Semantic Navigation on the Web of Data: Specification of
Routes, Web Fragments and Actions
Valeria Fionda
KRDB, Free University of
Bozen-Bolzano, Italy
Claudio Gutierrez
DCC, Universidad de Chile,
Santiago, Chile
Giuseppe Pirró
KRDB, Free University of
Bozen-Bolzano, Italy
ABSTRACT
The massive semantic data sources linked in the Web of Data
give new meaning to old features like navigation; introduce
new challenges like semantic specication of Web fragments;
and make it possible to specify actions relying on semantic
data. In this paper we introduce a declarative language to
face these challenges. Based on navigational features, it is
designed to specify fragments of the Web of Data and actions
to be performed based on these data. We implement it in
a centralized fashion, and show its power and performance.
Finally, we explore the same ideas in a distributed setting,
showing their feasibility, potentialities and challenges.
Keywords
Navigation, Web of Data, Linked Data, Semantic Web
1. INTRODUCTION
Classically the Web has been modelled as a huge graph
of links between pages [4]. This model included Web fea-
tures such as links without labels and only generated by
the owner of the page.1 Although Web pages are created
and kept distributively, their small size and lack of struc-
ture stimulated the idea to view searching and querying
through single and centralized repositories (built from pages
via crawlers). With the advent of the Web of Data, that is,
semantic data at massive scale [3, 16], these assumptions, in
general, do not hold anymore. First, links are semantically
labelled (thanks to RDF triples) thus can be used to orient
and control the navigation, are generated distributively and
can be part of any data source. Hence, it has become a
reality {using the words of Tim Berners-Lee{ that anyone
can say anything about anything and publish it anywhere.
Second, data sources have a truly distributed nature due to
their huge size, autonomous generation, and standard RDF
structure. This makes inconvenient and impractical to re-
organize them in central repositories as for Web pages.
1Even though the spec. XLink [7] allows to dene links in a
third page, it was never used massively.
The Web
The Web of Data
r
<2><3>
1 <4>
2
<5>3
<3><5>
4
5
Search/Query
<2><3>
<4>
1
2
<3><5>
<5>3
4
5
href
< ,q, >< ,p, >... < ,r, >< ,p, >...
< ,m, >< ,n, >...
< ,t, >...

p qn t m
Figure 1: Classical Web versus Web of Data. Size,
distributive character, and semantic description of
data gives navigation a prominent role.
In this setting, navigation along the nodes of the Web of
Data, using the semantics stored in each data source, be-
comes signicant. To model these issues, rather than as a
graph, the Web of Data is better represented as a set of nodes
plus data describing their semantic structure\hanging" from
each node (see Fig. 1). This model permits to better ex-
press the distributed creation and maintenance of data, and
the fact that its structure is provided by dynamical and dis-
tributed data sources. In particular, it re
ects the fact that
at each moment of time, and for each particular agent, the
whole network of data on the Web is unknown [19].
This new scenario calls for new models and languages to
query and explore this semantic data space. In particular we
highlight three functionalities: (1) a new type of navigation
emerges as an important feature, in order to traverse sites
and data sources; (2) closely tied to it, navigation charts or
specications, that is, semantic descriptions of fragments of
the Web; (3) specication of actions one would like to per-
form over this data (e.g., retrieving data, sending messages,
etc.) also becomes relevant. Navigation, specications of re-
gions, and actions appear as part of the basic functionalities
for exploring and doing data management over the Web of
Data. Ideally, one would like to have a simple declarative
language that integrates all of them.
In this paper we present such a language, which we call
NautiLOD, and show that it can be readily implemented
on the current Linked Open Data (LOD) network [16]. In
fact, we introduce the swget tool that exploits current Web
protocols and work on LOD data. Finally, we explore its dis-
tributed version and implement an application as proof-of-
concept to show its feasibility, potentialities and challenges.
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Figure 2: An excerpt of data that can be navigated from dbpedia:StanleyKubrick.
