A Neuro-Fuzzy Model for QoS Based Selection of Web Service

doi:10.4236/jsea.2010.36068

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J. Software Engineering & Applications, 2010, 3, 588-592

doi:10.4236/jsea.2010.36068 Published Online June 2010 (http://www.SciRP.org/journal/jsea)

A Neuro-Fuzzy Model for QoS Based Selection of

Web Service

Abdallah Missaoui1, Kamel Barkaoui2

1LSTS-ENIT, Tunis, Tunisia; 2CEDRIC-CNAM, Paris, France.

Email: abdallah.mi[email protected], [email protected]

Received January 5th, 2010; revised March 8th, 2010; accepted March 10th, 2010.

ABSTRACT

The automatic selection and composition of Web services rely strongly on the manner to deal with ambiguity inherent to

the description of functionalities of these services and the client’s requests. Quality of Service (QoS) criteria become

crucial in Web services selection and the problem of checking that a web service satisfies a given level of QOS is

considered in recent research works. This paper presents a QoS based automatic classification method of web services.

These services give generally similar functionalities and are offered by different providers. The main feature of our Web

service selection model is to take advantage of the neuro-fuzzy logic for coping with the imprecision of QoS constraints

values.

Keywords: Web Service, Selection, Neuro-Fuzzy, QoS, Constraint

1. Introduction

Web services are modular, self-contained, self-describing

software components which are distributed over the Web.

They can be readily located and checked-out online and

dynamically, using a new directory and corresponding

search mechanism known as Universal Description, Dis-

covery, and Integration (UDDI).

The requester accesses the description using a UDDI or

other types of registry, and requests the execution of the

provider’s service by sending a SOAP message to it (see

Figure 1).

SOAP and HTTP provide exactly what they were de-

signed for a simple, lightweight mechanism for interop-

Figure 1. Basic web services architecture

erability and distributed communication. However, SOAP

and HTTP do not provide the traditional enterprise quali-

ties of service typically needed for an enterprise.

Furthermore, SOAP was designed to be extensible,

and it can be extended to support any desired QoS fea-

ture by adding SOAP headers to the SOAP messages and

adding QoS features to the basic SOAP run-time facili-

ties.

In recent years, several service providers offer QoS

features to there customers. Then, multiple providers

may provide similar functionalities with different values

of non-functional properties.

Their non-functional properties need to be considered

during service selection. There are characterised as qual-

ity of service (QoS). In many practice cases of business

applications, it is recommended to be taken into account

during the provider selection.

The human faculty of cognition and perception is very

complex, but it possesses an efficient mechanism for in-

formation processing and expression [1,2].

This paper applies the neuro-fuzzy decision making

approach in the process of selection and choice of the

most appropriate web service with respect to quality of

service criteria.

This paper is organized as follows: Section 2 presents

web service QoS generic description. In Section 3, we

discuss and evaluate related works on web service selec-

tion adopting a common fuzzy logic approach. Section 4,

A Neuro-Fuzzy Model for QoS Based Selection of Web Service589

we enlighten our QoS requirement description model ex-

ploiting neuro-fuzzy logic in order to deal with the im-

precision of QoS constraints values. Comments and

recommendations for our model are explicitly presented

in Section 5. Finally, Section 6 draws a conclusion.

2. QoS Properties of Web Service

Many services are appearing on the Web, several requ-

esters are presented to a group of service providers offer-

ing similar services. Different service providers may have

different qualities of service.

QoS is one of the most important factors for user’s

choice of Web service. This will require sophisticated pa-

tterns of selection process. It is necessary to provide an

appropriate negotiation mechanism between clients and

service providers to reach mutually-agreed QoS goals.

QoS management in Web service architecture includes

the definition of QoS attributes and the specifications of

the following processes: QoS publication, discovery,

validation, and monitoring. Many works have studied

QoS management on web service. Several QoS languages

and architectures are proposed.

The proposed approaches for QoS management can be

classified into two groups: one based on extending web

service technologies including SOAP, WSDL and UDDI

to support QoS [3-5]. The second group use independent

entities to perform QoS management [6].

Quality of service is defined by the ability to provide

different priorities to different applications, users, or data

flow, or to guarantee a certain level of performance to a

data flow. A QoS property may include several sub-pro-

perties representing different evaluation criteria, e.g. avai-

lability, performance, accessibility. In addition, a QoS

property can be evaluated by many metrics and therefore

it is necessary to define the units of measurements.

QoS in web service architecture is a combination of

several qualities or properties of a service, such as:

 Response time: the interval between a user-

command and the reception of an action, a result or a

feedback from the service.

 Availability: availability is the percentage of

time that a service is available for use;

 Accessibility: Accessibility represents the de-

gree that a system is normatively operated to coun-

teract request messages without delay.

 Throughput: It means the max number of ser-

vices that a platform providing Web services can

process for a unit time.

