Difference between a SOAP message and a WSDL?

Question

I am confused about how SOAP messages and WSDL fit together? I have started looking into SOAP messages such as:

    POST /InStock HTTP/1.1 Host: www.example.org Content-Type: application/soap+xml; charset=utf-8 Content-Length: nnn  <?xml version="1.0"?> <soap:Envelope xmlns:soap="http://www.w3.org/2001/12/soap-envelope" soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">  <soap:Body xmlns:m="http://www.example.org/stock">   <m:GetStockPrice>     <m:StockName>IBM</m:StockName>   </m:GetStockPrice> </soap:Body>  </soap:Envelope> 

Are all SOAP messages WSDL's? Is SOAP a protocol that accepts its own 'SOAP messages' or 'WSDL's? If they are different, then when should I use SOAP messages and when should I use WSDL's?

Some clarification around this would be awesome.

Answer 1

A SOAP document is sent per request. Say we were a book store, and had a remote server we queried to learn the current price of a particular book. Say we needed to pass the Book's title, number of pages and ISBN number to the server.

Whenever we wanted to know the price, we'd send a unique SOAP message. It'd look something like this;

<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:GetBookPrice xmlns:m="http://namespaces.my-example-book-info.com">       <ISBN>978-0451524935</ISBN>       <Title>1984</Title>       <NumPages>328</NumPages>     </m:GetBookPrice>   </SOAP-ENV:Body> </SOAP-ENV:Envelope>  

And we expect to get a SOAP response message back like;

<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:GetBookPriceResponse xmlns:m="http://namespaces.my-example-book-info.com">       <CurrentPrice>8.99</CurrentPrice>       <Currency>USD</Currency>     </m:GetBookPriceResponse>   </SOAP-ENV:Body> </SOAP-ENV:Envelope> 

The WSDL then describes how to handle/process this message when a server receives it. In our case, it describes what types the Title, NumPages & ISBN would be, whether we should expect a response from the GetBookPrice message and what that response should look like.

The types would look like this;

<wsdl:types>    <!-- all type declarations are in a chunk of xsd -->   <xsd:schema targetNamespace="http://namespaces.my-example-book-info.com"     xmlns:xsd="http://www.w3.org/1999/XMLSchema">      <xsd:element name="GetBookPrice">       <xsd:complexType>         <xsd:sequence>           <xsd:element name="ISBN" type="string"/>           <xsd:element name="Title" type="string"/>           <xsd:element name="NumPages" type="integer"/>         </xsd:sequence>       </xsd:complexType>     </xsd:element>      <xsd:element name="GetBookPriceResponse">       <xsd:complexType>         <xsd:sequence>           <xsd:element name="CurrentPrice" type="decimal" />           <xsd:element name="Currency" type="string" />         </xsd:sequence>       </xsd:complexType>     </xsd:element>    </xsd:schema> </wsdl:types> 

But the WSDL also contains more information, about which functions link together to make operations, and what operations are avaliable in the service, and whereabouts on a network you can access the service/operations.

See also W3 Annotated WSDL Examples