has balanced parens.

3.11 URI query strings

Option #1

Posted by Noah at [usequery]

Sample URI is

"http://airline.wsdl/ticketagent?portType=TicketAgent&operation=listFlights&input=listFlightsRequest"

Option #2

Posted by Jonathan Marsh at [jonathanonusingquery]. Suggest a single name with a value that uses periods as separators.

http://airline.wsdl/ticketagent/?wsdl-input=TicketAgent.reserveFlight.reserveFlightRequest

Pros

  • Compliant with URI specification

  • Type is apparent

Cons

  • Overrides a portion of the URI space

  • No hierarchy of names

  • No prevention of namespace conflicts. Not sure how much of this is a problem for refering to WSD documents.

  • Extensions not supported (need namespaces). Perhaps extensions can be supported, but we have to say that no name clashes are allowed?

Notes:

I wonder if there would be a way of getting QNames into this for extensions.

This does seem close to re-inventing XPointer though.

Roy: this solution defeats the purpose of assigning names

A References

IssueRaise
Issue Raising (See http://lists.w3.org/Archives/Public/www-tag/2003Feb/0207.html.)
RymanExplanation
Ryman Explanation (See http://lists.w3.org/Archives/Public/www-ws-desc/2002Dec/0021.html.)
useURN
Use URNs (See http://lists.w3.org/Archives/Public/www-ws-desc/2002Oct/att-0084/01-URI-References.html.)
scuds
Schema Component Designators (See http://www.w3.org/TR/xmlschema-ref/.)
rddl-fragid
RDDL Frag Id (See http://lists.w3.org/Archives/Public/www-tag/2003Mar/0064.html.)
tagf2f
TAG F2F Minutes (See http://www.w3.org/2002/11/18-tag-summary.)
xptr-element
Xpointer element (See http://www.w3.org/TR/xptr-element/.)
xptr-framework
Xpointer framework (See http://www.w3.org/TR/xptr-framework/.)
xptr-ns
Xpointer Namespaces (See http://www.w3.org/TR/xptr-xmlns/.)
xpointer-full
Full XPointer (See http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219/.)
royonfragsasnames
Roy Fielding posting on fragments as names (See http://lists.w3.org/Archives/Public/www-tag/2003Jan/0148.html.)
usequery
Use Query string (See http://lists.w3.org/Archives/Member/w3c-ws-desc/2003Apr/0010.html (W3C Member only).)
noahonscuds
Noah Mendellsohn posting on schema component designators (See http://lists.w3.org/Archives/Public/www-tag/2003Apr/0081.html.)
metadatainuri
Metadata in URI (See http://www.w3.org/2001/tag/ilist#metadataInURI-31.)
mar24telcon
Mar 24 TAG telcon (See http://www.w3.org/2003/03/24-tag-summary.html#abstractComponentRefs-37.)
jonathanonusingquery
Jonathan Marsh proposal on using query string (See http://lists.w3.org/Archives/Public/www-ws-desc/2003Sep/0075.html.)