 Reliability: Reliability is the quality aspect of a

Web service that represents the degree of being ca-

pable of maintaining the service and service quality.

The number of failures per month or year represents

a measure of reliability of a Web service.

 Price: represents the money that the customer

should pay for this service. It is always associated

with the value of the service’s functionality, i.e. the

more a service costs, the more complicated functions

it provides.

 Security Level: represents the security level of

a service. It includes the existence and type of au-

thentication mechanisms the service offers, confi-

dentiality and data integrity of messages exchanged,

non-repudiation of requests or messages, and resil-

ience to denial-of-service attacks [7].

3. Related Work

With the strong popularity of the development of service

oriented application, quality of service becomes a central

interest of more and more researchers and enterprises. QoS

values are proportional to the reliability degree and per-

formance of service and thus play a very important role in

the provider choice. A large number of services are ex-

posed constraint information’s for comparison providers.

Many researches [5,8-10] have studied QoS issues to

improve two processes of discovery and selection of ser-

vices. Several QoS-aware web service selection mecha-

nisms have been developed in recent years in order to

perform the web service composition and to improve

performance applications based on services. This mecha-

nisms’ main objective is how to how properly select a set

of providers that most satisfy the constraints defined by

the user in his business processes.

Menascé studies the problem of finding services that

minimize the total execution time. It presents an opti-

mized heuristic algorithm that finds the optimal solution

without exploring the entire solution space. The solution

provided in [11] covers only the important case of execu-

tion constraints but not all QoS properties.

Pfeffer proposed a fuzzy logic based model for repre-

senting any kind of non-functional service properties. This

representation of user preferences enables the fast eva-

luation of the requested service composition by a fuzzy

multiplication of the service composition properties. Thus

service composition’ properties are measured during or

after execution [12].

Other works have been done in fuzzy logic based web

service selection. In [12-17], various methods have been

proposed for specifying fuzzy QoS constraints and for

ranking Web services based on their fuzzy representation.

There is a more suitable technique to quantify func-

tional properties: Linear Programming. These properties

are not fitting well for measuring the non-functional at-

tributes, because the majority of them are not easy to be

quantified in numerical form. In the meantime, user’s QoS

constraints regularly remain imprecise or ambiguous due

to various human mental states, and it is very difficult to

distinguish the priority order among QoS criteria.

Furthermore, in web services selection, the applied QoS

constraints are not explicitly defined. It is necessary to

relax the constraints to make an optimal solution. The use

A Neuro-Fuzzy Model for QoS Based Selection of Web Service

590

of fuzzy logic offers improvements in the overall satis-

faction level. The QoS information’s represented at ab-

stract level such that it could efficiently select the best

services.

However they are still initial efforts which need further

investigation for more complete solutions. In the follow-

ing, we specify several open issues that can be solved:

 When we use some kinds of fuzzy numbers like

triangular fuzzy they may not be easy to be defined

by end users.

 It is very important to correctly define the QoS

properties that we use in the selection process. These

criteria’s QoS have important effects on ranking

methods.

 How to improve fuzzy based web service dis-

covery and the representation of QoS to achieve ef-

fective web service selection?

 How to automatically set the weights of service

providers attributes?

4. Refinement of the Framework

Neuro-fuzzy technique is the combination of two artificial

intelligence (AI) methods: fuzzy logic techniques and

neural networks. Neuro-fuzzy system has the ability to

handle the nonlinear and complex systems. It is con-

structed based on the learning algorithm of neural net-

works technique to adjust the appropriate parameters for

fuzzy logic system [18].

In this paper, we aim to solve the selection of web ser-

vices in a global and flexible manner by introducing a

neuro-fuzzy way. For this purpose, we have developed a

neural-fuzzy system based on the Sugeno Approach [19].

This is known as the ANFIS (i.e., Adaptive Neuro-Fuzzy

Inference Systems). We assume that semantic match-

making has taken place to identify functionally equivalent

services. When several of them are available to perform

the same task, their quality aspects become important and

useful in the selection process.

An ANFIS is a multi-layered feed forward network, in

which each layer performs a particular task. The layers are

characterized by the fuzzy operations they perform. Fig-

ure 2 describes the global architecture of this neural-fuzzy

system. It shows a n-input, type-5 ANFIS. Three member-

ship functions are associated with each input.

We assume that the fuzzy inference system under con-

sideration has n inputs Q1, Q2,…,Qn (which are one service

attributes). Each input has five linguistic terms, for ex-

ample, the input Q1 possesses the terms {M11, M12,…, M15}.

For each input Qi, we have defined linguistic expres-

sions

Li = {Very Poor(vp), Poor(p), Medium(m), Good(g), Very

Good(vg)}

The common fuzzy if-then rule has the following type:

Rule 1: If (Q1 is M11) and (Q2 is M21) and … and (Qn is Mn1)

then f1 (Q1, Q2,…, Qn)

We denote the output of the ith node in layer k as Ok,i.

