Publishing Linked Data - Linked Data: A Geographic Perspective - page 108

description of the object; this is known as a dereferenceable URI. The third principle
states that when we retrieve the description of the object, we should be able to under-
stand it, so we use a standard language like RDF to encode that description. The
fourth principle recommends adding links between different datasets to be able to
move along these links from one dataset to another. This then positions each dataset
as a connected subpart of the whole Linked Data Web and enables applications such
as semantic search engines to crawl along these links and discover data across dif-
ferent datasets. Furthermore, as a link is encoded as the predicate of an RDF triple,
it is more than just a Web hyperlink, as it is typed, and hence has meaning in itself.
7.3 MAKING URIS DEREFERENCEABLE OR SLASH VERSUS HASH
There are two ways to dereference a URI, that is, to retrieve the description of the
actual object we are talking about, using HTTP. One is to use a 303 URI, which will
be of the form http://id.mereamaps.gov.me/topo/00012 , a “Slash URI”, and the other
is to use a Hash 1 URI of the form http://id.mereamaps.gov.me/topo#00012 . You will
see both mechanisms used, but they have slightly different effects, and you should
consider carefully, based on your particular use case, which is more appropriate.
7.3.1 S laSh or 303 UrI S
The Slash or 303 URIs are so called because when the client requests a URI of the
form http://id.mereamaps.gov.me/topo/00012 using an HTTP GET command, the
HTTP server responds with a code “303 See Other” plus the URI of a Web document
that describes the object, for example, http://data.merea.gov.me/topo/00012 . As sug-
gested by Heath and Bizer (2011), it can be helpful to follow the convention of using
“id” 2 in the URI when referring to the resource (real-world object) itself, “page”
when specifying a HTML Web page, and “data” when referring to the RDF/XML
document describing the resource. This means that the client is redirected to this
second URI, and now the client also needs to dereference it to get to the Web docu-
ment describing the object. So, the client then sends a second HTTP GET request on
the second URI, and the server replies with an HTTP response code 200 OK plus
the RDF document describing the original object. This somewhat convoluted pro-
cess of redirection means that the server is forced to do the hard work of finding the
individual resource (in the second query) rather than leaving it to the client to do so.
The advantage of this system is that the whole URI can be retrieved from the server.
7.3.2 h aSh UrI S
The drawback of the 303 or Slash URI method is that the client has to make two
requests, and the server has to respond twice. An alternative is to create URIs using
hashes (#) in the form http://id.mereamaps.gov.me/topo#00012 . In this case, the HTTP
protocol will strip away the bit after the #, which is known as the fragment identifier,
and then only the part before the # is requested from the server. This means that the
URI as a whole cannot be directly retrieved from the server, so there might not be a