791 research outputs found
Answering SPARQL queries modulo RDF Schema with paths
SPARQL is the standard query language for RDF graphs. In its strict
instantiation, it only offers querying according to the RDF semantics and would
thus ignore the semantics of data expressed with respect to (RDF) schemas or
(OWL) ontologies. Several extensions to SPARQL have been proposed to query RDF
data modulo RDFS, i.e., interpreting the query with RDFS semantics and/or
considering external ontologies. We introduce a general framework which allows
for expressing query answering modulo a particular semantics in an homogeneous
way. In this paper, we discuss extensions of SPARQL that use regular
expressions to navigate RDF graphs and may be used to answer queries
considering RDFS semantics. We also consider their embedding as extensions of
SPARQL. These SPARQL extensions are interpreted within the proposed framework
and their drawbacks are presented. In particular, we show that the PSPARQL
query language, a strict extension of SPARQL offering transitive closure,
allows for answering SPARQL queries modulo RDFS graphs with the same complexity
as SPARQL through a simple transformation of the queries. We also consider
languages which, in addition to paths, provide constraints. In particular, we
present and compare nSPARQL and our proposal CPSPARQL. We show that CPSPARQL is
expressive enough to answer full SPARQL queries modulo RDFS. Finally, we
compare the expressiveness and complexity of both nSPARQL and the corresponding
fragment of CPSPARQL, that we call cpSPARQL. We show that both languages have
the same complexity through cpSPARQL, being a proper extension of SPARQL graph
patterns, is more expressive than nSPARQL.Comment: RR-8394; alkhateeb2003
Context-Free Path Queries on RDF Graphs
Navigational graph queries are an important class of queries that canextract
implicit binary relations over the nodes of input graphs. Most of the
navigational query languages used in the RDF community, e.g. property paths in
W3C SPARQL 1.1 and nested regular expressions in nSPARQL, are based on the
regular expressions. It is known that regular expressions have limited
expressivity; for instance, some natural queries, like same generation-queries,
are not expressible with regular expressions. To overcome this limitation, in
this paper, we present cfSPARQL, an extension of SPARQL query language equipped
with context-free grammars. The cfSPARQL language is strictly more expressive
than property paths and nested expressions. The additional expressivity can be
used for modelling graph similarities, graph summarization and ontology
alignment. Despite the increasing expressivity, we show that cfSPARQL still
enjoys a low computational complexity and can be evaluated efficiently.Comment: 25 page
vSPARQL: A View Definition Language for the Semantic Web
Translational medicine applications would like to leverage the biological and biomedical ontologies, vocabularies, and data sets available on the semantic web. We present a general solution for RDF information set reuse inspired by database views. Our view definition language, vSPARQL, allows applications to specify the exact content that they are interested in and how that content should be restructured or modified. Applications can access relevant content by querying against these view definitions. We evaluate the expressivity of our approach by defining views for practical use cases and comparing our view definition language to existing query languages
Shape Expressions Schemas
We present Shape Expressions (ShEx), an expressive schema language for RDF
designed to provide a high-level, user friendly syntax with intuitive
semantics. ShEx allows to describe the vocabulary and the structure of an RDF
graph, and to constrain the allowed values for the properties of a node. It
includes an algebraic grouping operator, a choice operator, cardinalitiy
constraints for the number of allowed occurrences of a property, and negation.
We define the semantics of the language and illustrate it with examples. We
then present a validation algorithm that, given a node in an RDF graph and a
constraint defined by the ShEx schema, allows to check whether the node
satisfies that constraint. The algorithm outputs a proof that contains
trivially verifiable associations of nodes and the constraints that they
satisfy. The structure can be used for complex post-processing tasks, such as
transforming the RDF graph to other graph or tree structures, verifying more
complex constraints, or debugging (w.r.t. the schema). We also show the
inherent difficulty of error identification of ShEx
Constrained regular expressions for answering RDF-path queries modulo RDFS
alkhateeb2014aInternational audienceThe standard SPARQL query language is currently defined for querying RDF graphs without RDFS semantics. Several extensions of SPARQL to RDFS semantics have been proposed. In this paper, we discuss extensions of SPARQL that use regular expressions to navigate RDF graphs and may be used to answer queries considering RDFS semantics. In particular, we present and compare nSPARQL and our proposal CPSPARQL. We show that CPSPARQL is expressive enough to answer full SPARQL queries modulo RDFS. Finally, we compare the expressiveness and complexity of both nSPARQL and the corresponding frag- ment of CPSPARQL, that we call cpSPARQL. We show that both languages have the same complexity through cpSPARQL, being a proper extension of SPARQL graph patterns, is more expressive than nSPARQL
A Trichotomy for Regular Simple Path Queries on Graphs
Regular path queries (RPQs) select nodes connected by some path in a graph.
