The Internet Encyclopedia (Volume 3)

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758 WEBSERVICES

Document Exchange Layer—Deals with processing of the

business documents like digital signatures, encryption,

and reliable delivery.

Transport Layer—Identifies the transport protocols to be

used with the endpoint addresses, along with other

properties of the transport layer. The transport proto-

cols could be SMTP, HTTP, and FTP.

When a party discovers another party’s CPP they ne-

gotiate certain agreement and form a Collaboration Pro-

tocol Agreement (CPA). The intent of the CPA is not to

expose the business process internals of the parties but

to make visible only the processes that are involved in

interactions between the parties. Message exchange be-

tween the parties can be facilitated with the ebXML Mes-

saging Service (ebMS). A CPA and the business process

specification document it references define aconversation

between parties. A typical conversation consists of mul-

tiplebusiness transactionswhich in turn may involve a

sequence of message exchanges for requests and replies.

Although a CPA may refer to multiple business process

specification documents, any conversation is allowed to

involve only a single process specification document. Con-

ceptually, the B2B servers of parties involved are respon-

sible for managing CPAs and for keeping track of the

conversations. They also interface the operations defined

in a CPA with the corresponding internal business pro-

cesses.

Web Services Platforms

Web services platforms are the technologies, means, and

methods available to build and operate Web services. Plat-

forms have been developed and changed over the course

of time. A classification into four generations of platform

technology should help to structure the space:

First Generation: HTML and CGI—Characterized by Web

servers, static HTML pages, HTML FORMS for simple

dialogs, and the Common Gateway Interface (CGI) to

connect Web servers to application programs, mostly

Perl or Shell scripts. (See Figure 3.)

Second Generation: Java—Server-side dynamic genera-

tion of HTML pages and user session support; the Java

servlet interface became popular for connecting to ap-

plication programs.

Third Generation: Application server as Richer develop-

ment and run-time environments—J2EE as foundation

for application servers that later evolved towards the

fourth generation.

Service

A

CPP

Service

B

CPA CPP

ebXML

registry

Figure 3: ebXML service-to-service interaction.

FW LB

WS AS

WS AS

WS AS

WS AS

Back-End

Internet

front-end web server app server back-end

DB DB

Figure 4: Basic four-tier architecture for Web services.

Fourth Generation: Web services—Characterized by the

introduction of XML and WSDL interfaces for Web

services with SOAP-based messaging. A global service

infrastructure for service registration and discovery

emerged: UDDI. Dynamic Web services aggregation—

Characterized by flow systems, business negotiations,

agent technology, etc.

Technically, Web services have been built according to a

pattern of ann-tier architecture that consists of a front-

end tier, firewall (FW), load balancer (LB), a Web-server

tier (WS), an application (server) (AS) tier, and a back-

end tier for persistent data, or the database tier (DB). (See

Figure 4.)

First Generation: HTML and CGI

The emergence of the World Wide Web facilitated the

easy access and decent appearance of linked HTML mark-

up pages in a user’s browser. In the early days, it was

mostly static HTML content. Passive information services

that provided users with the only capability of naviga-

ting though static pages could be built. However, HTML

supported from the very beginning FORMS that allowed

users to enter text or select from multiple-choice menus.

FORMS were treated specially by Web servers. They were

passed onto CGI, behind which small applications, mostly

Perl or Shell scripts, could read the user’s input, perform

respective actions, and return a HTML page that could

then be displayed in the user’s browser. This primitive

mechanism enabled a first generation of services on the

Web beyond pure navigation through static contents.

Second Generation: Java

With the growth of the Web and the desire for richer ser-

vices such as online shopping and booking, the initial

means to build Web services quickly became too primi-

tive. Java applets also brought graphical interactiveness to

the browser side. Java appeared as the language of choice

for Web services. Servlets provided a better interface be-

tween the Web server and the application. Technology to

support dynamic generation of HTML pages at the server

side was introduced: JSP (Java Server Pages) by Sun Mi-

crosystems, ASP (Active Server Pages) by Microsoft, or

PHP pages in the Linux world enabled separation of pre-

sentation, the appearance of pages in browsers, from con-

tent data. Templates and content were then merged on

the fly at the server in order to generate the final page re-

turned to the browser. Since user identification was crit-

ical for business services, user log-in and user sessions

were introduced. Applications were becoming more com-

plex, and it turned out that there was a significant overlap

in common functions needed for many services such as

session support, connectivity to persistent databases, and

security functions.