SOAQE

Service Oriented Architecture Quality Evaluation

Riad Belkhatir

, Mourad Oussalah

and Arnaud Viguier

Department of Computing, University of Nantes, Rue de la Houssinière, Nantes, France

BeOtic, Rezé, France

Keywords: SOA, Quality, Evaluation, Attributes.

Abstract: This paper presents a semi-automated method for evaluating SOAs called SOAQE, correcting defects

observed so far with existing methods such as lacks of pertinence and accuracy for evaluation results.

SOAQE takes as a starting point the McCall model, describing software quality, which led to an

international standard for the evaluation of software quality (ISO/IEC 9126-1, 2001). This model is

organized around three types of quality attributes (factors, criteria and metrics). The SOAQE method

consists in decomposing the whole architecture and evaluating it according to the McCall model, i.e. a list

of quality factors arising from business needs grouping criteria composed by metrics. Our experimentations

led us to quantify numerically a first determining factor for SOAs, the ‘dynamism’ and some attributes of its

structure: namely the ‘loose coupling’ criterion and its constituent metrics (‘physical, semantic and

syntactic’).

1 INTRODUCTION

We can distinguish a few architectural paradigms for

distributed systems, and, among the most

noteworthy ones, three have contributed to the

evolution of the concerns. These are

chronologically, object oriented architectures

(OOA), component based architectures (CBA) and

service oriented architectures (SOA).

First developers were quickly aware of code

repetitions in applications and sought to define

mechanisms limiting these repetitions. OOA is

focused on this concern and its development is one

of the achievements of this research. OOA provides

great control of the reusability (reusing a system the

same way or through a certain number of

modifications) which paved the way to applications

more and more complex and consequently to the

identification of new limits in terms of granularity.

These limits have led to the shift of the concerns

towards the composability (combining in a sure way

its architectural elements in order to build new

systems or composite architectural elements).

Correlatively, the software engineering community

developed and introduced CBA to overcome this

new challenge and thus, the CBA reinforces control

of the composability and clearly formalizes the

associated processes. By extension, this

formalization establishes the base necessary to

automation possibilities. At the same time, a part of

the software community took the research in a new

direction: the dynamism concern (developing

applications able to adapt in a dynamic, automatic

and autonomous ways their behaviors to answer the

changing needs of requirements and contexts as well

as possibilities of errors) as the predominant aspect.

In short, SOA has been developed on the basis of the

experience gained by objects and components, with

a focalization from the outset on ways of improving

the dynamism.

The SOAQE method presented in the present

paper allows evaluating SOA by combining the

computerized approach and the human intervention

to eliminate lacks identified with past methods such

as ATAM, SAAM or ARID (Clements,Kazman and

Kein, 2001). The main idea resides in automating

the process to avoid time-wasting evaluations. The

process consists in three principal stages detailed

later in this paper. In the current paper, we first

relate in the section 2 the interests of the evaluation,

we analyze the existing works and we present the

SOAQE method in the section 3; and finally we

relate the experimentation which has been done by

the lab team in the section 4. We conclude this paper

199

Belkhatir R., Oussalah M. and Viguier A..

SOAQE - Service Oriented Architecture Quality Evaluation.

DOI: 10.5220/0003997701990202

In Proceedings of the 7th International Conference on Evaluation of Novel Approaches to Software Engineering (ENASE-2012), pages 199-202

ISBN: 978-989-8565-13-6

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

with a conclusion about the perspectives of our

work.

2 MEASURING THE QUALITY

The scientific community utilizes models of quality

introducing what we call the software attributes

decomposition.

2.1 Mc Call Model

One of the models that have been published is the

McCall model in 1977 (see figure 1) decomposing

quality attributes in three stages. This model led to

the IEEE standard: ISO/IEC 9126 (ISO/IEC 9126-1,

2001). A certain number of attributes, called external

(applicable to running software), are considered as

key attributes for quality. We call them quality

factors (Clements,Kazman and Kein, 2001). These

attributes are decomposed in lower level attributes,

the internal attributes (which do not rely on software

execution), called quality criteria and each criterion

is associated to a set of attributes directly

measurable and which are called quality metrics.

Figure 1: Mc Call model applied to the "Maintainability".

