=Paper=
{{Paper
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==paper==
https://ceur-ws.org/Vol-169/paper1.pdf
Towards an Ontology-enabled Service-Oriented
Architecture
Maksym Korotkiy
Vrije Universiteit Amsterdam, Department of Computer Science
De Boelelaan 1081a, 1081 HV Amsterdam, The Netherlands
maksym@cs.vu.nl
Abstract. In the field of Semantic Web Services (SWS), the OWL-S and WSMF
approaches provide us with ontology-based frameworks for WSDL web services.
These description frameworks enable automation of high-level tasks such as dis-
covery, invocation and composition of web services. In our work we focus on the
software architectural aspects of ontology-enabled services to define an Ontology-
enabled Service-Oriented Architectural style (Onto-SOA). The proposed style
provides for a general integration mechanism for ontologies and SOA in a manner
consistent with the SOA constraints. Onto-SOA is independent from an ontology
language and a particular web service technology. Therefore, it can be easily
combined with other styles to devise software architectures for existing SWS
frameworks as well as novel approaches to integration of ontologies and SOA.
1 Introduction
The Service-Oriented Architectural style (SOA) is gaining momentum. Gartner predicts
that SOA will become a prevailing software engineering practice in coming years1 . SOA
attracts industry attention by providing for a flexible and cost effective way to re-use
functionality captured in loosely coupled, business-aligned services. WSDL [1] web
services is the most well-known, however, not the only possible, implementation of
SOA.
The Semantic Web [2] is a vision of the next generation of the Web that makes re-
sources published on the Web “understandable” by machines, thus improving usability
of these resources. One of the main means in achieving the vision is ontologies, the
responsibility of which is to be a carrier of semantics shared among agents.
The significant potential of the combination of web services and the Semantic Web
has been recognized by the Semantic Web Services research area, with OWL-S [3] and
WSMF [4] approaches being the two most well-known representatives. Both OWL-S
and WSMF are aimed at providing an extensive ontology-based description framework
for WSDL web services. In our work we analyze the combination of ontologies and
web services from the viewpoint of software architectures [5] and one of our goals
is to define an Ontology-enabled Service-Oriented Architectural style (Onto-SOA) to
provide for a general, technology- and ontology (language) independent framework for
integration of ontologies and service-oriented architectures.
1
http://www.gartner.com/resources/114300/114358/114358.pdf
2 Maksym Korotkiy
On the more practical side, our work is motivated by the problem of applying on-
tologies in design of applications for the e-Science domain. In the e-Science domain a
large variety of knowledge intensive computational resources exists: experiment design
and execution environments, model simulation and statistical analysis services. On-
tologies have a great potential to improve (re-)usability of these resources. In order to
realize this potential, end-user applications have to be developed. However, at present,
an application developer faces both complexity of the state-of-the-art approaches and
very limited guidance available on how these approaches can be applied to development
of ontology-enabled software. We address this problem by further operationalizing the
proposed Onto-SOA style into a pragmatic framework ready to be applied in ontology-
enabled applications.
The rest of the paper is devoted to a brief introduction to SOA (Sec. 2) followed
by the analysis of relations between SOA, web services (Sec. 3) and Semantic Web
Services (Sec. 3). During this analysis we outline a gap between SOA and current ap-
proaches to ontology-enabled web services. We address the gap by defining Onto-SOA
(Sec. 4). Finally, we conclude with a summary and directions for future work.
2 Service-Oriented Architectural Style (SOA)
An architectural style is “a coordinated set of constraints on architectural elements
and relationships among those elements within any architecture that conforms to that
style” [6]. The general architectural elements are processing components, connectors
and data. Processing components can transform data elements. Connectors provide for
an abstract mechanism that mediates communication, coordination or cooperation be-
tween components [7]. From the processing component perspective, connectors transfer
data without modifying it. However, internally a connector can contain a complex sub-
system that can subject the data to a number of intermediate transformations.
SOA is a style that is constrained to induce a number of desirable characteristics
on a compliant architecture. In the literature, SOA is often characterized as a style that
supports loosely coupled, business-aligned, networked services to enable flexibility and
interoperability, in a technology-independent manner.
Services in SOA represent coarsely-grained expertise from an application (business)
domain. A service makes this problem-solving expertise available to a consumer. SOA
achieves loose coupling between its processing components (a service consumer and
a service) by requiring that connectors are simple, generic and application indepen-
dent, and that descriptive messages constrained by an extensible schema are delivered
through these connectors. A connector is application independent and generic if it does
not introduce real dependency and minimizes artificial dependency between a service
and consumer.
