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Semantic Web Services 171
SOAP
SOAP [52] is a protocol for exchanging XML-based messages over computer networks. It
normally uses existing transport protocols HTTP/HTTPS. SOAP is fundamentally stateless,
one-way messaging, but by combining the one-way exchanges, more complex communication
patterns can be created. A core SOAP has a very simple structure. It is an XML
element with two child elements for the header and the body. One of the most common
messaging patterns in SOAP is the RPC (Remote Procedure Call). In this pattern the client
sends a request message to the server which sends immediately the response.
WSDL
SOAP offers a basic communication pattern for web services. To be able to successfully
interact with a service, we need to know which messages must be exchanged. For this
purpose we use WSDL, an XML based language for description of the interfaces of Web
services.
UDDI
UDDI is a way how to find a web service. It is a centralized registry of web services. It has
three components:
– White pages: name and contact details.
– Yellow pages: categorization based on standard taxonomies.
– Green pages: technical data about services.
UDDI can be accessed using SOAP API. Using it, one can perform querying and updating
of the registry. In Figure 6-3, you see how the SOAP, WSDL and UDDI work together to
provide means for using web services.
Service
broker
UDDI
WSDL
WSDL
f(x)
SOAP
Service
requestor
Service
provider
Figure 6-3. Web service standards’ collaboration.