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XML defines a universal format for exchanging application data. The universal XML specification alone, however, is not enough to provide developers with the infrastructure they need to create easy and elegant web services. Although XML provides an efficient format for reading and writing program data, XML alone does not provide a standard format for structuring and interpreting that data. The SOAP specification fills that role. SOAP is a standard protocol for exchanging XML-based messages that pass between the web-service client and server.

SOAP is designed to support communication between so-called SOAP nodes. (A SOAP node is basically a computer or application that supports SOAP.) The SOAP specification defines the structure of a message that passes from the SOAP sender to the SOAP receiver. Along the way, the message might pass through intermediate nodes that process the information in some way (see Figure 20.2). An intermediate node might provide logging, or it might modify the message somehow in transit to its final destination.

Figure 20.2

Figure 20.2 A SOAP message passes from the sender to the receiver and may pass through intermediate nodes.

At the conceptual level, a SOAP message from the client says "Here is some input. Process this and send me the output." The functionality of the application derives from a series of these XML-based SOAP messages in which the endpoints send information and receive responses. The formal structure of the SOAP message allows the software developer to easily create a SOAP-based client application that interacts with the server. For instance, a rental company that provides car rental reservations through a web-based server application could easily make the specifications available for a developer to write a custom client application that could connect to the server and reserve a car.

The structure of a SOAP message consists of an optional header and a message body. The header contains callouts, definitions, and meta-information that will be used by any node along the message path. The body includes data intended for the message recipient. For example, in the case of the car reservation service, the message body might contain data from the client describing the car the customer would like to rent and the date the vehicle must be available.

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