Since the connectors are generic, all application-specific semantics must be ex-
pressed in descriptive messages that communicate a problem description from a service
consumer to a service provider. The messages specify what is to be solved but not how
it should be solved. The reason for this is that a service provider possesses problem-
solving expertise that is missing in the service consumer.
Towards an Ontology-enabled Service-Oriented Architecture 3
In order to enable “understanding” between communicating components, messages
must have unified syntax and structure and must be expressed in a vocabulary shared
by communicating components. The vocabulary is defined in a schema, extensibility of
which is very important due to an open range of possible application domains.
In SOA, loose coupling is achieved by enforcing application independent connector
elements. All application-specific semantics is to be contained in data elements (mes-
sages). A service can be uniquely characterized by a set of messages it can properly
interpret. These messages are constrained by a certain schema that can be seen as an
abstract definition of a service. From a service consumer point of view, a service is
known via this abstract definition only. Since a service represents problem-solving ex-
pertise in an application domain, the schema effectively plays a role of a specification
of this expertise. Such view on a schema in SOA allows us to make a natural transition
from a schema to an ontology and treat an ontology as an integral component of SOA.
3 Web Services
In the literature the term web services most often refers to WSDL/SOAP-based services.
However, in our work we employ it in more general sense to refer to a derivation of
SOA that employs standard Web transport protocols (HTTP, for example) and XML
language to express messages. The main reason for this is that WSDL/SOAP is not the
only possible way to apply SOA on the Web.
WSDL/SOAP Web Services presently are the most popular application of SOA on the
Web. WSDL provides a description framework for web services and is aimed at service
invocation primarily. SOAP [1] provides for a standard way to structure messages that
can be carried over a variety of transport protocols with HTTP being most frequently
used one.
In the very beginning SOAP supported the RPC (Remote Procedure Call) commu-
nication style only2 and later on the document-based flavor of SOAP was introduced.
The document-based SOAP messages are capable of carrying any XML-based content
and are not restricted to the RPC communication style only.
From the SOA perspective, RPC introduces an additional artificial dependency be-
tween a consumer and a service and, therefore, hinders loose coupling between these
components. Furthermore, the RPC style messages tend to be prescriptive rather than
descriptive: with the RPC-message a consumer prescribes a service how to solve a prob-
lem rather than describing what is to be solved. In RPC web services, message semantics
often becomes workflow-dependent and that hinders integration of services. With SOAP
1.2 the RPC style has become optional, thus, enabling us to avoid it in WSDL/SOAP
web services. However, RPC still attracts most of the attention in the research commu-
nity.
Document-based WSDL/SOAP web services are fully compatible with SOA. Nor-
mally they rely on the XML Schema language to describe a structure of an XML doc-
ument. However, XML schemas do not allow us to capture semantics of message ele-
2
http://www.xml.com/pub/a/ws/2001/04/04/soap.html
4 Maksym Korotkiy
ments. WSDL extension elements allow us to use different schema languages capable
of capturing semantics, however, there is little guidance available on that.
WSDL/SOAP web services introduce a significant amount of conceptual and archi-
tectural elements [8]. A significant part of it is devoted to the RPC communication style
that is to be avoided in SOA. Moreover, in many cases a document-based invocation that
is performed via the well-established communication interface (HTTP protocol and the
request-response message exchange pattern) is sufficient to define an operational SOA.
Such a SOA can be both simpler and sufficient for many practical applications.
Representational State Transfer (REST) Web Services are based on the REST ar-
chitectural style [6] that is designed around concepts such as Resource and URI. The
interface among agents in REST is limited to the HTTP protocol that provides for both
the transport layer and actions applicable to resources. REST web services normally
employ XML to express messages and XML Schema as a vocabulary definition mech-
anism. However, since the infrastructure underlying REST web services is simple, it
becomes rather straightforward to employ an ontology-based schema definition mech-
anism in REST web services. Another important feature of REST is that this style has
been designed to fulfill requirements of the Web [9]. REST web services, therefore, gain
many architectural properties of the Web which have already proven to be successful.
WSDL/SOAP web services require a significant infrastructure and have a high
adoption barrier. On the other hand, REST web services require little infrastructure in
addition to what is already provided by the Web, are fully compatible with the SOA con-
straints, can be easily made to employ an ontology-based schema definition languages
and sufficient for many applications.
Despite the fact that REST web services are not the standard web services and less
widely publicized, they are known in some cases to be preferred over WSDL/SOAP
web services. For example, Amazon provides interfaces for both WSDL/SOAP and
REST web services, and 85% of the usage is on the REST interface3 .
