Databases Reference
In-Depth Information
idea of content negotiation is that HTTP clients send HTTP headers with each request
to indicate what kinds of documents they prefer. Servers can inspect these headers and
select an appropriate response. If the headers indicate that the client prefers HTML then
the server will respond by sending an HTML document If the client prefers RDF, then
the server will send the client an RDF document.
There are two di
erent strategies to make URIs that identify real-world objects de-
referencable [137]. Both strategies ensure that objects and the documents that describe
them are not confused and that humans as well as machines can retrieve appropriate
representations.
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303 URIs.
Real-world objects can not be transmitted over the wire using the HTTP
protocol. Thus, it is also not possible to directly de-reference URIs that identify real-
world objects. Therefore, in the 303 URI strategy, instead of sending the object itself
over the network, the server responds to the client with the HTTP response code 303
See Other and the URI of a Web document which describes the real-world object.
This is called a
303 redirect
. In a second step, the client de-references this new URI and
retrieves a Web document describing the real-world object.
Hash URIs.
A widespread criticism of the 303 URI strategy is that it requires two HTTP
requests to retrieve a single description of a real-world object. One option for avoiding
these two requests is provided by the hash URI strategy. The hash URI strategy builds
on the characteristic that URIs may contain a special part that is separated from the base
part of the URI by a hash symbol (#). This special part is called the fragment identifier.
When a client wants to retrieve a hash URI the HTTP protocol requires the fragment
part to be stripped o
before requesting the URI from the server. This means a URI that
includes a hash cannot be retrieved directly, and therefore does not necessarily identify
a Web document. This enables such URIs to be used to identify real-world objects and
abstract concepts, without creating ambiguity [137].
Both approaches have their advantages and disadvantages. Section 4.4. of the
W3C Interest Group Note
Cool URIs for the Semantic Web
compares the two ap-
proaches [137]: Hash URIs have the advantage of reducing the number of necessary
HTTP round-trips, which in turn reduces access latency. The downside of the hash URI
approach is that the descriptions of all resources that share the same non-fragment URI
part are always returned to the client together, irrespective of whether the client is inter-
ested in only one URI or all. If these descriptions consist of a large number of triples,
the hash URI approach can lead to large amounts of data being unnecessarily transmit-
ted to the client. 303 URIs, on the other hand, are very flexible because the redirection
target can be configured separately for each resource. There could be one describing
document for each resource, or one large document for all of them, or any combination
in between. It is also possible to change the policy later on.
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2.3
RDF Data Model
The RDF data model [1] represents information as sets of statements, which can be
visualized as node-and-arc-labeled directed graphs. The data model is designed for the