The edge labels of such a path have to form a word that matches a given regular
expression. We investigate the evaluation of RPQs with an additional constraint
that prevents multiple traversals of the same nodes. Those regular simple path
queries (RSPQs) find several applications in practice, yet they quickly become
intractable, even for basic languages such as (aa)* or a*ba*.
In this paper, we establish a comprehensive classification of regular
languages with respect to the complexity of the corresponding regular simple
path query problem. More precisely, we identify the fragment that is maximal in
the following sense: regular simple path queries can be evaluated in polynomial
time for every regular language L that belongs to this fragment and evaluation
is NP-complete for languages outside this fragment. We thus fully characterize
the frontier between tractability and intractability for RSPQs, and we refine
our results to show the following trichotomy: Evaluations of RSPQs is either
AC0, NL-complete or NP-complete in data complexity, depending on the regular
language L. The fragment identified also admits a simple characterization in
terms of regular expressions.
Finally, we also discuss the complexity of the following decision problem:
decide, given a language L, whether finding a regular simple path for L is
tractable. We consider several alternative representations of L: DFAs, NFAs or
regular expressions, and prove that this problem is NL-complete for the first
representation and PSPACE-complete for the other two. As a conclusion we extend
our results from edge-labeled graphs to vertex-labeled graphs and vertex-edge
labeled graphs.Comment: 15 pages, conference submissio
Semantics and Validation of Shapes Schemas for RDF
We present a formal semantics and proof of soundness for shapes schemas, an
expressive schema language for RDF graphs that is the foundation of Shape
Expressions Language 2.0. It can be used to describe the vocabulary and the
structure of an RDF graph, and to constrain the admissible properties and
values for nodes in that graph. The language defines a typing mechanism called
shapes against which nodes of the graph can be checked. It includes an
algebraic grouping operator, a choice operator and cardinality constraints for
the number of allowed occurrences of a property. Shapes can be combined using
Boolean operators, and can use possibly recursive references to other shapes.
We describe the syntax of the language and define its semantics. The
semantics is proven to be well-defined for schemas that satisfy a reasonable
syntactic restriction, namely stratified use of negation and recursion. We
present two algorithms for the validation of an RDF graph against a shapes
schema. The first algorithm is a direct implementation of the semantics,
whereas the second is a non-trivial improvement. We also briefly give
implementation guidelines
Leveraging Semantic Web Technologies for Managing Resources in a Multi-Domain Infrastructure-as-a-Service Environment
This paper reports on experience with using semantically-enabled network
resource models to construct an operational multi-domain networked
infrastructure-as-a-service (NIaaS) testbed called ExoGENI, recently funded
through NSF's GENI project. A defining property of NIaaS is the deep
integration of network provisioning functions alongside the more common storage
and computation provisioning functions. Resource provider topologies and user
requests can be described using network resource models with common base
classes for fundamental cyber-resources (links, nodes, interfaces) specialized
via virtualization and adaptations between networking layers to specific
technologies.
This problem space gives rise to a number of application areas where semantic
web technologies become highly useful - common information models and resource
class hierarchies simplify resource descriptions from multiple providers,
pathfinding and topology embedding algorithms rely on query abstractions as
building blocks.
The paper describes how the semantic resource description models enable
ExoGENI to autonomously instantiate on-demand virtual topologies of virtual
machines provisioned from cloud providers and are linked by on-demand virtual
connections acquired from multiple autonomous network providers to serve a
variety of applications ranging from distributed system experiments to
high-performance computing
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