ENHANCED DISCOVERY OF WEB SERVICES
Using Semantic Context Descriptions
Simone A. Ludwig
School of Computer Science, Cardiff University, Cardiff, UK
S.M.S. Reyhani
Department of Information Systems and Computing, Brunel University, Uxbridge, UK
Keywords: Context information, Semantics, Ontology, Web Service Discovery
Abstract: Automatic discovery of services is a crucial task for the e-Science and e-Business communities. Finding a
suitable way to address this issue has become one of the key points to convert the Web into a distributed
source of computation, as they enable the location of distributed services to perform a required
functionality. To provide such an automatic location, the discovery process should be based on a semantic
match between a declarative description of a service being sought and a description being offered. This
problem requires not only an algorithm to match these descriptions, but also a language to declaratively
express the capabilities of services. This paper presents a context-aware ontology selection framework
which allows an increase in precision of the retrieved results by taking contextual information into account.
1 INTRODUCTION
Recently, more and more organisations are
implementing IT systems across different
departments. The challenge is to find a solution that
is extensible, flexible and fits well with existing
legacy systems. Replacing legacy systems to cope
with the new architecture is not only costly but also
introduces a risk to fail. In this context, the
traditional software architectures prove ineffective in
providing the right level of cost effective and
extensible Information systems across the
organisation boundaries. Service Oriented
Architecture (SOA) (McGovern, 2003) provides a
relatively cheap and more cost-effective solution
addressing these problems and challenges.
Dynamic discovery is an important component
of SOA. At a high level, SOA is composed of three
core components: service providers, service
consumers and the directory service. The directory
service is an intermediary between providers and
consumers. Providers register with the directory
service and consumers query the directory service to
find service providers. Most directory services
typically organise services based on criteria and
categorise them. Consumers can then use the
directory services' search capabilities to find
providers. Embedding a directory service within
SOA accomplishes the following:
• Scalability of services
• Decoupling consumers from providers
• Allowing updates of services
• Providing a look-up service for consumers
• Allowing consumers to choose between
providers at runtime rather than hard-coding
a single provider.
Although the concepts behind SOA were
established long before web services came along,
web services play a major role in SOA. This is
because web services are built on top of well-known
and platform-independent protocols (HTTP
(Hypertext Transfer Protocol) (HTTP, 2004), XML
(Extensible Markup Language) (XML, 2004), UDDI
(Universal Description, Discovery and Integration)
(UDDI, 2000), WSDL (Web Service Description
Language) (WSDL, 2004) and SOAP (Simple
Object Access Protocol) (SOAP, 2004)). It is the
combination of these protocols that make web
services so attractive. Moreover, it is these protocols
that fulfil the key requirements of a SOA. That is, a
SOA requires that a service be dynamically
193
A. Ludwig S. and Reyhani S. (2005).
ENHANCED DISCOVERY OF WEB SERVICES - Using Semantic Context Descriptions.
In Proceedings of the Second International Conference on e-Business and Telecommunication Networks, pages 193-196
DOI: 10.5220/0001407501930196
Copyright
c
 SciTePress

discoverable and invokeable. This requirement is
fulfilled by UDDI, WSDL and SOAP.
However, SOA in its current form only performs
service discovery based on particular keyword
queries from the user. This, in majority of the cases
leads to low recall and low precision of the retrieved
services. The reason might be that the query
keywords are semantically similar but syntactically
different from the terms in service descriptions.
Another reason is that the query keywords might be
syntactically equivalent but semantically different
from the terms in the service description. Another
problem with keyword-based service discovery
approaches is that they cannot completely capture
the semantics of a user’s query because they do not
consider the relations between the keywords. One
possible solution for this problem is to use ontology-
based retrieval.
In this paper, ontologies are used for
classification of services based on their properties.
This enables retrieval based on service types rather
than keywords. This approach uses context
information to discover services using context and
services descriptions defined in ontologies.
2 FRAMEWORK
Related work has shown the need for more
expressiveness of service descriptions revealing the
limitation of a syntactic approach to service
discovery. To follow this direction proposed by
related work towards a semantic based approach for
service discovery the context-aware ontology
selection framework is proposed. Additional
requirements have driven this framework towards a
context-aware ontology selection framework
described and are summarised as follows:
1. High Degree of Flexibility and
Expressiveness
2. Support for Subsumption
3. Support for Data Types
4. Matching Process should be Efficient
5. Flexible and Modular Structure
6. Lookup of Matched Services
The architecture shown in Figure 1 comprises of
clients, matchmaker, context and service ontologies,
registries, and web servers hosting the web services.