Semantic Web Services The state-of-the-art approaches to Semantic Web Services [10]
such as OWL-S and WSMF employ ontologies to provide semantically rich descriptions
of WSDL/SOAP services. Such descriptions provide us with meta-data about a service
and can be applied to automate web service-related tasks such as discovery, invocation,
composition etc. The fact that a number of related but independent tasks are addressed
within a single framework makes its application cumbersome if only some of these
tasks are important for a given SOA.
From the SOA perspective, SWS define a relatively complex semantic connector
element responsible for establishing a connection between a service and its consumer
in a way that insures semantical compatibility of messages. The connector element
can perform complex operations, however, the service still operates on semantics-free
messages (SOAP RPC in most cases). This makes us to believe that the state-of-the-
art approaches to SWS do not address direct exchange of semantically rich messages
between processing components in SOA. For example, if there is a web service designed
3
http://www.oreillynet.com/pub/wlg/3005
Towards an Ontology-enabled Service-Oriented Architecture 5
to operate over RDF [11] messages then the-state-of-the art SWS approaches are of
little use here, and if the service is not a WSDL/SOAP-based one then there is even less
possibilities to apply the existing SWS approaches.
4 Onto-SOA: an Ontology-enabled SOA
We propose Onto-SOA – SOA that assumes a direct exchange of semantically rich
messages between processing components. Onto-SOA is general meaning that it can
be combined with any SOA-compatible architecture (WSDL/SOAP or REST) and with
any ontology language. The style can be gradually refined into derivative styles that will
support additional service-related activities.
We derive the Onto-SOA style from SOA by introducing an additional constraint on
architectural data elements (messages): An ontology language must be used to express a
schema underlying messages. This constraint explicitly addresses the semantical inter-
operability of messages. In Onto-SOA we assume that both a service and its consumer
are aware about an ontology underlying messages.
Implementing an Onto-SOA with an Ontology Language To check whether the
newly-derived Onto-SOA is compatible with an underlying SOA, we have to validate
the introduced constraint against the general SOA requirement on schema extensibility.
In most of the modern applications of SOA, XML and XML Schema provide a
unified syntax and vocabulary definition mechanism to messages. However, the XML
Schema language addresses structural aspects only, leaving semantics of a defined vo-
cabulary implicit. This leaves a problem of semantical interoperability among process-
ing components unsolved but, on the other hand, does not restrict the flexibility in defin-
ing domain-specific vocabularies.
An ontology can provide a domain vocabulary, semantics of which is precisely de-
fined in terms of ontological primitives of the underlying language. The ontology lan-
guages such as RDFS [11] and OWL [11] have fixed semantics and do not allow a user
to extend it. The user is limited to model an application domain in terms provided by an
ontology language. This means that the more restrictive an ontology language is – the
less flexible an Onto-SOA based on this language becomes and, therefore, the more it
diverges from the general requirements to SOA.
This allows us to conclude that in order to achieve the level of flexibility required by
SOA, semantics of an ontology language must be extensible. However, at present, such
languages are not available and we cannot foresee whether they will appear in future.
If an extensible ontology language is unavailable then the least restrictive one (such as
RDFS, for example) might provide a fair approximation.
In order to operationalize the introduced Onto-SOA, we have devised MoRe – a
derivation of Onto-SOA (and a corresponding implementation) that combines the RDF/S
languages with the elements of REST web services. MoRe has been applied to provide
a number of services for the domain of units of measure captured in the UnitDim4
ontology. The services are employed within the Unit Converter tool [12].
4
Rijgersberg, H., Top, J.: UnitDim: an ontology of physical units and quantities.
http://www.atoapps.nl/foodinformatics. Sec. News (2004)
6 Maksym Korotkiy
5 Conclusions and Future Work
We propose Onto-SOA – an Ontology-enabled Service-Oriented Architectural style that
addresses a direct exchange of semantically-rich (ontology-based) messages between a
service and its consumer. Onto-SOA is independent from a particular web service tech-
nology (WSDL/SOAP, for example) and, therefore, can be employed within any SOA-
compatible architecture. Onto-SOA makes no assumptions about an ontology language
employed as a schema definition mechanism. This enables us to combine Onto-SOA
with any ontology languages.
In our future work we intend to further analyze the applicability of the modern
ontology languages such as RDF/S and OWL within the proposed Onto-SOA style.
Further applications the MoRe framework – an operational RDFS/HTTP derivation of
Onto-SOA – within the e-Science domain will allow us to validate the main ideas behind
Onto-SOA and to provide additional guidance to software and ontology developers in
designing an ontology- and service-enabled architectures.
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