Concepts

Two key parts of the Web Services infrastructure are WSDL, a language for describing Web Services; and SOAP, a protocol that enables applications an Web Services to communicate.

About WSDL

The Web Services Description Language (WSDL) is an XML language for describing the syntax of Web Service interfaces and their locations. The WSDL specification calls it "an XML format for describing network services as a set of endpoints operating on messages containing either document-oriented or procedure-oriented information." Within that context, the diagram below illustrates the elements that are present in a WSDL document, and shows how they are related.

WSDL element hierarchy.

Programmers or automated development tools can create WSDL files to describe a service and can make the description available over the Internet. Client-side programmers and development tools can use published WSDL descriptions to obtain information about available Web Services and to build and create proxies or program templates that access available services.

A WSDL document has a definitions element that contains the types, message, portType, binding, and service elements as described in the following table.

definitions	Defines one or more services. A `definition` element supports the following attributes: `name` is optional. `targetNamespace` is the logical namespace for information about this service. WSDL documents can import other WSDL documents, and setting `targetNamespace` to a unique value ensures that the namespaces do not clash. `xmlns` is the default namespace of the WSDL document, and it is set to `http://schemas.xmlsoap.org/wsdl/`. All the WSDL elements, such as `<definitions>`, `<types>` and `<message>` reside in this namespace. `xmlns:xsd` and `xmlns:soap` are standard namespace definitions that are used for specifying SOAP-specific information as well as data types. `xmlns:tns` stands for this namespace.
types	Provides information about any complex data types used in the WSDL document. When simple types are used, the WSDL document does not need this section.
message	An abstract definition of the data being communicated.
operation	An abstract description of the action supported by the service.
portType	An abstract set of operations supported by one or more endpoints.
binding	Describes how the operation is invoked by specifying concrete protocol and data format specifications for the operations and messages.
port	Specifies a single endpoint as an address for the binding, thus defining a single communication endpoint.
service	Specifies the port address(es) of the binding. The service is a collection of network endpoints or ports.

The following listing shows the WSDL code for the Exchange Web Service used by the FBS.

<?xml version = '1.0' encoding = 'windows-1252'?>
<!--Generated by the Oracle9i JDeveloper Web Services WSDL Generator-->
<!--Date Created: Tue Apr 23 15:09:13 IST 2002-->
<definitions
   name="Exchange"
   targetNamespace=
    "http://tempuri.org/jdeveloper/generated/oracle/otnsamples/ibfbs/admin/ejb/Exchange"
   xmlns="http://schemas.xmlsoap.org/wsdl/"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema"
   xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
   xmlns:tns=
    "http://tempuri.org/jdeveloper/generated/oracle/otnsamples/ibfbs/admin/ejb/Exchange"
   xmlns:ns1=
    "http://tempuri.org/jdeveloper/generated/oracle/otnsamples/ibfbs/admin/ejb/Exchange/schema">
   <types>
      <schema
         targetNamespace=
          "http://tempuri.org/jdeveloper/generated/oracle/otnsamples/ibfbs/admin/ejb/Exchange/schema"
         xmlns="http://www.w3.org/2001/XMLSchema"
         xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/"/>
   </types>
   <message name="processOrder0Request">
      <part name="toAddress" type="xsd:string"/>
      <part name="symbol" type="xsd:string"/>
      <part name="tradeDate" type="xsd:dateTime"/>
      <part name="tradeType" type="xsd:string"/>
      <part name="qty" type="xsd:int"/>
   </message>
   <message name="processOrder0Response">
      <part name="return" type="xsd:int"/>
   </message>
   <portType name="ExchangePortType">
      <operation name="processOrder">
         <input name="processOrder0Request" message="tns:processOrder0Request"/>
         <output name="processOrder0Response" message="tns:processOrder0Response"/>
      </operation>
   </portType>
   <binding name="ExchangeBinding" type="tns:ExchangePortType">
      <soap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http"/>
      <operation name="processOrder">
         <soap:operation soapAction="" style="rpc"/>
         <input name="processOrder0Request">
            <soap:body use="encoded" namespace="oracle.otnsamples.ibfbs.admin.ejb.Exchange" 
                       encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
         </input>
         <output name="processOrder0Response">
            <soap:body use="encoded" namespace="oracle.otnsamples.ibfbs.admin.ejb.Exchange" 
                       encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
         </output>
      </operation>
   </binding>
   <service name="Exchange">
      <port name="ExchangePort" binding="tns:ExchangeBinding">
         <soap:address location="http://incq212e.idc.oracle.com:8888/ibfbs/ExchangeService"/>
      </port>
   </service>
</definitions>

About SOAP

The Simple Object Access Protocol (SOAP) is a lightweight, XML-based protocol for exchanging information in a decentralized, distributed environment. SOAP supports different styles of information exchange, including:

Remote Procedure Call style (RPC), which allows for request-response processing, where an endpoint receives a procedure oriented message and replies with a correlated response message.
Message-oriented information exchange, which supports organizations and applications that need to exchange business or other types of documents where a message is sent but the sender may not expect or wait for an immediate response.

SOAP has the following features:

Protocol independence
Language independence
Platform and operating system independence
Support for SOAP XML messages incorporating attachments (using the multipart MIME structure)

A SOAP message consists of a SOAP envelope that encloses two data structures, the SOAP header and the SOAP body, and information about the name spaces used to define them. The header is optional; when present, it conveys information about the request defined in the SOAP body. For example, it might contain transactional, security, contextual, or user profile information. The body contains a Web Service request or reply to a request in XML format. The high-level structure of a SOAP message is shown in the following figure.

SOAP message structure.

Here is a SOAP message that corresponds to the WSDL listed above:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"      
  SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">      
  <SOAP-ENV:Body> 
     <m:processOrder xmlns:m="http://tempuri.org/jdeveloper/generated/oracle/otnsamples/ibfbs/admin/ejb/Exchange">      

     <m:toAddress>abcd@yahoo.com</m:toAddress>
     <m:symbol>ORCL</m:symbol>
     <m:tradeDate>2001-12-15T13:20:00-05:00</m:tradeDate>
     <m:tradeType>B</m:tradeType>
     <m:qty>50</m:qty>
     </m:processOrder> 
  </SOAP-ENV:Body> 
</SOAP-ENV:Envelope>

SOAP messages, when used to carry Web Service requests and responses, can conform to the WSDL definition of available Web Services. WSDL can define the SOAP message used to access the Web Services, the protocols over which such SOAP messages can be exchanged, and the Internet locations where these Web Services can be accessed. The WSDL descriptors can reside in UDDI or other directory services, and they can also be provided via configuration or other means such as in the body of SOAP request replies.

The SOAP specification provides a standard way to encode requests and responses. It describes the structure and data types of message payloads using XML Schema. The way that SOAP is used for the message and response of a Web Service is:

The SOAP client uses an XML document that conforms to the SOAP specification and which contains a request for the service.
The SOAP client sends the document to a SOAP server, and the SOAP servlet running on the server handles the document using, for example, HTTP or HTTPS.
The Web service receives the SOAP message, and dispatches the message as a service invocation to the application providing the requested service.
A response from the service is returned to the SOAP server, again using the SOAP protocol, and this message is returned to the originating SOAP client.

SOAP provides a way to leverage the industry investment in XML. Also, since SOAP is typically defined over "firewall friendly" protocols such as HTTP and SMTP, the industry investment in firewall technology is leveraged as well. Thus, by defining SOAP as an essential part of Web Services, the industry will likely enjoy volume production use of Web Services far sooner than if other strategies had been employed.