RDF/NextStepWorkshop/Report

From Semantic Web Standards

Executive Summary

The Resource Description Framework (RDF), the first layer of the Semantic Web, became a W3C Recommendation in 1999. A major revision was published in 2004, adding a few features, clarifying the syntax and semantics, and retaining compatibility with existing deployment. In the years since then, RDF has been widely implemented and has been adopted in various industries for a wide range of applications. Now, in June 2010, the W3C held a workshop to gather feedback and begin to determine if another revision of RDF is warranted and, if so, which elements should be added or clarified.

The workshop submissions, presentations, and discussions indicated a strong demand for a few features to be added in a compatible manner. Participants also expressed considerable resistance to doing anything which would disrupt or confuse existing deployment efforts. While some participants expressed a strong desire to change certain elements of the design of RDF, there was general agreement that the negative impact from doing so, in nearly all cases, made it unwise.

The work items summary table lists each of the possibilities discussed for an imagined new RDF Working Group. Next to each item is an indication of the level of support it had, expressed via straw poll of the workshop participants. The work items which had strong support and no stated opposition were:

  • Adding support for graph identification (such as with named graphs)
  • Fixing known errors and shortcomings in the specifications
  • Standardizing Turtle (N3), including support for embedded/named graphs
  • Standardizing an RDF serialization based on JSON

While the demand was much weaker, there was also wide support and little or no opposition to several more work items, as shown in the table. Based on this data, these items should be left as optional "time permitting" deliverables for a Working Group, allowing them to be addressed if sufficiently motivated participants rapidly develop a design acceptable to the Working Group.

Workshop partipants made it clear that any Working Group chartered should be clearly directed to respect, support, and advance existing deployment, avoiding any changes which would negatively impact current RDF users.

Background

The Resource Description Framework (RDF), including the general concepts, its semantics, and an XML Serialization (RDF/XML), was published as a W3C Recommendation in 2004. Since then, RDF has become the core architectural block of the Semantic Web, with a significant deployment in terms of tools and applications.

As a result of the R&D activities and the publication of newer standards like SPARQL, OWL, POWDER, or SKOS, but also due to the large scale deployment and applications, a number of issues regarding RDF came to the fore. Some of those are related to features that are not present in the current version of RDF but which became necessary in practice (eg, the concept of “Named Graphs”). Others result from the difficulties caused by the design decisions taken in the course of defining the 2004 version of RDF (eg, restrictions whereby literals cannot appear as subjects). Definition of newer standards have also revealed difficulties when applying the semantics of RDF (eg, the exact semantics of blank nodes for RIF and OWL, or the missing connection between URI-s and the RDF resources named by those URI-s for POWDER). New serializations formats (eg, Turtle) have gained a significant support by the community, while the complications in RDF/XML syntax have created some difficulties in practice as well as in the acceptance of RDF by a larger Web community. Finally, at present there is no standard programming API to manage RDF data; the need may arise to define such a standard either in a general, programming language independent way or for some of the important languages (Javascript/ECMAscript, Java, Python,…)

The RDF Next Steps Workshop was held to consider whether a revision of the 2004 version of RDF is necessary. This report constitutes the workshop's findings.

Workshop Overview

Workshop Format

The workshop was conducted over 2 days with a series of presentations with consisting of a selection of the accepted submissions. Based on the submission a issues where identified on a whiteboard and at the end of the day grouped. Based on the identified groupings four break-out groups where formed, which on the 2nd day detailed and discussed the identified issues. The results of the discussion were reported back to the plenary. Discussion during the plenary and most break-out groups were scribed in IRC and the whiteboard discussion were recorded.

Workshop Program Day 1

The first session of the workshop ("Setting the scene") started with two presentations urging caution wrt to possible changes by Richard Cyganiak and Jeremy Carroll, but identified a number of issues which they would like to be changed. The 2nd presentation by David Beckett identified a number of issues with current RDF Syntax formats, followed by a presentation by David Booth identifying a number of issues with the current RDF specification. In the second session of the day a number of specific communities reported on their requirements and issues. Jun Zhao reported on the requirements from the W3C Provenance Working group, Elisa Kendall reported on behalf of the OMG a experiences the OMG Ontology PSIG has with the current RDF specification. In the final presentation in this session Axel Polleres reported requirements from a number of SPARQL users regarding a combined XML and RDF querying mode. In the third session a number of presenters reported on specific semantic issues: Alejandro Mallea reported on the mismatch of the current RDF Blank Nodes specification and their usage in SPARQL. Jie Bao highlighted the usefulness of contexts and import facilities for RDF, and Axel Polleres presented the need for annotations of RDF statements. Finally Peter Patel-Schneider presented his motivation to revise the RDF semantics towards a pure data structuring language. In the last session of the day additional contributions were presented: Fabien Gandon presented the need for named graphs and other graph extensions including RDF/XML syntax extensions. Andy Seaborne highlighted the need to a change propagation mechanism for large RDF graphs. James Leigh

