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Modeling and specification of Web services composition using

UML-S

Abstract

As Web services composition arouses a growing interest, most research works address

implementation

and

execution

issues.

Therefore,

many

composition

languages

(BPEL,

XLANG, WSFL, WSCI, to name a few of them) have been proposed in the past few years.

However,

a

weakness

of

these

languages

is

that

they

are

difficult

to

use

in

early

stages

of

development, such as specification. In this paper, an extensio

n to UML 2.0 called ”UML

-S:

UML for Services” is introduced. UML

-S allows for a Model Driven Engineering (MDE) of

Web services and their interactions.

1

1. Introduction

Many companies are now using the Web as a platform to communicate with their partners.

The Web and its technologies allows them to provide Web services to individuals as well as other

businesses.

The main challenges in the Web services paradigm are their discovery and their composition.

In other words, one must be able to find a suitable Web service for a given task. This process is

called the discovery [18, 19]. The second challenge is the one that is addressed in this paper. It is

known

as

Web

services

composition

[4,

10].

In

Web

services

composition,

already

defined

services are used together to achieve a larger task, resulting in a new composite and value-added

Web

service.

To

accomplish

this

purpose,

a

common

approach

is

to

allow

the

Web

services

to

interact in a scenario through the use of messaging mechanisms.

Although a lot of research works deal with Web services interactions, most of them address

language,

implementation

or

application

issues,

neglecting

early

stages

of

the

development

process,

such

as

specification.

To

address

this

issue,

an

extension

to

UML

2.0

called

”UML

-S:

UML

for

Services”

is

introduced.

UML

-S

allows

for

modeling

Web

services

as

well

as

their

interactions.

The Unified Modeling Language (UML) has been defined by the Object Management Group

(OMG) [2] to express graphically system development models.

UML-S enables the developers to build composite Web services by following the principles

of

the

Model- Driven

Architecture

(MDA).

As

a

consequence,

it

is

possible

to

generate

platform-specific code from high-level UML-S models.

This

paper

is

structured

as

follows.

Section

2

provides

a

survey

of

existing

approaches

to

model

Web

services

interactions.

In

section

3,

the

requirements

for

a

good

Web

services

composition modeling language are put forward. UML-S is then presented in details in section 4.

After that, a case study is provided in section 5 to observe UML-S in action.

Finally, section 6

draws the conclusions and presents future work.

2.

Related Work

The

Business

Process

Management

Initiative

(BPMI)

has

developed

the

Business

Process

Modeling

Notation

(BPMN).

This

notation

is

particularly

useful

to

visualize

BPEL

[23]

is

now

maintained

by

the

OMG

.

Unfortunately,

one

could

reproach

to

2

BPMN

its

lack

of

formalism,

as

explained

by

Wohed

et

at.

in

[24].

Although

BPMN

is

an

interesting solution, we preferred to extend UML 2.0 to achieve the same purpose. Indeed, UML

was already used as a Process Modeling Language (PML) [9, 13, 16].As a matter of fact, UML

has

some

very

interesting

features

as

a

PML:

it

is

standard,

graphical,

popular

and

it

contains

several diagrams which allows to model different views of a system.

UML

was

already

considered

to

describe

Web

services

composition.

In

[20],

an

approach

using UML activity diagrams to do so was presented by Skogan et provide a way to model

the coordination and the sequencing of the interactions between Web services. They also explain

how UML activity diagrams can be converted into BPEL [1] or in WorkSCo [3].However, in this

approach, methods input/output and data transformation are modeled in notes (i.e. comments) on

the side of the workflow, which can get quite confusing when the composition flow gets complex.

Chunming

Gao

et

al.

also

present

in

[11]

a

non- graphical

way

to

model

Web

services

composition

with

some

mobility

and

time

constraint.

To

do

so,

they

introduce

Discrete

Time

Mobile

Ambient

calculus

(DTMA),

an

extension

to

the

formal

model

called

Mobile

ambients

calculus [5]. Using DTMA, they focus on modeling BPEL operations. Due to their non-graphical

nature, languages such as DTMA are less user-friendly than graphical like UML.

Another approach to Web services composition modeling was proposed by De Castro et al in

[6]. In their work, they make use of the behavior modeling method of MIDAS, a Model-Driven

Architecture

(MDA)

framework

[14].

MIDAS

is

a

model-driven

methodology

for

the

development of Web Information Systems (WIS) based on the MDA [21], proposed by the OMG

[2].

They

introduced

Web

services

composition

through

UML

activity

diagrams

in

their

paper.

However, the model is not detailed as much as necessary to allow code generation as BPEL. Some

features could also be added such as data transformation and flow control mechanisms.

In [12], Hamadi et al. put forwards Petri nets [17] based algebra for composing Web services.

Petri nets are a well-known process modeling technique. The pros of using such Petri net based

algebra is that it allows the verification of properties and the detection of inconsistencies. However,

Web services need to be expressed using algebra constructs before being translated into a Petri net

representation, adding consequently another necessary stage in the process.

UML-S

transformation

rules

from

WSDL

2.0

and

to

WS-BPEL

2.0

were

provided

in

[8].

UML-S activity diagrams verification and validation using Petri nets was also detailed in [15].

3

3.

Web services composition model requirements

In this part, we state what the requirements for a good Web services composition modeling

language.

First

of

all,

it

is

better

to

extends

an

already

existing,

well-known

standard

if

it

is

adapted instead of coming up with a new model. UML modeling language is the de facto industry

standard. Therefore it is a good candidate to be extended for Web services composition modeling.

Moreover, UML is widely used and its graphical models are easily understandable. The modeling

language should allow to represent Web services interfaces as well as the dynamism induced by

their composition. UML class diagram is particularly adapted to represent interfaces. Additionally,

UML

activity

diagram

is

a

excellent

candidate

to

model

Web

services

composition,

due

to

its

strength to represent the dynamic. A good modeling language can also be judged by its simplicity

and its clarity. Graphical languages such as UML are known for being user- friendly. Finally, it is

worth noting that a composite Web service simply calls other services and makes them interact.

Therefore,

there

is

not

a

lot

of

programming

involved

compared

to

usual

Web

services.

As

a

consequence,

a

composite

Web

service’s

code

can

be

generated

in

its

totality

from

high

level

graphical

models

such

as

UML’s.

4.

UML

-S:

UML

for

Services

The

main

contribution

of

this

paper is ”UML

-

S: UML for Services”, an extension to

UML 2.0 that allows for

modeling Web

services

as

well

as

their

interactions.

In

UML-S,

both

class

diagrams

and

activity

diagrams

are

used to model and specify respectively Web services and their interactions. In part 4.1, we present

UML-S extended class diagram. After that, the activity diagram proposal is detailed in part 4.2.

4

5

建模和使用

UML-S Web

服务组合的规范

摘要

随着

W eb

服务组合引起了越来越多的兴趣，

大部分研究工作解决了实 现和执

行的问题。因此，在过去的几年里许多组成语言（

BPE L

，

XLANG

，

WSFL

，

WSCI

，