Friday, August 04, 2006
Linked Data - Design Issues
Linked Data - Design Issues: "Browseable graphs
So now we have looked at ways of making a link, let's look at the choices of when to make a link.
One important pattern is a set of data which you can explore as you go link by link by fetching data. Whenever one looks up the URI for a node in the RDF graph, the server returns information about the arcs out of that node, and the arcs in. In other words, it returns any RDF statements in which the term appears as either subject or object.
Formally, call a graph G browsable if, for the URI of any node in G, if I look up that URI I will be returned information which describes the node, where describing a node means:
1. Returning all statements where the node is a subject or object; and
2. Describing all blank nodes attached to the node by one arc.
(The subgraph returned has been refered to as 'minimum Spanning Graph (MSG [@@ref] ) or RDF molecule [@@ref], depending on whether nodes are considerd identified if they can be expressed as a path of function, or reverse inverse functional properties. A concise bounded description, which only follows links from subject to object, does not work.)
In practice, when data is stored in two documents, this means that any RDF statements which relate things in the two files must be repeated in each. So, for example, in my FOAF page I mention that I am a member of the DIG group, and that information is repeated on the DIG group data. Thus, someone starting from the concept of the group can also find out that I am a member. In fact, someone who starts off with my URI can find all the people who are in the same group."
So now we have looked at ways of making a link, let's look at the choices of when to make a link.
One important pattern is a set of data which you can explore as you go link by link by fetching data. Whenever one looks up the URI for a node in the RDF graph, the server returns information about the arcs out of that node, and the arcs in. In other words, it returns any RDF statements in which the term appears as either subject or object.
Formally, call a graph G browsable if, for the URI of any node in G, if I look up that URI I will be returned information which describes the node, where describing a node means:
1. Returning all statements where the node is a subject or object; and
2. Describing all blank nodes attached to the node by one arc.
(The subgraph returned has been refered to as 'minimum Spanning Graph (MSG [@@ref] ) or RDF molecule [@@ref], depending on whether nodes are considerd identified if they can be expressed as a path of function, or reverse inverse functional properties. A concise bounded description, which only follows links from subject to object, does not work.)
In practice, when data is stored in two documents, this means that any RDF statements which relate things in the two files must be repeated in each. So, for example, in my FOAF page I mention that I am a member of the DIG group, and that information is repeated on the DIG group data. Thus, someone starting from the concept of the group can also find out that I am a member. In fact, someone who starts off with my URI can find all the people who are in the same group."
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