Journal of Geographic Information System, 2010, 2, 40-44
doi:10.4236/jgis.2010.21008 Published Online January 2010 (http://www.scirp.org/journal/jgis)
A QoS-aware Method for Web Services Discovery
Bian WU, Xincai WU1,2
1Research Center for GIS Software and Application Engineering, Ministry of Education, Wuhan, China
2School of Earth Sciences and Resou rces, China University of Geosciences, Beijing, China
Abstract: The non-functional QoS (quality of service) information helps us to select a proper Web-service
from the web applications, by using component services such as UDDI[1](Univ ersal Description, Discovery,
and Integration) and MDS(Monitoring and Discovery System). MDS is a suite of web services to monitor
and discover resources and service on Grids, but MDS only based on function aspects. This paper studies on
an approach to provide the QoS information and a discovery model by using MDS and gives a system
deployment and implementation plan. The simulation results show that the method is effective in service
discovery.
Keywords: quality of service (QoS), web services, monitoring and discovery system (MDS)
1. Introduction
Web services are frequently used to build a distributed
system which can be accessed over Internet. It is loosely
coupled to enhance productivity, simplify using, get
reusability and improve system expansibility. But with
the widespread proliferation, the QoS becomes an
important factor in distinguishing the success of a web
service application.
QoS is the ability to provide different priority to
different applications and users to guarantee a certain
level of performance to data flow. Typical QoS properties
associates with a web service execution cost and time,
availability, successful execution rate, reputation, and
usage frequency [2]. The QoS covers a range of
non-functional properties of Web-service such as
response time, performance, security, availability and
reliability. The service providers provide differentiated
capacity model for different service types to ensure
appropriate QoS levels for different customers'
applications. For example, a complex spatial analysis
Web service might require good throughput, while a
simple searching Web service might require a good
response performance.
In order to describe the contract between the service
provider, the customer and the Grid system, several
specifications defined by XML based language have
been proposed such as WSDL, which is complement to
the service description implemented, WS-Agreement
specification [3] which is used within a framework
allowing the management of web services and their
compositions.
In this work we propose to modify the WSDL file in
order to increase QoS support to the web service. Then
we propose a method of web services discovery on GT4
MDS.
1.1. Web Service QoS Requirements
The major requirements[4] for supporting QoS in Web
services are as follows:
 Regulatory is the aspect of the Web service in
compliance with the standards such as SOAP, UDDI,
WSDL, etc and the established service level agreement.
Strict adherence to correct versions of standards by
service providers is necessary for proper invocation of
Web services by service consumers.
 Availability is the quality aspect of whether the
Web service is ready for immediate use. It represents the
probability that a service is available. Larger values
represent that the service is always ready to use while
smaller values indicate unpredictability of whether the
service will be available at a particular time.
 Reliability is the quality aspect of a web service
that represents the capability of maintaining the service
and service quality. The number of successfully used per
month represents a measure of reliability of a web
service.
 Performance is the quality aspect of Web service,
which is measured in terms of throughput and latency,
and response time. Throughout represents the number of
Web service requests served at a given time period.
Latency is the round-trip time between sendin g a request
and receiving the response. Higher throughput, lower

B. WU ET AL.
41
latency and less response time values represent good
performance of a GIS Web service.
 Security is th e quality aspect of the web services of
providing confidentiality by authenticating the parties
involved, encrypting messages, and providing access
control. Because distributed GIS system is deployed and
invoked over the public Internet, security becomes more
important. The service providers have different approa-
ches and levels of providing security depending on the
service requester.
2. QoS-Based Web Services Discovery
Method
MDS is the information services component of the Glob-
us Tookit4 and provides information about the available
resources on the Grid and their status [5]. It is a suite of
web services to monitor and discover resources and
services on Grids[6]. MDS builds on query, subscription,
and notification protocols and interfaces defined by the
Web Services Resource Framework (WSRF) and Web
Services Notification families of specifications and
implemented by the GT4 Web Services Core. Discovery
is the process of finding a suitable resource to perform a
task. This process may involve both finding which
resource are suitable and choosing a suitable member
from that set. The discovery process requires the ability
to collect information from multiple, perhaps distributed
information sources. To meet this need, MDS provides
so-called aggregator services that collect recent state
information from registered information sources. The
MDS architecture is shown in Figure 1.
2.1. QoS Properties in WSDL
Web Service Description Language (WSDL) is an XML
format for describing network services as a set of endp-
oints operating on messages containing either document-
oriented or procedure-oriented information[7]. Specifica-
lly, it defines via its portType component, the Web serv-
ice's abstract interface, specifying the operations that the
service supports and for each operation, the format of the
messages that the service sends and receives. The client
programs read the WSDL files to connect and comm-
unicate with the service. The current WSDL file contains
the following information: the number of inputs and
outputs, the variable type of each input and output, the
order the inputs are given and outputs are returned, and
how the Web service should be invoked. But from the
WSDL file, we can not get the QoS information. To
solve this problem we propose to extend the WSDL to
support the QoS information description.
The hierarchical QoS semantics model[8] which is an
extension of WSDL document structur e (shows in Figure.
2), can be used to describe the quality information of
Web Services. Specifically the method for describing the
quality information of GIS Web Services properties is to
increase QoS elements through the extension of WSDL
protocol. Add atQualityInfo property in the WSDL tport
element to describe the five QoS attributes of Web
services, the tport elements are regulatory, response time,
availability, performance, and security level.
Figure 1. MDS architecture