For more details and access to the indvidual presentations and minutes, see the workshop program and the workshop minutes.

Identified Issues - RDF Core
Issue Maybe Now Maybe Later Maybe Never
Named graphs X
Turtle X
Follow your nose X
JSON X
Atom X
XSLT-friendly XML X
XML Schema-friendly XML X
Skolemize bnodes X
Disentangle RDF/RDFS namespaces X
URI->IRI X
Binary RDF X
n-ary predicates X
Revise semantics X
List construct X
RDFa profiles X
Weakly deprecate aspects X
Identified Issues - RDF Infrastructure
Issue Maybe Now Maybe Later Maybe Never Discuss on Day 2
Rule-based querying X
Identity for non-RDF resources X
Evolution (provenance) X
Provenance vocabularies X
Annotations X X
Standard APIs X
Unified RDF/XML/RDB query layer X
Change vocabularies X
Isolation API X
Identity vocabulary X X

Photographs of the issues tracked by white board at the workshop are available at:

Issues were separated into three groups: "maybe now", "maybe later" and "maybe never". Each issue was identified by one or more participants, placed into a category by the person or people presenting the issue and later moved into a final category by straw poll. Issues were further separated into either "RDF Core" or "RDF Infrastructure" categories that were orthogonal to the three temporal groups. Workshop participants were polled at the end of the first day to determine the relative importance of each issue. The most important issues were thus identified by group consensus and slated for detailed discussion on the second day. RDF Core issues were treated as a higher priority for a potential working group to consider; RDF Infrastructure issues were elevated for discussion only if they were seen as urgently important for immediate treatment by a working group.

Each issue considered in the "maybe now" category was scheduled for discussion and documentation on the second day.

Workshop Program Day 2

Workshop activities on the second day centered around four broad areas, each assigned to a breakout group. The topics identified at the end of Day 1 for further discussion were distributed among the four breakout groups as follows:

Breakout Group 1: Syntaxes

Chair: Dave Beckett

Topics:

  • Turtle
  • JSON
  • RDFa Profiles

Breakout Group 2: Semantics

Chair: Paul Gearon

Topics:

  • BNodes
  • Revising RDF Semantics
  • Weak Deprecation
  • Literals as subjects

Breakout Group 3: Graph Metadata

Chair: Elisa Kendall See also: IRC minutes

Topics:

  • IRC: #rdfn-meta
  • Named Graphs
  • Annotations

Breakout Group 4: Linked Data

Chair: David Booth

Topics:

  • Identity Vocabulary
  • Follow Your Nose

Results

Workshop participants created a very rough draft of a working group charter. Please note that not every aspect of that document has been reviewed as of this writing. Each work item addressed was discussed at some length, including those which may not have much support.

The summary table of results illustrates the consensus of the workshop participants and suggests a priority order for a later working group to address the issues discussed. The issues of most importance, and the ones where consensus was strongest were (in weighted order):

Recommended Next Steps

The outcome of the workshop clearly indicated that the community takes issue with important aspects of RDF as it is currently standardized. The workshop recommends that a W3C working group be chartered at the earliest convenience to address those issues. Workshop participants urge the W3C to use the workshop summary table as guidance in the production of the working group charter.

Acknowledgements

The RDF Next Steps workshop was graciously hosted by Stanford University and the National Center for Biomedical Ontology. Stanford and NCBO provided facilities, refreshments and lunch for both days of the workshop. Natasha Noy and Chris Macintosh are to be commended for their efforts organizing workshop logistics.

The workshop was co-chaired by Ivan Herman, Stefan Decker and David Wood. Day 2 breakout sessions were chaired by Dave Beckett, Paul Gearon, Elisa Kendall and David Booth. Thanks to all the participants for working late and for volunteering their time to improve RDF.