An Introduction to Web Services - A closer look at SOAP, WSDL and UDDI

Up to this point we have learned about three elements of web service platforms: SOAP, WSDL and UDDI. A developer usually publishes the WSDL for their services in the registry. A client can query the registry using UDDI and get the WSDL from the registry. After that the client can establish a connection with the service and send SOAP requests to the service, whose response will also be SOAP messages.

SOAP is used for communication between applications via the Internet. SOAP is platform independent. SOAP is also language independent; it is based on XML. Each SOAP message has a mandatory SOAP envelope, and multiple optional attachments. A SOAP envelope consists of an optional SOAP header, a mandatory body, and optional fault sections. Usually a SOAP body contains the original requests or the responses.

WSDL is an XML document, which is used to describe web services. WSDL is also used to locate web services. WSDL elements include the following:

Types

• Data type definitions

Messages

• Abstract, typed definitions of data being exchanged (Request/Response)

Operations

• Abstract descriptions of an action
• Refer to input and/or output messages

Port type

• Collection of operations
• Abstract definition of a service

Binding

• Concrete protocol and data format for a particular

Port

• Defines a single communication endpoint
• Endpoint address for binding

Service

• Aggregate set of related ports

UDDI is a directory for storing information about web services. UDDI is also a directory of web service interfaces described by WSDL. UDDI communicates via SOAP.

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