B. WU ET AL.
42
Figure 2. WSDL document structure
The Web services function elements correspond with
tport in the general WSDL documents. So add the
qualityInfo attributes into the tport element of WSDL to
describe the QoS[9]. An example is as follows:
< wsdl: qualityInfo name="approve"
qualityID=
"DAJD0891-28da-48B3-234J-24IA2394C342">
< wsdl: constraintInfo name="regulatory" value="75"
probability="9"
constraintID="D 2JD2 371-4H7 P-15A3- 414f-1 5I323 r4
C342"/>
< wsdl: constraintInfo name="ResponseTime" value=
"27" probability="28"
constraintID="Q EJD 1851- RF36-4 1G6-4 14f-1 5I323 r4
C342"/>
< wsdl: constraintInfo name="availability" va lue="65"
probability="39"
constraintID="D BYT5769- 2T4P-W1A 3-414 f-15I 323r
4C342"/>
< wsdl: constraintInfo name="performance" value=
"23" probability="27"
constraintID="FUJJ3861-1G5P-1EA7-414f-15I323r4
C342"/>
< wsdl: constraintInfo name="security level" value=
"7" probalility="56"
constraintID="D 2JR3431 -Y68L-D2 A3-414 f-15 I323r4
C342"/>
< /wsdl: qualityInfo>
<types>
<xsd:schema targetNamespace="http://localhost/exam-
ples/core/QoS_instance"
xmlns:tns="http://localhost/examples/core/OoS_instan
ce"
xmlns:xsd="http//localhost/2012/XMLSchema">
...
<xsd: element name = "regulatory">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="tns:Value" minOccurs="1" maxO
ccurs="9"/>
</xsd:sequence>
<xsd:complexType>
</xsd:element>
...
</xsd:schema>
</types>
...
<xsd:element name="Value" type="xsd:int"/>
...
2.2. Modifying Query Aggregator Source
Configuration File
Besides editing WSDL documents. another necessary
step is to increase QoS support to the Web Services'
XML schema documents, and through the extension of
the GetMultipleResourcePropertiesPollType which is
one of the parameters for Query Aggregator Source in
configuration file. The QoS properties can be retrieved
from a Index Service as resource properties.
The Aggregator Framework is a software framework
used to build services that collect and aggregate data.

B. WU ET AL.
43
Index Services built on the Aggregator Framework are
sometimes called aggregator services. The Index
Services collect XML-formatted data via Aggregator
Source. The Query Aggregator Source is a java class
that implements an interface to poll a WSRF service for
resource property information. An example is as follows:
<Content
xmlns:agg="http://mds.globus.org/aggregator/types"
xsi:type="agg:AggregatorContent">
<agg:AggregatorConfig
xsi:type="agg:AggregatorConfig">
<agg:GetMultipleResourcePropertiesPollType>
<agg:GridGISServices>Spatial_Operations</agg:GridGI
SServices>
<agg:ResourcePropertyNames>rp1_namespace:regulator
y</agg:ResourcePropertyNames>
<agg:ResourcePropertyNames>rp1_namespace:response
_time</agg:ResourcePropertyNames>
<agg:ResourcePropertyNames>rp1_namespace:availabili
ty</agg:ResourcePropertyNames>
<agg:ResourcePropertyNames>rp2_namespace:performa
nce</agg:ResourcePropertyNames>
<agg:ResourcePropertyNames>rp3_namespace:security_
level</agg:ResourcePropertyNames>
</agg:GetMultipleResourcePropertiesPollType>
</agg:AggregatorConfig>
<agg:AggregatorData/>
</Content>
Then Query Aggregator Source will request five QoS
properties in the Grid services. There is no limit on the
number of “ResourcePropertyName” parameters that can
be specified.
Qos properties provide support for service publishing
and selection, but how to select web services that can
well meet the requirements of Web services' consumers
becomes a problem.
2.3. Web Services Discovery from MDS
The users use the web browser to send a HTTP request,
including some web form arguments, to the web server
container. The web server invokes the MDS servlet, wh-
ich uses the form arguments to determine what plugins to
use to retrieve the request GML data and the XSLT
transform tools to apply to it. Then use the standard
WSRF resource property query interfaces to retrieve
service WSDL information from an Index. Every GT4
web services container includes a default MDS-Index
service with which any GT4 services running in that
container are automatically registered. Thus, each Grid
system has an index that allows one to discover what
services are available. GT4 is configured to use MDS
mechanisms to good effect for discovering and monit-
oring of G T4 services. Ever y GT4 web services supports
a minimal set of QoS properties and thus can be
registered easily into one or more aggregators for
monitoring and discovery. The MDS use servlet passes
arguments to the transaction tool, which then retrieves
the appropriate data and XSLT transform. The MDS
servlet applies the XSLT transformation to the XML data
and returns the result to the web server, which sends it
back to the client's web browser. The discovery model is
shown in Figure 3.
Figure 3. Discovery model

B. WU ET AL.
44
Figure 4. Te sting performance
3. Deployment and Testing
Testing executions wer e pe rformed on a Intel Core 2 Duo
T9300@ 2.5GHz machine, with DDR 666MHz 3.0G of
RAM and Windows XP sp3 operation system as web
server. Java as Developing language, IDE: ecplise5.0,
Apache Tomcat as web services container IE7.0 as web
browser, GD4 as Grid container and Grid system
environment Figure 4 shows the testing results.
4. Conclusion
The work proposed in this paper provides an approach to
describe Web services QoS information in WSDL and a
discovery model in which the functional and non-
functional requirements is to be taken into account for
the selection. The test results illustrating the method have
high performance in practices.
5. Acknowledgments
We would like to thank Professor Fujiang Liu for his
valuable comments and suggestions which have helped
us to improve the quality, organization, and presentation
of our manuscript.
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