These last few years, the term “Web Service” has called the attention of several analysts and architects, mainly those mostly interested in business to business (B2B). The concept has been created, implemented and now is being used. The expectations are significant, high investment, powerful frameworks, gains in productivity, portability and in independence. In this article, part of this important technology that promised, and is already fulfilling, to challenge the manner in which the systems are developed will be presented.
Introduction to Web Service
The basic requirement of a company is to provide services whether through the salesmen of an organization, the cost and purchase sector, the service providers etc; . Each company offers services for communication between itself and other persons, which can be natural individuals or legal entities, internal or external to the company.
Some of these services can be automated. For example, no need to have a sales representative if your customer has already in hand, the price and all the other relevant data to develop a purchase request. This request can and, in several cases, is already done through computational interfaces. The customer accesses its site, develops the request according to its needs and confirms the purchase. This is a web service, this means, a service that is published in web so that any person can make use of it.

Web Services were created to construct applications of this type, applications that are services in internet. However, creation of graphical user interfaces is not part of the Web Service concept and this part is left for development by other companies or persons. It is common to find texts state that Web Services make available services only for the developers, or that Web Services are nothing else that a call of methods using XML. These definitions are correct.

Web Service is the ideal technology for communication between system and is frequently used in B2B applications. Communication between the services is standardized, enabling platform independence and programming language. For example, a system in Java used in reservation of air tickets and executing in a Linux server can access, with transparency the hotel reservation service provided in .Net executing on a Microsoft server.

To communicate with the Web Service, implementation of the SOAP (Simple Object Access Protocol)  protocol defined in W3C is required. This protocol is responsible for the independence that the Web Service requires. Currently several implementation in different languages are available.
In the figure below is a diagram showing the exchange of messages between the customer and the server in a SOAP communication. There are two applications communicating, a Client Wrapper and a Server Wrapper that are making available the transparency for the applications. Between them only the XML passes through, following the SOAP protocol over HTTP.
A Web Service will be published and to allow other individuals to use it,  it is important to define how such Service is, how it should be accessed and the values that will be returned. These definitions are described in a XML file according to the Web Service Description Language (WSDL) standardization. This file should be constructed to that the service user can understand the operation of the Web Service and of course, will be accessed by the public.
The Web Services can also be used to implement architectures directed to services, such as Service-Oriented Architectures (SOA). In this model of architecture the main requirements turn into services and are accessed by other services, modularizing and increasing the coherency of the application components.