The components are now explained in more
detail:
• Clients provide an interface for the users to
describe their service requests. The client also
lists the matches and provides the facility to
call the web services retrieved.
• Registries contain the service information.
Service descriptions are in the form of service
name, service attributes (inputs and outputs)
and service description.
• Web Servers host the web services.
• Matchmaker consists of the matching module
including the matching algorithm and a
reasoner for the ontology matching process.
The matching algorithm is explained in
further detail in the following section.
• Ontologies (context and services) describe
the domain knowledge such as book shop
services and provide a shared understanding
of the concepts used to describe services.
Contextual information is crucial to ensure a
high quality service discovery process
(Gruber, 1992).
Figure 1. Matching Architecture.
The interactions of a service request are the
following: The user contacts the matchmaker where
the matching algorithm is stored. The matchmaker
contacts the context ontology and reasons depending
on a set of rules defined. The same is carried out for
the services ontology. Having additional match
values the registry is then queried to retrieve
services descriptions which match the request and
returns the service details to the user via the
matchmaker. The parameters stored in the registry
are service name, service attributes, service
description and contact details. Having the URL of
the service the user can then call the web service and
interact with it.
The matching algorithm reads the service request
parameters (context attributes and service attributes)
from the client first. Then the context ontology is
parsed and rules are applied to match the context
keyword by providing the context attributes. Having
the context keyword and the service attributes allows
to query the services ontology which in turn returns
the service matches. This list is then forwarded to
the registry module where the lookup is performed
retrieving the necessary contact details for each
service.
ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES
194

3 APPLICATION SCENARIO
An application scenario was chosen to
demonstrate the usability of the approach. It is
assumed that many e-shopping web services are
available on the Web. These can be any kind of
services e.g. Amazon, eBay, etc., wrapped as web
services offering different goods to buy such as
Books, Bikes and CDs. It is furthermore assumed
that in most cases a client searches for a service not
knowing the service name. The user only specifies a
service request with a few keywords describing the
service needs. For this scenario a context ontology
was created supplying the categories of services for
e-shopping. The context ontology contains
categories representing Food, Clothes, Bikes, Cars,
Shoes, Books and CDs. The underlying classes
contain many associative relations to each of the
categories. Each of the classes belonging to one of
the categories contains attributes describing the class
further. E.g. class Business (belonging to context
Books) contains the attributes computer, reading,
etc. For a special application domain two identical
attributes in more than one class could be
eliminated. However, if context ontologies would be
reused from other sources this ambiguity can not be
disqualified. The prototype implementation solves
this problem by taking the additional context
parameters into account to eliminate the “wrong”
context. If the user only specifies one context
parameter which matches two categories then the
prototype returns a mismatch statement.
The context ontology (Context Ontology, 2005)
is written in OWL (W3C Working Draft, 2004)
description containing class and subclass
relationships. The structure of the e-shopping
services ontology is the following: The first level
contains the corresponding categories of the context
ontology. The second level represents the actual
service implementation with the attributes below.
For example, one service specification outlines the
Books web service. Different service
implementations are BookBuy, Bookshop, BuyBooks,
Books and BookSale.
In the services ontology (Services Ontology,
2005) not only class and subclass relationships are
declared but also data type property relationships
describing the attributes of the service.
In order to demonstrate how the process from
service request to service response works is shown
next. The user issues a service request consisting of
context and service attributes. The context attributes
(e.g. computer and reading) are taken first and the
context ontology is queried using these search
attributes resulting in the context keyword Books
which is used for the service search part. The
services ontology is then reasoned using the context
keyword and the service attributes specified in the
service request query. The retrieved services are
BookBuy, Bookshop, BuyBooks, Books and
BookSale. After these services are matched the
service details are retrieved from the registry and
returned to the user.
4 EVALUATION
The evaluation is done by calculating precision
and recall rates. Precision is the fraction of
advertised services which are relevant, i.e. the
highest number is returned when only relevant
services are retrieved. Recall is the fraction of
relevant services which have been retrieved, i.e. the
highest number is returned when all relevant
services are retrieved.
For the evaluation of precision and recall values
a comparison of a keyword-based approach with the
prototype approach was conducted. The focus for
this evaluation was on book services.
