=Paper=
{{Paper
|id=None
|storemode=property
|title=Towards Service-Oriented Resource Discovery by means of Semantic Web Reasoning
|pdfUrl=https://ceur-ws.org/Vol-862/REDp2.pdf
|volume=Vol-862
}}
==Towards Service-Oriented Resource Discovery by means of Semantic Web Reasoning==
https://ceur-ws.org/Vol-862/REDp2.pdf
Towards Service-Oriented Resource Discovery by means
of Semantic Web Reasoning
Alexey Cheptsov
High Performance Computing Center Stuttgart (HLRS), University of Stuttgart, Nobelstrasse
19, 70569 Stuttgart, Germany
cheptsov@hlrs.de
Abstract. Reasoning is one of the essential tools of the modern Semantic Web.
A number of applications for resource discovery on the Web such as random
indexing enjoy a prominent place in face of the novel Semantic Web Reasoning
trends. However, the reasoning algorithms are dealing with significant chal-
lenges when scaled up to the problem sizes addressed by the modern Semantic
Web application. As such, they are not well-optimized to be applied to the
emerging Internet-scale knowledge bases. We introduce a solution to building
highly efficient and scalable reasoning applications based on the Large Knowl-
edge Collider – a service-oriented incomplete reasoning platform breaking the
scalability barriers of the existing solutions. We discuss the application of in-
complete reasoning for the resource discovery tasks and demonstrate a service-
oriented realization for the query expansion and subsetting algorithms based on
the random indexing knowledge extraction technique.
Keywords: Random Indexing, Semantic Web Reasoning, Large Knowledge
Collider.
1 Introduction
The large- and internet-scale data applications is a primary challenge for the Semantic
Web, and in particular for reasoning algorithms, used for processing exploding vol-
umes of data, exposed currently on the Web. Reasoning is the process of making im-
plicit logical inferences from the explicit set of facts or statements, which constitute
the core of any knowledge base. The key problem for most of the modern reasoning
engines such as Jena [1] or Pellet [2] is that they can not efficiently be applied for the
real-life data sets that consist of tens, sometimes of hundreds of billions of triples (a
unit of the semantically annotated information), which can correspond to several
petabytes of digital information. Whereas modern advances in the Supercomputing
domain allow this limitation to be overcome, the reasoning algorithms and logic need
to be adapted to the demands of rapidly growing data universe, in order to be able to
take advantages of the large-scale and on-demand infrastructures such as high per-
formance computing or cloud technology. On the other hand, the algorithmic princi-
67
pals of the reasoning engines need to be reconsidered as well in order to allow for
very large volumes of data. Service-oriented architectures (SOA) can greatly contrib-
ute to this goal, acting as the main enabler of the newly proposed reasoning tech-
niques such as incomplete reasoning [3]. This paper focuses on a service-oriented
solution for constructing Semantic Web applications of a new generation, ensuring
the drastic increase of the scalability for the existing reasoning applications, as elabo-
rated by the Large Knowledge Collider (LarKC)1 EU project.
The paper is organized as follows. In Section 2, we collect our consideration to-
wards enabling the large-scale reasoning and its application for the resource discovery
tasks. In Section 3, we discuss LarKC – a service-oriented platform for development
of fundamentally new reasoning application, with much higher scalability barriers as
by the existing solutions. In Section 4, we introduce some successful resource discov-
ery applications implemented with LarKC, such as Random Indexing. In Section 5,
we discuss our conclusions and highlight the directions for future work in highly scal-
able semantic reasoning.
2 Semantic Reasoning on the Web Scale
Despite the majority of data on the Web is available as an unstructured text, e.g. gen-
erated from the content kept in RDBM, the application areas of the modern Semantic
Web spawn a wide range of domains, from social networks to large-scale Smart Cities
projects in the context of the future internet [4][5]. However, data processing in such
applications goes far beyond a simple maintenance of the collection of facts; based on
the explicit information, collected in datasets, and simple rule sets, describing the
possible relations, the implicit statements and facts can be acquired from those data-
sets.
