Aaaaaa

Introduction to Web Services
by Hartwig Gunzer, Sales Engineer, Borland March 2002

Preface
Everybody is talking about Web Services, and almost every company creates another definition for the term, though all are similar. This paper starts with the history of distributed programming to show that Web Services are not a reinvention of the wheel but an evolutionary step. Afterwards, an overviewof Web Services as well as a formal definition is presented. Therefore, the definition of Web Services from WebServices.org is discussed instead of creating yet another definition. The rest of the paper explains the three main technologies used in connection with Web Services: SOAP, WSDL, and UDDI.

Table of Contents
Preface The past The present The future: Web Services SOAP WSDL UDDIConclusion References 1 2 2 4 5 9 14 16 17

Throughout the chapters these technologies are applied to a single example, which is a function that returns a ticker symbol for a given company name. This paper is useful for managers needing a broad presentation of Web Services, and it is useful for developers who need technical details. Since SOAP, WSDL, and UDDI are based on XML, the reader should havesome basic knowledge about XML, XML schemas, and XML namespaces. The references section at the end of the paper shows where to find this information.

Web Services The present
As we have discussed, there are basically three middlewares you can choose from: DCOM, CORBA, and RMI. All of them have their advantages as well as their disadvantages. This chapter discusses the main features without goingtoo much into detail. It will point out how Web Services fit into the picture.

The past
With the emergence of computer networks, the paradigm of distributed computing was born. Applications were split first into two parts with one part, the client, initiating a distributed activity, and the other part, the server, carrying out that activity. This decentralization minimized bottlenecks bydistributing the workload across multiple systems. It provided flexibility to application design formerly unknown on centralized hosts. But this two-tier architecture had its limits. For failover and scalability, issues a third tier was introduced, separating an application into a presentation part, a middle tier containing the business logic, and a third tier dealing with the data. This threetiermodel of distribution has become the most popular way of splitting applications. It makes application systems scalable. The foundation for the communication between the distributed parts of an application is the remote procedure call (RPC). To keep developers from the low-level tasks like data conversion or the byte order of different machines, a new layer of software hit the market. This middlewaremasks the differences between various kinds of hosts. It sits on top of the host’s operating system and networking services and offers its services to the applications above. The first middlewares, like DCE, were based on a procedural programming model and were superseded by the introduction of the object oriented programming model by middlewares like CORBA,® DCOM, or RMI, which are the mostpopular middlewares at present

CORBA®
CORBA is an open standards-based solution to distributed computing. The Object Management Group (OMG™), an industry consortium, developed the specification for CORBA and specified the Internet InterORB Protocol (IIOP®), the standard communication protocol between Object Request Brokers (ORB™). The primary advantage of CORBA is that clients and servers can bewritten in any programming language. This is possible because objects are defined with a high level of abstraction provided by the Interface Definition Language (IDL™). After defining an IDL file, a compiler takes care of the mapping of the IDL file to a specific programming language. The communication between objects, clients, and servers are handled through ORBs. If you need high performance in...