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Web Services
Introduction
Web Services have been described as the next evolution in distributed computing and is considered by many to be one of the most important innovations in distributed computing in recent years. A Web Service does not refer to a technology or platform but describes a set of interrelated supporting technologies and architectures. At its' most basic level a Web Service can be described as a "a Network of applications that use SOAP, UDDI and WSDL to exchange information in the form of XML documents". This definition, while technically correct, does not provide a full understanding of exactly what Web Services are and does not explain why they are important.
Web Services allow the components of an application to be used by another application that is physically located in another part of the world. This is accomplished via the World Wide Web using the Web's underlying set of protocols, such as the Hyper Text Transport Protocol (HTTP), Simple Object Access Protocol (SOAP) and eXtensible Markup Language (XML). Web services are in effect the building blocks for creating open distributed systems that are designed in such a way as to be modular, flexible and platform independent. Cheap University Papers on Web Services
Web Services, through the use of standardized and reusable components, allow companies to communicate with their customers and business partners. This technology allows companies to develop and deploy their products and services more rapidly with lower costs, improved efficiency and greater productivity. By using Web Services, companies are better able to leverage the distributed nature of modern corporate networks.
Web Services - Distributed Computing
Prior to the introduction of Web Services, communication between two applications, for the most part, took place using a tightly coupled mechanism in which one program called the other through an Application Programming Interface (API). Examples of such mechanisms are the Common Object Request Broker Architecture (CORBA) or the Distributed Component Architecture (DCOM).
Architectures such as DCOM failed to become the standard way of building distributed systems simply because they were too complex and tended to be operating system specific. Instead of focusing on specific areas these architectures tried address every possible requirement. This, in turn, required ever more sophisticated communication layer software to be installed on machines participating in these distributed environments.
Web Services, however, provide a means of integrating applications in such a way that applications can work together without changing anything in the communication layer. In a company whose distributed environment has many client machines this is a huge benefit as deploying changes to these clients can be time consuming, expensive and resource intensive.
Interoperability (meaning information is transferred in a way that satisfies the needs of both parties) had always been a major importance for companies and organisations. Recently two areas of system integration has become of even greater importance. Firstly integrating systems across the enterprise know as Enterprise Application Integration (EAI), and communication between systems belonging to different organisations called Business to Business Integration (BBi).
Of the two areas EAI is the most common problem that businesses face. For example, a typical company may have applications running on a Window NT network, an AS400 payroll system and several UNIX based systems. These systems need to collaborate by passing information between the various systems. BB represents the business transactions that companies need to transmit between each other. For example, a company may need to purchase supplies from another and in order to do so they need to exchange information. This ideally should take place between their respective IT systems at a network level rather than say over the telephone.
The use of Web Services over the Internet together with the availability of Web Servers and Web browsers on all major platforms provides a potential solution to these problems. This is because Web Services can allow Windows clients to exchange information with applications running on UNIX or AS400 platforms or even Linux machines.
In general, the use of distributed networking Web Services allows systems to move away from tightly coupled monolithic systems. Provided the supporting infrastructure and architectures are in place, systems can now be more open, flexible and modular with the Web Service components of many systems providing part of the overall delivery of the required services.
Web Services - Architecture
Web services combine the advantages of component-based development and the open standards of the Web. Like components, Web services may be viewed as black boxes that can be reused without worrying about how the service is implemented. Unlike current component technologies, Web services are not accessed via protocols specific to a particular development model or technology. Instead, Web services are accessed via commonly used Web protocols and data formats, such as the Hypertext Transfer Protocol (HTTP) and the eXtensible Markup Language (XML).
A Web Service interface defines the messages the Web Service accepts and generates. Consumers of the Web Service, such as applications, can be implemented on any platform in any programming language, as long as they can create and consume the messages defined for the Web Service interface.
The external architecture of how Web Service interacts with current technologies may be depicted as follows
From a developers perspective the internal architecture of a Web Service may be illustrated as follows
This architecture is very similar to the n-tier client-server application architecture. The architecture is divided into four logical layers.
The database layer stores information required by the business functionality that makes up the Web Service. Below the data layer is the data access layer, which presents a logical view of the physical data to the business layer. The data access layer isolates the business layer from the impact of any changes made to the underlying databases of data stores and thereby ensures the integrity of the data. The business layer implements the business logic of the Web Service. The business layer interfaces with and maps directly to the functions provided by the Web Service. Finally the Listener is that component of the Web Service that interacts with client applications.
The listener is also responsible for receiving incoming messages containing requests for services. It evaluates the message, and then passes the request to the appropriate method in the business layer. If the Web Service returns a response, the listener is also responsible for formatting the response from the business layer into a message and sending it that back to the client. The listener also handles requests for interface specifications and other documents about the Web Service.
There are important specifications and technologies that are required when building or consuming Web services. These specifications and technologies focus on five requirements for service-based development
·A standard way to represent data
·A common, extensible, message format
·A common, extensible, service description language
·A way to discover services located on a particular Web site
·A way to discover service providers
Web Services - Supporting Infrastructure
Web Services are not merely applications or components that have been designed as Web Services. Web Services also includes, and relies upon, other technologies which in some cases are not specific to Web Services e.g. XML. These supporting technologies or components together make Web Services a viable form of distributed computing.
