The Internet Encyclopedia (Volume 3)

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WEBSERVICESTODAY 757

and discovery, e-marketplaces offer their participants a

number of value-added services, including the following:

Enabling inter-Web service interaction after the discovery

(the actual interaction may happen with or without the

direct participation of the e-marketplace);

Enabling supply and demand mechanisms through tradi-

tional catalogue purchasing and request for purchase

(RFP), or through more dynamic auctions and ex-

changes;

Enabling supply-chain management through collabora-

tive planning and inventory handling; and

Other value-added services, such as rating, secured pay-

ment, financial handling, certification services, and no-

tification services.

Thus, e-marketplaces can be developed as an entity that

uses public UDDI registries. The e-marketplaces are cat-

egorized as vertical and horizontal depending on their

target market. The vertical e-marketplaces, such as Ver-

ticalNet, GlobalNetXChange, and Retailer Market Ex-

change, target a specific industry sector where partici-

pants perform B2B transactions. In particular, Chemdex,

E-Steel, DirectAg.com, and many more have been success-

ful in their respective markets. By contrast, horizontal ex-

changes, such as eBay, are directed at a broad range of

clients and businesses.

Web Services Orchestration

By specifying a set of operations in their WSDL document,

Web services make visible to the external world a certain

subset of internal business processes and activities. There-

fore, the internal business processes must be defined and

some of their activities linked to the operations before

publication of the document. This in turn requires mod-

eling a Web service’s back-end business processes as well

as interactions between them. On the other hand, Web ser-

vices are developed to serve and utilize other Web services.

This kind of interaction usually takes a form of a sequence

of message exchanges and operation executions, termed

conversation. Although conversations are described inde-

pendently of the internal flows of the Web services, they

result in executions of a set of backend processes. A Web

service and its ensuing internal processes together form

what is called aglobal process.

Intra-Web Service Modeling and Interaction

The Web Services Flow Language (WSFL) (Leymann,

2001), the Web Services Conversation Language (WSFL)

(W3C, 2002), the Web Service Choreography Interface

(WSCI) (BEA, 2002) and XLANG (Thatte, 2001) are some

of many business process specification languages for Web

services.

WSFL introduces the notion of activities and flows

which are useful for describing both local business pro-

cess flows and global message flows between multiple Web

services. WSFL models business processes as a set of ac-

tivities and links. An activity is a unit of useful work while

a link connects two activities. A link can be a control link

where a decision of what activity to follow is made, or a

data link specifying that a certain datum flows from an

activity to another. These activities may be made visible

through one or more operations grouped as endpoints. As

in WSDL, a set of endpoints defines a service. WSFL de-

fines global message flows in a similar way. A global flow

consists of plug links that link up operations of two ser-

vice providers. Complex services involving more than two

service providers are created by recursively defining plug

links.

XLANG developed by Microsoft extends the XML

Schema Definition Language (XSDL) to provide a mecha-

nism for process definition and global flow coordination.

The extension elements describe the behavioral aspects

of a service. A behavior may span multiple operations.

Action is an atomic component of a behavior definition.

An action element can be anoperation,adelayelement,

or araiseelement. A delay element can be of type de-

layFor or delayUntil. delayFor and delayUntil introduce

delays in execution for a process to wait for something

to happen (for example, a timeout) and to wait till an

absolute date-time has been reached, respectively. Raise

elements are used to specify exception handling. Excep-

tions are handled by invoking the corresponding handler

registered with a raise definition. Finally, processes com-

bine actions in different ways: some of them are sequence,

switch, while, all, pick, and empty.

Inter-Web Service Modeling and Interaction

Web services must negotiate and agree on a protocol in

order to engage in a business transaction on the Web.

X-EDI, ebXML, BTP, TPA-ML, cXML, and CBL have been

proposed as an inter-Web service interaction protocol. We

focus on ebXML as it is by far the most successful one.

(See Figure 2.)

In ebXML (http://www.ebxml.org/) parties to engage in

a transaction have Collaboration Protocol Profiles (CPP’s)

that they register at ebXML registries. A CPP contains the

following:

Process Specification Layer—Details the business transac-

tions that form the collaboration. It also specifies the

order of business transactions.

Delivery Channels—Describes a party’s message receiving

and sending characteristics. A specification can con-

tain more than one delivery channel.

A

B C

X

Y

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Pt. o2

o3

Pt. o5

o7

GFA

B C

X

Y

Z

P t’.o 2

o 3

P t’. o1

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o 7

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Figure 2: Intra and inter-Web service modeling and interac-

tion.