Many data collections as well as application built on top of them allow for rule-
based inferencing to obtain new, more important facts. The process of inferring logi-
cal consequences from a set of asserted facts, specified by using some kinds of logic
description languages (e.g., RDF/RDFS and OWL2), is in focus of semantic reason-
ing. The goal is to provide a technical way to determine when inference processes is
valid, i.e., when it preserves truth. This is achieved by the procedure which starts
from a set of assertions that are regarded as true in a semantic model and derives
whether a new model contains provably true assertions.
The latest research on the Internet-scale Knowledge Base Technologies, combined
with the proliferation of SOA infrastructures and cloud computing, has created a new
wave of data-intensive computing applications, and posed several challenges to the
Semantic Web community. As a reaction on these challenges, a variety of reasoning
methods have been suggested for the efficient processing and exploitation of the se-
mantically annotated data. However, most of those methods have only been approved
for small, closed, trustworthy, consistent, coherent and static domains, such as syn-
thetic LUBM [6] sets. Still, there is a deep mismatch between the requirements on the
1
http://www.larkc.eu/
2
http://www.w3.org/TR/owl-ref/
68
real-time reasoning on the Web scale and the existing efficient reasoning algorithms
over the restricted subsets.
Whereas unlocking the full value of the scientific data has been seen as a strategic
objective in the majority of ICT- related scientific activities in EU, USA, and Asia
[7], the “Big Data” problem has been recognized as the primary challenger in seman-
tic reasoning [8][9]. Indeed, the recent years have seen a tremendous increase of the
structured data on the Web with scientific, public, and even government sectors in-
volved. According to one of the recent IDC reports [10], the size of the digital data
universe has grown from about 800.000 Terabytes in 2009 to 1.2 Zettabytes in 2010,
i.e. an increase of 62%. Even more tremendous growth should be expected in the
future (up to several tens of Zettabytes already in 2012, according to the same IDC
report [10]).
The “big data” problem makes the conventional data processing techniques, also
including the traditional semantic reasoning, substantially inefficient when applied for
the large-scale data sets. On the other hand, the heterogeneous and streaming nature
of data, e.g. implying structure complexity [11], or dimensionality and size [12],
makes big data intractable on the conventional computing resource [13]. The problem
becomes even worse when data are inconsistent (there is no any semantic model to
interpret) or incoherent (contains some unclassifiable concepts) [14].
The broad availability of data coupled with increasing capabilities and decreasing
costs of both computing and storage facilities has led the semantic reasoning commu-
nity to rethink the approaches for large-scale inferencing [15]. Data-intensive reason-
ing requires a fundamentally different set of principles than the traditional mainstream
Semantic Web offers. Some of the approaches allow for going far beyond the tradi-
tional notion of absolute correctness and completeness in reasoning as assumed by the
standard techniques. An outstanding approach here is interleaving the reasoning and
selection [16]. The main idea of the interleaving approach (see Fig. 1a) is to intro-
duce a selection phase so that the reasoning processing can focus on a limited (but
meaningful) part of the data, i.e. perform incomplete reasoning.
a) b)
Fig. 1. Incomplete reasoning, the overall schema (a) and the service-oriented vision (b)
69
As discussed before, the standard reasoning methods are not valid in the existing
configurations of the Semantic Web. Some approaches, such as incomplete reasoning,
offer a promising vision how a reasoning application can overcome the “big data”
limitation, e.g. by interleaving the selection with the reasoning in a single “work-
flow”, as shown in Fig. 1a. However the need of combining several techniques within
a single application introduces new challenges, for example related to ensuring the
proper collaboration of team of experts working on a concrete part of the workflow,
either it is identification, selection, or reasoning. Another challenge might be the
adoption of the already available solutions and reusing them in the newly developed
applications, as for example applying selection to the JENA reasoner [1], whose
original software design doesn’t allow for such functionality. The SOA approach can
help eliminate many of the drawbacks on the way towards creating new, service-
based reasoning applications. Supposed that each of the construction blocks shown in
Fig. 1a is a service, with standard API that ensures easy interoperability with the other
similar services, quite a complex application can be developed by a simple combina-
tion of those services in a common workflow (see Fig. 1b).