Figure 2 shows the schematic diagram of the ANFIS

structure, which consists of five layers.

Layer 1: Every node i in this layer transform the crisp

values to a fuzzy one

1, 1

1()





for





1, 2,...,5i and

1, 2

2()





for





1, 2,...,5i and, …, and

1, ()

iMn





for





1, 2,...,5i

where QK is the input to node K and Mki (and



1,...,kn







1,...,5i) is a linguistic label (very poor, poor, fair,

good, very good) associated with this node. In other words,

O1,i is the membership grade of a fuzzy set









1 5

,...,

M M



11,...MM 1521 25

,,..., ...MM M

and it specifies the degree which the given input QK

(





1,...,kn) satisfies the quantifier M.

We use the following generalized Bell function as the

membership function (MF)

()









where ai, bi and ci are the parameters set of MF. The

bell-shaped function varies depending on the values of

these parameters. Where the parameters a and b vary the

width of the curve and the parameter c locates the center





Figure 2. The structure of the neural fuzzy selector

A Neuro-Fuzzy Model for QoS Based Selection of Web Service591

of the curve. The parameter b should be positive. The pa-

rameters in this layer are referred to as premise para-

meters. The generalized Bell-shaped function is shown in

Figure 3.

Layer 2: Every node in this layer is a fixed node labeled

∏. The weighting factor, wk, of each rule is calculated by

evaluating the membership expressions in the antecedent

of the rule. This is accomplished by first converting the

input values to fuzzy membership values by utilizing the

input membership functions and then applying the and

operator to these membership values.

The and operator corresponds to the multiplication of

input membership values.

2,1 2

( )()...()

iiM MMn

ii ni

OwQ QQ







Each node output represents the firing strength of a

rule.

Layer 3: Every node in this layer is a fixed node labeled

N. The function of the fixed node is used to normalize the

input firing strengths.









1,...,in

Layer 4: Every node in Layer 4 is a parameterized

function, and the adaptive parameters are called “conse-

quent parameters”.

The node function is given by:

4,1 12 111

(... )

iii i

iiiin n

OwfwpQpQpQp



  

Layer 5: The single node in this layer is a fixed node

labeled ∑, which computes the overall output as the

summation of all inputs:

5,1

Owf







Thus, the ANFIS network is constructed according to

the TSK fuzzy model. This ANFIS architecture can then

update its parameters according to the backpropagation

algorithm [20].

This algorithm minimizes the error between the output

Figure 3. Generalized bell-shaped (a = 2, b = 4, c = 6)

of the ANFIS and the desired output.

Our neuro-fuzzy system allows classifying service

providers in several categories: very poor, poor, fair,

good, very good. It allows automating the selection pro-

cess in the dynamic composition of services.

According to the QoS requirements of web service

providers and the functions of Neuro-fuzzy system, we

believe that each service invoked is appropriate candidate

to increase the composition ability of web services and to

decrease the burden of composition cognition and the

minimal development cost.

5. Comments and Recommendations

In fuzzy inference system (FIS), The MF of the conse-

quent of each rule is a constant of a fuzzy MF. There are

two steps to construct this system: the specification of an

appropriate number of input/output and the specification

of the shape of MFs. The main problem is that structure

identification requires human expertise to solve the pa-

rameter estimation. In our selector system we used a dif-

ferent approach, which take advantage of adaptive neural

networks algorithms during fitting procedures. MF pa-

rameters are fitted to a dataset through a learning algo-

rithm.

A significant number of samples of service providers

are needed in order to have better result and to avoid

having too many defect values during selection process.

The database must be as complete as possible, including

samples of providers attributes over a broad range. The

number of samples depends on the context and on the

runtime environment.

On the other hand, fuzzy logic sets are based on trans-

parence, linguistic rules and establish a framework to

include human expertise into modelling. The number of

rules is decided by an expert who is familiar with the

system to be modeled. In our work, however, no expert is

available and the number of membership functions as-

signed to each input qualities is chosen empirically by

examining the desired input-output data.

We merged the fuzzy logic approach with the ability of

learning algorithms from neural networks to adjust the

model.

6. Conclusions

Web service composition is an emerging area involving

important technological challenges. Some of the main

challenges are to correctly describe QoS of Web services,

to compose them adequately and automatically, and to

discover suitable providers and QoS composition issues.

Neuro-fuzzy logic can be seen as a promising formal

technique for representing imprecise QoS constraints. In

this paper, we have presented a solution to use neuro-

fuzzy approach in Web service discovery and selection.

We have proposed methods for ranking and selecting web

services based on a neuro-fuzzy specification of fuzzy

A Neuro-Fuzzy Model for QoS Based Selection of Web Service

592

QoS constraints. The user’s constraints are formalized as

fuzzy sets and the Qos criteria’s are expressed as fuzzy

expressions.

This model can be seen as a contribution towards a

more complete solution for web service composition in-

tegrating fully QoS features.

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