2.2 Lack of Precision

Current methods of evaluation stop the quality

attributes decomposition at the “quality factors

step” and remain too vague when it comes to giving

accurate measures to quality. These methods are not

precise because they cannot go further in the

decomposition and consequently they cannot be

automated to the point of defining a finite value for

each attribute. They lead inevitably to the

establishment of a brainstorming between

stakeholders (see section 3) for the purpose of the

institution of a utility tree (SEI, 2010). Because

stakeholders do not have tools to quantitatively

measure each quality factor chosen, they shall set up

scenarios aiming to respectively solicit separately

each criterion, and factor. An approximated

evaluation of the architecture is then realized after

having studied the system behavior while carrying

out the scenarios set.

3 SOAQE

It is precisely where our work differs from those

existing insofar as we wish to obtain a precise

quantitative measurement for each quality factor

with the SOAQE method. We especially aim to

automate the process in order to avoid hand-operated

evaluations pushing to

solicit stakeholders for the

whole evaluation.

3.1 Principle of the Method

The process consists in three principal stages

corresponding each to a decomposition step of our

quality attributes.We first identify relevant quality

factors for our architecture.Then we isolate the

quality criteria defining them.And finally we define

quality metrics composing each criterion in order to

quantify them numerically.

3.2 Steps in More Detail

The main idea of the SOAQE process is to evaluate

in three steps the whole architecture from every

metric to the set of quality factors obtained after

having previously identified the business objectives.

Our work is based on the architect point of view and

the attributes selected are the ones considered as the

most relevant among all existing.

3.2.1 Quality Factors

The CBA is defined with reusability and

composability (Crnkovic, Chaudron and Larsson,

2006

). Basing on previous analysis, we define the

SOA with the Reusability, the Composability and the

Dynamism.These three attributes, that we identified

as quality factors for SOA represent the qualitative

quintessence which has directed the definition of the

object, component and service paradigms.

The first step of the SOAQE method consists in

choosing a first quality factor to study in depth and

there exist a lot which could come out after the

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analysis of the business objectives. But we have

naturally chosen to work on those identified as the

qualitative quintessence for SOA. These three quality

attributes (dynamism, composability and reusability)

define each of our three paradigms to varying degrees.

Moreover, there exist a hierarchical ranking

propelling “dynamism” on top of SOA concerns, and

this is precisely why we chose to especially focus

deeply on this quality factor. We may record that each

of the two others attributes is of major importance for

the three paradigms considered (object, component or

service oriented architectures).

3.2.2 Quality Criteria

With regards to our work and after having identified

the determining factor quality for SOA (i.e. the

dynamism), we were interested in the second step of

the SOAQE method, namely, discovering the criteria

defining the factor on which we have gone through.

Further down the road, any factor is composed

by a set of criteria that could be looked at as part of

our work. These criteria belong to different families

of attributes and a few of them are common to the

three quality factors chosen for our paper. But we

deliberately concentrated our work on technical

criteria of each of the quality factors studied because

we adopted the point of view of an architect that is

itself a technical stakeholder. In this light, we

identified six criteria common to each of our three

factors. These technical criteria gather elements

having significant impacts on global quality, from

the development process to the system produced: the

loose coupling (potential of dependences reduction

between services), the explicit architecture

(paradigm ability to define clear architectural

application views), the expressive power (potential

of paradigm expression in terms of creation capacity

and optionalities), the communication abstractions

(paradigm capacity to abstract services functions

communications), the upgradability (paradigm

ability to make evolve its services), and the owner's

responsibility (corresponds to the responsibilities

sharing out between services providers and

consumers). Each of these quality criteria is given

varying degrees of consideration according to the

quality factor in question. Our previous works

(Hock-Koon, 2011) allowed organizing

hierarchically (under three distinct levels) these

quality criteria for each of the three quality factors

(dynamism, reusability and composability).

Consequently, we obtain the triptych of the

figure 2 while considering all paradigms. While

focusing on the dynamism, our previous work

(Hock-Koon, 2011) allowed to conclude that the

“loose coupling” criterion is of biggest importance

for this factor (see figure 2), this is why we chose to

concentrate in depth, on it; whereas, the criterion

“expressive power” is of less importance.

3.2.3 Quality Metrics

The coupling is relatively well known by the

community, and thus, our lab team members decided

to look into the matter (Hock-Koon, 2011).

Figure 2: Expression of reusability, composability and

dynamism perspectives.