Components have traditionally been accessible only through local area networks by means of object-model specific protocols such as DCOM. This limitation is partly due to the complexity involved in allowing external access to the internal network, as well as the dependence on both parties using a particular protocol.
Web Services remove this limitation by allowing access to components by means of the Internet and HTTP while still maintaining security. The supporting framework for Web Services provides the following
·Open Internet protocols and XML
Web Services use open Internet protocols such as XML, HTTP, and Simple Mail Transfer Protocol (SMTP) to transmit messages. This allows any client application to us a Web Service regardless of the operating system it is running on or the language it has been written in. XML Web services are one of the fundamental building blocks in the move to distributed computing on the Internet. Applications are constructed using multiple XML Web Services from various sources that work together regardless of where they reside or how they were implemented.
·SOAP
SOAP is an industry standard for using XML to represent data and commands in an extensible way. If XML is the language of Web Services, SOAP can be viewed as its grammar. Web Services do not, however, have to use the SOAP and HTTP mechanisms can be used instead. HTTP, however, is not as powerful as SOAP but can be used by all HTTP clients. As mentioned above, past attempts to design a common communications protocol that would be widely adopted have failed due to their size and complexity. SOAP is much smaller and simpler to implement than many of the previous protocols.
·Messages defined by Web Services Description Language (WDSL)
Web Services provide an interface specification that client applications can rely on to accurately describe the service's methods and arguments. WDSL is the XML-based way of describing this interface specification that can be read by all client applications that use XML. A WSDL file is an XML document that describes a set of SOAP messages and how these messages can be exchanged. The WSDL file message format is based on the XML Schema standard which means it is both independent of any specific programming and and is based on a commonly accepted standard. This makes it suitable for describing XML Web services interfaces that are accessible from a wide variety of platforms and programming languages.
·Service descriptions available through discovery (UDDI)
A discovery specification provides a standardized way for Web Service providers to publish Web Service interface specification. It also describes how developers and tools can discover these interface specification documents. Universal Discovery Description and Integration (UDDI) provides a distributed directory of Web Services that enables the registration and discovery of these Web Services. A UDDI directory entry is an XML file that describes a business and the services it offers.
The UDDI directory also includes several ways to search for the services required to build applications. For example, you can search for providers of a service in a specified geographic location or for business of a specified type. The UDDI directory will then supply information, contacts, links, and technical data to allow you to evaluate which services meet your requirements.
Web Services -Conclusion
In conclusion it worth listing the reasons that will probably make the technology a success. Web Services are not tied to a specific platform or technology. They focus on the protocols used to exchange messages (i.e. XML, SOAP and WSDL), not the implementation that supports those protocols. In other words, you can build Web services on any platform, using any programming language.
Another advantage of Web services over current distributed systems is that they are based on an existing technology infrastructure and are, besides the lower cost of entry, therefore easier for companies to implement. SOAP and WSDL are based on XML, for which widespread support and standards already exists. In addition, Web services messages are normally exchanged over the TCP/IP network, which is supported by just about every current software platform and programming language. Because Web services builds on an existing infrastructure of XML and TCP/IP, the adoption of Web services should be quick and widespread.In general, the major advantages of Web Services are
Interoperability This is regarded as the most important benefit of Web Services. Web Services allow systems written in different languages and residing on different platforms to collaborate and share information. The Web Services that enable this interoperability of disparate computer systems will most likely result in a longer life span of these systems with a consequent better return on investment. Web services also allow developers to use various programming languages such as Java and C++, without having to concern themselves with the problems of system compatibility. In addition, thanks to the use of standards-based communications methods, Web Services are virtually platform-independent.
Usability Web Services allow the business logic of many different systems to be exposed as services over the Web. This allows applications to make use of the Web services that they require without the need to code the same business logic on different clients. Instead of re-inventing the wheel for each client, developers need only include additional application-specific business logic on the client-side. This in turn allows developers to write services and/or client-side code using the tools and languages of their choice
Reusability Web Services provide, not only the ability to code business logic as "black boxes" or components, but also minimize the problems associated with deploying these applications across a company's Intranet as well as the Internet itself (know as zero-coding deployment). In other words it enables the reuse of code as components in the form of Web Services across the enterprise as well as between businesses.
Security Web Services can be deployed over current Internet technologies such as HTTP without the need to write code to cater for specific security technologies as such as firewalls or proxy servers. This is because, through the use of existing standards, underlying security such as Secure Socket Layer (SSL) is already built-in.
References
http//www.ticideon.com/Services.aspWeb Service Development.
http//www.goto-silicon-valley.com/articles/barbara-angius-saxby/web-services4.shtml - Web Service Floor Wax or Dessert Topping.
http//archive.infoworld.com/articles/tc/xml/01/05/14/010514tcwebserve.xml Next Generation e-bizJames R. Borck.
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