G&G in XML Web Services programming, with SOAP, XML, UDDI, WSDL, in offshore application development

G&G has extensive experience in XML Web services. We have long experience in working with XML Web services on several projects. In this section we provide a general overview of this platform from various development perspectives.

With Web Services, we are on the verge of a new programming model. A set of standards has been developed that gives us programmatic access to the application logic of the web. This application logic is accessible to clients on every platform, and in every programming language. Using this model, we can build applications that integrate components using standard Internet protocols. At the core of the Web Services model is SOAP (Simple Object Acc Protocol), the protocol that allows messages to be transmitted as XML documents and invokes the capabilities of Web Services. The SOAP standard is the key to Web Services.

This section delves into SOAP basics, and the concepts needed to start using SOAP in applications. We will try to cover the fundamentals of SOAP and its design, and then will also drill down into the details of SOAP messages, transports, and conventions.

SOAP is a specification for using XML documents as messages. The SOAP Specification contains:

• A syntax for defining messages as XML documents, which are referred to as SOAP messages
• A model for exchanging SOAP messages
• A set of rules for representing data within SOAP messages, known as SOAP encoding (or section 5 encoding due to the section of the specification it appears in)
• A guideline for transporting SOAP messages over HTTP
• A convention for performing remote procedure calls (RPC) using SOAP messages

SOAP and Web Services

With all the buzz and acronyms surrounding the topic of Web Services, it can get a little confusing. The list of protocols and technologies related to Web Services grows everyday. Of all the Web Services acronyms, SOAP is probably the most important. It is rapidly becoming the standard protocol for accessing a Web Service, and accessing the service is key. For Web Services to work as a technology, there must be well-defined approaches for discovering a service (UDDI - Universal Description Discovery and Integration) and determining its capabilities (WSDL - Web Service Definition Language). For any individual Web Service to succeed, however, these technologies are optional: written documentation or even a conversation over coffee can define the location of a service and its methods. However, without a protocol to access the methods, the service is useless. SOAP is the best choice today for that protocol. Although SOAP is a great choice for a Web Services messaging protocol, it is not the only choice. Web Services can simply operate on HTTP GET, or only expose functionality through XML-RPC. This does not make these components any less of a Web Service than a component that works with SOAP. Generally, however, SOAP is the messaging protocol of choice for Web Services. There is widespread acceptance of SOAP both by vendors and independent developers, and the tools and implementations that work with SOAP are improving all the time. The first version of the SOAP Specification that was available to the public was released in 1999, and it was a result of collaboration between developers at Microsoft, DevelopMentor, and UserLand Software. The current version, SOAP 1.1, was released in May, 2000 as a Note by the W3C with additional contributions from IBM and Lotus. Since then more than twenty different implementations have been started covering a wide variety of languages and platforms.

The SOAP Message Exchange Model

The SOAP specification defines a model for exchanging messages. It relies on three basic concepts: messages are XML documents, they travel from a sender to a receiver, and receivers can be chained together. Working with just these three concepts, it is possible to build sophisticated systems that rely on SOAP.

XML Documents As Messages

The most fundamental concept of the SOAP model is the use of XML documents as messages. SOAP messages are XML. This provides several advantages over other messaging protocols. XML messages can be composed and read by a developer with a text editor, so it makes the debugging process much more simple than that of a complex binary protocol. As XML has achieved such widespread acceptance, there are tools to help us work with XML on most platforms.