Resource discovery is an essential feature of the Semantic Web, which involves
tasks of decentralized and autonomous control, distributed service discovery etc. Rea-
soning can greatly contribute to solving these issues by for example improving the
fine-grained service matchmaking, resource ranking, etc. in typical resource discovery
workflows [29].
Although utilizing reasoning in the resource discovery workflows is not a new con-
cept for the Semantic Web [17][18], there was quite a big gap in realizing the single
steps of the reasoning algorithms (Fig. 1b) as a service. This was due to many rea-
sons, among them complexity of the data dependency management, ensuring interop-
erability of the services, heterogeneity of the service’s functionality. Realizing a sys-
tem where a massive number of parties can expose and consume services via ad-
vanced Web technology was also a research highlight for Semantic Web. An example
of very successful research on offering a part of the semantic reasoning logic as a
service is the SOA4ALL3 project, whose main goal was to study the service abilities
of development platforms capable of offering semantic services. Several useful ser-
vices wrapping such successful reasoning engines as IRIS [19] and several others had
been developed in the frame of this project. Nevertheless, the availability of such
services is only an intermediate step towards offering reasoning as a service, as a lot
of efforts were required to provide interoperability of those services in the context of
a common application. Among others, a common platform is needed that would al-
low the user to seamlessly integrate the service by annotating their dependencies,
manage the data dependencies intelligently, being able to specify parts of the execu-
tion that should be executed remotely, etc.
An outstanding effort to develop such a platform was performed in the LarKC
(Large Knowledge Collider) [20] project. In the following sections, we discuss the
main ideas, solutions, and outcomes of this project.
3
http://www.soa4all.eu/
70
3 Large Knowledge Collider Approach
In order to create a technology for creation of trend-new applications for large-scale
reasoning, several leading Semantic Web research organizations and technological
companies have joined their efforts around the project of the Large Knowledge Col-
lider (LarKC), supported by the European Commission. The mission of the project
was to set up a distributed reasoning infrastructure for the Semantic Web community,
which should enable application of reasoning far beyond the currently recognized
scalability limitations [22], by implementing the interleaving reasoning approach. The
current and future Web applications that deal with “big data” are in focus of LarKC.
The LarKC’s design has been guided by the primarily goal to build a scalable plat-
form for distributed high performance reasoning. Fig. 2 shows a conceptual view of
the LarKC platform’s architecture and the proposed development life-cycle. The ar-
chitecture was designed to holistically cover the needs of the three main categories of
users – semantic service (plug-in) developers, application (workflow) designers, and
end-users internet-wide. The platform’s design ensures a trade-off between the flexi-
bility and the performance of applications in order to achieve a good balance between
the generality and the usability of the platform by each of the categories of users.
Below we introduce some of the key concepts of the LarKC architecture and dis-
cuss the most important platform’s services and tools for them.
Semantic
SemanticWebWebService
Service LarKC platform
Farm
Farm/ /Plug-in
Plug-inMarketplace
Marketplace
Plug-in Plug-in
developers Deciders Identifiers Selectors Registry Data
Deciders Identifiers Selectors
Layer
(OWLIM)
Transformers
Transformers Reasoners
Reasoners
Plug-in
Managers Execution
Remote
Framework
Invocation
Plug-in … Plug-in Workflow
Framework
Workflow Support
Plug-in (GAT)
System
designers
Monitoring
End-points Service
Application
end-users Infrastructure
High-
RDF data base performance
computer
Storage
Storage Computation
Computation Monitoring
Monitoring
Fig. 2. Architecture of LarKC.
