Jesse Weaver Presents Named Graphs

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Presentation given at CSCI 6966 Advanced Semantic Web (Fall 2008) - Lesson 11

• File:ASW-Named-Graphs-Jesse-Weaver.ppt -- File was originally an OpenOffice Presentation, but since the wiki doesn't accept files with the opd extension, I saved it as ppt. As a result, the formatting was slightly affected.

• Speaker: Jesse Weaver
• Title: Named Graphs
• Authors: Jeremy J. Carroll, Christian Bizer, Pat Hayes, Patrick Stickler
• Conference: Journal of Web Semantics (2005)
• URL: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B758F-4H9YC8X-1&_user=659639&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000035878&_version=1&_urlVersion=0&_userid=659639&md5=ba75487c523b0e9608be7f83d0358f25
• Date of Presentation: 2008/11/13

Questions

ID	Question	Name	Answer
Carroll2005named question 1 by lebo	When comparing TriX to RDF/XML, the authors state, "The URI at which an RDF/XML document is published is used for three different purposes: as a retrieval address, with an operational semantics; as a means of identifying the document; and as a means of identifying the graph described by the document. There is potential for confusion between these three uses." The URIRef for a named graph performs only the latter function (identifying the graph). When encountering a URIref in a set of triples (e.g., ":G1 pr:disallowedUsage pr:Marketing ." in section 5.3), how does a crawler know that the URIRef identifies a named graph? When a crawler determines that a URIRef names a named graph (e.g., 'ng') , how does it obtain the rdfgraph(ng)? Would the process require anything different from the proposed methods for obtaining descriptions for a "plain old" rdfs:Resource?	Tim Lebo	Good questions. ONE: I believe the answer is not explicit in the article, but it seems to be implied. Consider the quote from section 8.1: "We will consider two such intentions expressed by the properties swp:assertedBy and swp:quotedBy. These take a named graph as a subject ...." Then, note in Fig. 1 that the subject of swp:assertedBy is labeled as an rdfg:Graph. From these two pieces of information, it seems that a named graph can be identified as being rdf:type rdfg:Graph. More formally, for a named graph ng, ng is in the class extension of rdfg:Graph (ICEXT(I(rdfg:Graph))). Thus, if a URIref u is countered, and it is the subject of a triple u rdf:type rdfg:Graph, then the crawler knows it's a named graph. TWO: I believe the assumption is that the crawling will occur over a set of named graphs and that the crawler will somehow have knowledge of how to obtain rdfgraph(ng) from name(ng). Consider first that in named graphs, all graphs have a name (i.e., there is no "default graph" like in SPARQL). Therefore, if a crawler is already crawling a graph, that means it already had some way of obtaining it. In whatever way the crawler obtained that graph, it would obtain other graphs. The last paragraph of section 3.5 seems to indicate that method of retrieval is independent of the named graphs approach. What is important is that a name is associated with each graph and that there exists a way to obtain the graph (and such a way is not specified in the article).
Jesse Weaver Presents Named Graphs GTW 1	After discussing in section 4.3 SPARQL's use of a default (unnamed) dataset, and suggesting that such a set "may have the side effect of reintroducing some of the difficulties that named graphs address, section 5.2 identifies the graph element in TriX as having an "optional uri child element which names the graph." Why should this be optional if the authors believe default or unnamed graphs mixed with named graphs causes problems? Similarly, in discussing TriG, section 5.3 says "each graph should be named with a URIref." Is "should" meant to be read as having normative force? If not, why?	Gregory Todd Williams	My best guess is that they allow an unnamed graph in TriX in order to be compatible with the existing RDF standard. If one wanted to convert a graph from RDF/XML representation to TriX, what would one put as the name of the graph (since there is no equivalent in RDF/XML)? I believe the statement "each graph should be named with a URIref" is just a recommendation in order to comply with the named graphs approach presented in the paper (which does not include an unnamed graph). So, in short, unnamed graphs are probably allowed for compatibility with RDF, but they are not recommended so that such a TriX/TriG representation will comply with the named graphs approach.

Attendees

Tim Lebo