Table 1: Relevant Services.
service1 service2 service3 service4 service5
context
attributes
computer
reading
title heading name writing title
author writer authors maker composer
number issue no product id
category class family concept category
price cost amount worth value
publisher owner proprietor publisher owner
service
attributes
pages page
number
page pages pages
Table 1 shows the relevant services. All
attributes shown in the table are the service attribute
parameters used for this evaluation. Matches are
indicated in bold.
Table 2: Irrelevant Services.
service6 service7 service8 service9 service10
context
attributes
graph
picture
title issue name product composer
number owner proprietor pages id
price
isbn issn book
value
pages
drink shop meal
pages
book pixel colour point book
shop font paragraph space food
service
attributes
colour space bold font colour
Table 2 shows the irrelevant services. The
attributes indicated in bold match with the extended
context ontology taken for this experiment; however
the context parameters do not match the Book
category. The number of service attributes is the
same for relevant and irrelevant services.
ENHANCED DISCOVERY OF WEB SERVICES - Using Semantic Context Descriptions
195

The context parameters define the category of
the service which results in the two tables (Table 1
and 2) being relevant services and irrelevant
services. The user wants to find Book shop services
and specifies a service request 1 (context
parameters: computer, reading; service parameters:
title, author, number, category, price, publisher,
pages) with the parameters specified for service 1 in
Table 1. Service request 2 is specified with the
parameters of service 2 (Table 1) and so on. The
context parameters of the service request are always
computer and reading.
Figure 2 shows the results of the precision and
recall values. The precision and recall results of the
keyword-based approach range between 20% and
70%, whereby the prototype approach achieved a
precision and retrieval rate of 100% in this
experimental setup. As the recall and precision rates
from the prototype show higher values than the rates
from the keyword-based approach, it shows that the
user receives a better subset of services that are
relevant and in addition, the user receives no
services that are irrelevant.
Figure 2: Evaluation of Precision and Recall Values.
Due to the fact that this research is conducted in
a limited application domain, the set of advertised
services, query and ontology are highly adapted and
therefore a result of 100% is retrieved. In a real-
world application scenario this correlation might not
always be that high, especially if a context ontology
from third-parties is used.
The accomplished result of service matches does
not state that in every application scenario always
values of 100% are achieved but it indicates the
improvement in quality of service discovery results
by using this semantic approach. Precision and recall
measures showed the increase of quality of service
matches, which was achieved by the customisation
of the context and services ontologies.
5 CONCLUSION
The use of contextual information results in a
better service discovery process due to an increased
precision of the matched services. The contextual
information enhances the expressiveness of the
matching process, i.e. by adding semantic
information to services, and also serves as an
implicit input to a service that is not explicitly
provided by the user. The prototype approach
facilitates interoperability as the context and service
properties are defined and specified in associated
ontologies. Re-writing of code or interface wrapping
does not need to be done in order to make systems
interoperable. The development and maintenance is
much easier due to the modular structure and
encapsulation of context matching, service matching
and registry selection. A drawback of this approach
is that users registering services need to know the
category their services belong to. Cases where a
service falls into more than one category need to be
registricted in order to allow an automatic and
precise discovery and selection of service matches.
REFERENCES
McGovern, J., Tyagi, S., Stevens, M., Mathew, S., 2003.
The Java Series Books - Java Web Services
Architecture. Chapter 2, Service Oriented
Architecture.
HTTP - Hypertext Transfer Protocol, 2004. W3C,
http://www.w3.org/Protocols/
.
Extensible Markup Language (XML), 2004. W3C.
http://www.w3.org/XML/.
UDDI Technical White Paper, 2000.
http://www.uddi.org/pubs/Iru_UDDI_Technical_Wh
ite_Paper.pdf.
Web Services Description Language (WSDL), 2004.
Version 1.1, W3C. http://www.w3.org/TR/wsdl
.
SOAP Version 1.2, 2004. W3C.
http://www.w3.org/TR/soap/.
Gruber, T.R., 1992. ONTOLINGUA: A Mechanism to
Support Portable Ontologies, Version 3.0, Technical
Report KSL 91-66, Department of Computer
Science, Stanford University.
W3C Working Draft, 2004. “Requirements for a Web
Ontology Language”.
http://www.w3.org/TR/webont-req/
.
Context Ontology, 2005.
http://users.cs.cf.ac.uk/Simone.Ludwig/ontologies/w
ebservices/ContextOntology.owl.
Services Ontology, 2005.
http://users.cs.cf.ac.uk/Simone.Ludwig/ontologies/w
ebservices/ServicesOntology.owl.
ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES
196