71
1. Plug-ins
Plug-ins are standalone services implementing some specific parts of the reasoning
logic as discussed previously, whether it is selection, identification, transformation, or
reasoning algorithm, see more at [21]. In fact, plug-ins can implement much broader
functionality as foreseen by the incomplete reasoning schema (Fig. 1), hence enabling
the LarKC platform to target much wider Semantic Web user community as originally
targeted, e.g. for machine learning or knowledge extraction. The services are referred
as plug-ins because of their flexibility and ability to be easily integrated, i.e. plugged
into a common workflow and hence constitute a reasoning application. To ensure the
interoperability of the plug-ins in the workflows, each plug-in should implement a
special plug-in API, based on the annotation language [23]. Most essentially, the API
defines the RDF schema (set of statements in the RDF format) taken as input and
produced as output by each of the plug-ins. The plug-in development is facilitated by
a number of special wizards, such as Eclipse IDE wizard or Maven archetype for
rapid plug-in prototyping. The ready-to-use plug-ins are uploaded and published on
the marketplace – a special web-enabled service offering a centralized, web-enabled
repository store for the plug-ins4.
2. Workflows
The workflow designers get access to the Marketplace in order to construct a work-
flow from the available plug-ins, combined to solve a certain task. In terms of LarKC,
workflow is a reasoning application that is constructed of the (previously developed
and uploaded on the Marketplace) plug-ins. The workflow’s topology is characterised
by the plug-ins included in the workflow as well as the data- and control flow connec-
tions between these plug-ins.
The complexity of the workflow’s topology is determined by the number of in-
cluded plug-ins, data connections between the plug-ins (also including multiple splits
and joins such as in Fig. 3a or several end-points such as in Fig. 3b), and control flow
events (such as instantiating, starting, stopping, and terminating single plug-ins or
even workflow branches comprising several plug-ins). Same as for plug-ins, the input
and output of the workflow is presented in RDF, which however can cause compati-
bility issues with the user’s GUI, which are not obviously based on an RDF-compliant
representation. In order to confirm the internal (RDF) dataflow representation with
the external (user-defined) one, the LarKC architecture foresees special end-points,
which are the adapters facilitating the workflow usage in the tools outside of the
LarKC platform. Some typical examples of end-points, already provided by LarKC,
are e.g. SPARQL end-point (SPARQL query as input and set of RDF statements as
output) and HTML end-point (HTTP request without any parameters as input and
HTML page as output).
For the specification of the workflow configuration, a special RDF schema was
elaborated for LarKC, aiming at simplification of the annotation efforts for the work-
4
Visit the LarKC Plug-in Marketplace at http://www.larkc.eu/plug-in-marketplace/
72
flow designers. Fig. 4a shows a simple example of the LarKC workflow annotation.
Creation of the workflow specification can greatly be simplified by using upper-level
graphical tools, e.g. Workflow Designer that offers a GUI for visual workflow con-
struction (Fig. 4b) [28]. The elaborated schema makes specification of the additional
features such as remote plug-in execution extremely simple and transparent for the
users and can be used for tuning the front-end graphical interfaces of the applications
to adapt them to the user needs.
a)
b)
Fig. 3. Examples of LarKC workflows: a) workflow with non-trivial branched dataflow (con-
taining multiple splits/joins), b) workflow with multiple end-points
73
a)
b)
Fig. 4. Further example of LarKC workflows: a) RDF schema for workflow annotation, b)
Workflow Designer GUI with the specification of the remote host
74
3. Applications
Workflows are already standalone applications that can be submitted to the plat-
form and executed by means of such tools as Workflow Designer discussed above.
Nevertheless, workflows can also be wrapped into much more powerful user inter-
faces, adapted to the needs of the targeted end-user communities, e.g. Urban Comput-
ing [24], and using LarKC as a back-end engine. The service-oriented approach
makes possible hiding the complexity of the LarKC platform, by enabling its whole
power to the end-users through such interfaces. We present an exemplarily LarKC
application in Section 4.
4. Platform services
All above-described activities related to plug-in creation, workflow design, and ap-
plication development are facilitated by an extensive set of the platform services, as
shown in Fig. 2. A detailed description of the main LarKC services can be found in
our previous publication [21].
4 Application Scenario – Random Indexing
Random indexing [25] is a distributional statistic technique used in resource discovery
for extracting semantically similar words from the word co-occurrence statistics in the
text data, based on high-dimensional vector spaces (Fig. 5).
Random indexing offers new opportunities for a number of large-scale Web appli-
cations performing the search and reasoning on the Web scale [26]. Prominent appli-
cation using random indexing is subsetting (Fig. 6a) and query expansion (Fig. 6b).