We found three quality metrics for the latter

which must be considered for the third step of the

SOAQE method (the semantic coupling: {high, low

or non-predominant} based on the high-level

description of a service defined by the architect, the

syntactic coupling: {high, low} measures

dependencies in terms of realization between

abstract services and concrete services and the

physical coupling: {γ, β and α with 0≤γ≤β≤α}

focusing on the implementation of the service). This

implementation corresponds to a particular instance

of the service where a choice has been made

concerning concrete services to use. A unique

solution has been chosen to fulfill each of the needs

expressed by abstract services. It reuses existing

researches (Perepletchikov, Ryan, Frampton and

Tari, 2007) and it is based on measurements such as

methods calls, messages exchanged, the number of

linked services, commune objects and so forth.

These metrics shall make it possible to identify

physical dependencies between concrete services.

4 EXPERIMENTATION

For the experimentation, we tempted to quantitative-

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ly measure the key quality attributes discussed in the

previous sections of this paper; notably, the quality

factor “dynamism”, the “loose coupling” criteria and

the “physical, syntactic and semantic coupling”

metrics. That being said, it is important to note that

the SOAQE method must be reproduced for every

quality factor identified after having analyzed the

objectives of the company and the set of criteria and

metrics belonging to that quality factor.

4.1 Loose Coupling

Taking as a starting point an existing formula of the

field of “Preliminary analysis of risks” (see formula

3.1) (Mortureux, 2002) our works led to the

identification of a mathematical formula (see

formula 3.2) combining the three couplings studied:

semantic, syntactic and physical.

NB: The simplified formula (see formula 3.1)

usually used in the automotive industry, makes it

possible to measure the default risk of a car

component A is the Criticality of the car component,

B is the Probability of occurrence of a failure on this

component and C is the Probability of non-detection

of this failure.

We associate this concept of risk with our vision

of the coupling. Correlatively, the quintessence of

the coupling is the expression of the dependences

which can exist between two elements and the

principle of dependence defines that one element

cannot be used without the other. Reducing the risk

that the role defined by a service cannot be assured

anymore is decreasing the dependence of the

application in relation to this service and thus

reducing its coupling. The calculation of this risk

takes into account all the characteristics influencing

the coupling by redefining the three variables A, B

and C according to the semantic, syntactic and

physical couplings. The global coupling corresponds

to the sum of the three couplings calculated

individually beforehand. The lower this result is, the

more the coupling is weak.

NB: The criticality A

∈

[(a),(b),(c)] is affiliated to

the semantic coupling. ‘a’ if the service is only

associated to non predominant couplings, ‘b’ for

non predominants and low couplings and ‘c’ for non

predominants,low and high couplings, while ‘Ps’ is

the probability of failure of a service.

5 CONCLUSIONS

The finality of our work is to design a conceptual

framework and, in fine, a semi-automated prototype

(based on past methods, such as ATAM or SAAM)

which could quantify with an accurate value the

quality of the whole service oriented architecture.

Another pursued goal consists in bringing to the

customer "less abstract" documents than those

proposed today. The quality concept remaining a

relative one, we will target the sectors requiring a

special attention by directly addressing the various

development lab teams charged with the relevant

functions.

REFERENCES

Crnkovic, I., Chaudron, M., Larsson, S., 2006.

Component-based development process and

component lifecycle, ICSEA’06, International

Conference on Software Engineering Advances.

Clements, P., Kazman, R., Klein, M., 2001. Evaluating

Software Architectures: Methods and case studied,

published by Addison-Wesley Professional.

Hock-Koon, A., 2011. Contribution à la compréhension et

à la modélisation de la composition et du couplage

faible de services dans les architectures orientées

services. Thesis (PhD). University of Nantes.

Mortureux, Y., 2002. Preliminary risk analysis.

Techniques de l'ingénieur. Sécurité et gestion des

risques.

Perepletchikov, M., Ryan, C., Frampton, K., Tari, Z.,

2007. Coupling metrics for predicting maintainability

in service-oriented designs. ASWEC’07. Australian

Software Engineering Conference

SEI. The Carnegie Mellon Software Engineering Institute.

2011 [Online]. Available From: www.sei.cmu.edu/

architecture/start/glossary/

ISO/IEC 9126-1:2001: Software Engineering – Product

Quality – Part 1: Quality Model. International

Organization For Standardization, Geneva,

Switzerland [Online]. Available From:

Http://Www.Iso.Org/Iso/Iso_Catalogue/Catalogue_Tc/

Catalogue_Detail.Htm?Csnumber=22749

APPENDIX

Figure 3: Defaut risk of a car component (3.1) and global coupling of an architecture (3.2) formulas.

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