NautiLOD by example. To help the reader to get a more
concrete idea of the language, we present some examples
using an excerpt of real-world data shown in Fig. 2. (The
formal syntax and semantics is introduced in Section 3).
Example 1.1. (Aliases via owl:sameAs) Specify what
is predicated from Stanley Kubrick in DBPedia and also con-
sider his possible aliases in other data sources.
The idea is to have <owl:sameAs>-paths, which start from
Kubrick's URI in DBPedia. Recursively, for each URI u
reached in this way, check in its data source the triples
hu; owl:sameAs; vi. Select all v's found. Finally, for each
of such v, return all URIs w in triples of the form hv; p; wi
found in v's data source. The specication in NautiLOD is
as follows:
(<owl:sameAs>)  =<_>
where <_> denotes a wild card for RDF predicates. In Fig.
2, when evaluating this expression starting from the URI
dbp:StanleyKubrick we get all the dierent representations
of Stanley Kubrick provided by dbpedia.org, freebase.org
and linkedmdb.org. From these nodes, the expression <_>
matches any predicate. The nal result is: fdbp:DavidLynch,
dbp:New York, dbp:FilmEditing,lmdb:Producer,lmdb:/film/334,fb:Path
of Glory,http://en.wikipedia.org/wiki/Stanley_Kubrickg. Note that
the naive search for Kubrick's information in DBPedia, would
only give {http://en.wikipedia.org/wiki/Stanley_Kubrick, New York,
David Lynch, Film Editing}.
A more complex example, which extends standard naviga-
tional languages with actions and SPARQL queries is:
Example 1.2. URIs of movies (and their aliases), whose
director is more than 50 years old, and has been in
uenced,
either directly or indirectly, by Stanley Kubrick. Send by
email the Wiki pages of such directors as you get them.
This specication involves influence-paths and aliases as
in the previous example; tests over the dataset associated to
a given URI (if somebody in
uenced by Kubrick is found,
check if it has the right age), a test expressed in NautiLOD
using ASK-SPARQL queries; and actions to be taken using
data form the data source. The NautiLOD specication is:
(<dbpo:influenced>)+[Test]/Act/<dbpo:director>/
/(<owl:sameAs>)?
where the test and the action are as follows:
Test= ASK ?p <dbpo:birthDate> ?y. FILTER(?y<1961-01-01).
Act= sendEmail(?p)[SELECT ?p WHERE {?x <foaf:page> ?p.}].
In the expression, the symbol + denotes that one or more
levels of in
uence are acceptable, e.g., we get directors like
David Lynch and Quentin Tarantino. From this set of re-
sources, the constraint on the age enforced by the ASK query
is evaluated on the data source associated to each of the re-
sources already matched. This lter leaves in this case only
dbp:DavidLynch. At this point, over the elements of this
set (one element in this case), the action will send via email
the page (obtained from the SELECT query). The action
sendEmail, implemented by an ad-hoc programming proce-
dure, does not in
uence the navigation process. Thus, the
evaluation will continue from the URI u =dbp:DavidLynch,
by navigating the property dbpo:director (found in the
dataset D obtained by dereferencing u). For example, in
D we found the triple hu; dbpo:director; dbp:BlueVelveti.
Then, from dbp:BlueVelvet we launch the nal part of
the expression, already seen in Example 1.1. It can be
checked that the nal result of the evaluation is: (1) the set
{dbp:BlueVelvet, fb:BlueVelvetg, that is, data about the
movie Blue Velvet from dbpedia.org and freebase.org; (2)
the set of actions done; in this case one email sent.
Contributions of the paper. The following are main the
contributions of this paper:
(1) First: we dene a general declarative specication lan-
guage, called NautiLOD, whose navigational features ex-
ploit regular expressions on RDF predicates, enhanced with
existential tests (based on ASK-SPARQL queries) and ac-
tions. It allows both: to specify a set of sites that match the
semantic description, and to orient the navigation using the