Fig. 5. Schema of the co-occurrence statistical analysis of text corpora.
75
a) b)
Fig. 6. Application of Random Indexing: a) subsetting b) query expansion.
Query expansion [30] is used in information retrieval with the aim to expand the
document collection returned as a result to a query, thus covering the larger portion of
the documents. Subsetting (also known as selection) [31], on the contrary, deprecates
the unnecessary items from a data set in order to achieve faster processing. Both pre-
sented problems are complementary, as change properties of the query to best adapt it
to the search needs.
The main complexity of the random indexing algorithms lies in the following:
• High dimensionality of the underlying vector space.
A typical random indexing search algorithm performs traversal over all the entries of
the vector space. This means, that the size of the vector space to the large extent de-
fines the search performance. The modern data stores, such as Linked Life Data or
Open Phacts consolidate many billion of statements and result in vector spaces of a
very large dimensionality. Random indexing over such large data sets is computation-
ally very costly, with regard to both execution time and memory consumption. The
latter is of especial drawback for use of random indexing packages on the mass com-
puters. So far, only relatively small parts of the Semantic Web data have been indexed
and analyzed.
• High call frequency.
Both indexing and search over the vector space is typically a one-time operation,
which means that the entire process should be repeated from scratch every time new
data is encountered.
The implementation as a LarKC plug-in allows random indexing to take advan-
tages of the LarKC data and execution model, being seamlessly integrated with the
other plug-ins and building up a common workflow. This allows random indexing to
be coupled with reasoners to improve the resource discovery algorithm. On the other
hand, the reasoning process can also benefit from the integration, for example by
using random indexing to expand the initial query and improve the quality of the ob-
tained results, such as shown in Fig. 7.
76
LarKC is the technology that not only enables the large-scale reasoning approach
for the already existing applications, but also facilitates their rapid prototyping with
low initial investments, leveraging the SOA approach through the unique platform
solutions. Furthermore, LarKC delivers a complete eco-system where the researches
from very different domains can team up in order to develop new challenging
mashup-applications, e.g. for the resource discovery, hence having a dramatic impact
on a lot of problem domains.
Query expansion part
Reasoning part
Fig. 7. Realization of query expansion in the Linked Life Data reasoning workflow.
5 Conclusions
We proposed a technology that allows a resource discovery process to be enhanced by
integration with the reasoning. The technology is based on the Large Knowledge Col-
lider (LarKC). LarKC is very promising platform for creation of new-generation se-
mantic reasoning applications. The LarKC’s main value is twofold. On the one hand,
it enables a new approach for large-scale reasoning based on the technique for inter-
leaving the identification, the selection, and the reasoning phases. On the other hand,
through over the project’s life time (2008-2011), LarKC has evolved in an out-
standing, service-oriented platform for creating very flexible but extremely powerful
applications, based on the plug-in’s realization concept. The LarKC plug-in market-
place has already comprised several tens of freely available plug-ins, which imple-
ment new know-how solutions or wrap existing software components to offer their
functionality to a much wider range of applications as even originally envisioned by
their developers. Moreover, LarKC offers several additional features to improve the
77
performance and scalability of the applications, facilitated through the parallelization,
distributed execution, and monitoring platform. LarKC is an open source develop-
ment, which encourages collaborative application development for Semantic Web.
Despite being quite a young solution, LarKC has already established itself as a very
promising technology in the Semantic Web world. Some evidence of its value was a
series of Europe- and world-wide Semantic Web challenges won by the LarKC appli-
cations. It is important to note that the creation of LarKC applications, including the
ones discussed in the paper, was also possible and without LarKC, but would have
required much more (in order of magnitude) development efforts and financial in-
vestments.
We believe that the availability of such platform as LarKC will make a lot of de-
velopers to rethink their current approaches for resource discovery as well as semantic
reasoning towards their tighter coupling and wider adoption of the service-oriented
paradigm.
6 Acknowledgment
This work has been performed for the LarKC project (http://www.larkc.eu), partly
funded by the European Commission's IST activity of the 7th Framework Programme
(ICT-FP7-215535).
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