WSDL Essentials

This is Chapter 6: WSDL Essentials from the book Web Services Essentials (ISBN:0-596-00224-6) written by Ethan Cerami, published by O'Reilly & Associates.

WSDL is a specification defining how to describe web services in a common XML grammar. WSDL describes four critical pieces of data:

Interface information describing all publicly available functions

Data type information for all message requests and message responses

Binding information about the transport protocol to be used

Address information for locating the specified service

In a nutshell, WSDL represents a contract between the service requestor and the service provider, in much the same way that a Java interface represents a contract between client code and the actual Java object. The crucial difference is that WSDL is platform- and language-independent and is used primarily (although not exclusively) to describe SOAP services.

Using WSDL, a client can locate a web service and invoke any of its publicly available functions. With WSDL-aware tools, you can also automate this process, enabling applications to easily integrate new services with little or no manual code. WSDL therefore represents a cornerstone of the web service architecture, because it provides a common language for describing services and a platform for automatically integrating those services.

This chapter covers all aspects of WSDL, including the following topics:

An overview of the WSDL specification, complete with detailed explanations of the major WSDL elements

Two basic WSDL examples to get you started

A brief survey of WSDL invocation tools, including the IBM Web Services Invocation Framework (WSIF), SOAP::Lite, and The Mind Electric's GLUE platform

A discussion of how to automatically generate WSDL files from existing SOAP services

An overview of using XML Schema types within WSDL, including the use of arrays and complex types

The WSDL Specification

WSDL is an XML grammar for describing web services. The specification itself is divided into six major elements:

definitions: The definitions element must be the root element of all WSDL documents. It defines the name of the web service, declares multiple namespaces used throughout the remainder of the document, and contains all the service elements described here.
types: The types element describes all the data types used between the client and server. WSDL is not tied exclusively to a specific typing system, but it uses the W3C XML Schema specification as its default choice. If the service uses only XML Schema built-in simple types, such as strings and integers, the types element is not required. A full discussion of the types element and XML Schema is deferred to the end of the chapter.
message: The message element describes a one-way message, whether it is a single message request or a single message response. It defines the name of the message and contains zero or more message part elements, which can refer to message parameters or message return values.
portType: The portType element combines multiple message elements to form a complete one-way or round-trip operation. For example, a portType can combine one request and one response message into a single request/response operation, most commonly used in SOAP services. Note that a portType can (and frequently does) define multiple operations.
binding: The binding element describes the concrete specifics of how the service will be implemented on the wire. WSDL includes built-in extensions for defining SOAP services, and SOAP-specific information therefore goes here.
service: The service element defines the address for invoking the specified service. Most commonly, this includes a URL for invoking the SOAP service.

To help you keep the meaning of each element clear, Figure 6-1 offers a concise representation of the WSDL specification. As you continue reading the remainder of the chapter, you may wish to refer back to this diagram.

**Figure 6-1.** **The WSDL specification in a nutshell**

In addition to the six major elements, the WSDL specification also defines the following utility elements:

documentation: The documentation element is used to provide human-readable documentation and can be included inside any other WSDL element.
import: The import element is used to import other WSDL documents or XML Schemas. This enables more modular WSDL documents. For example, two WSDL documents can import the same basic elements and yet include their own service elements to make the same service available at two physical addresses. Note, however, that not all WSDL tools support the import functionality as of yet.

TIP: WSDL is not an official recommendation of the W3C and, as such, has no official status within the W3C. WSDL Version 1.1 was submitted to the W3C in March 2001. Original submitters included IBM, Microsoft, Ariba, and a half dozen other companies. Most probably, WSDL will be placed under the consideration of the new W3C Web Services Activity's Web Services Description Working Group, which will decide if the specification advances to an official recommendation status. The WSDL Version 1.1 specification is available online at http://www.w3.org/TR/wsdl.

Basic WSDL Example: HelloService.wsdl

To make the previously described WSDL concepts as concrete as possible, let's examine our first sample WSDL file.

Example 6-1 provides a sample HelloService.wsdl document. The document describes the HelloService from Chapter 4.

As you may recall, the service provides a single publicly available function, called sayHello. The function expects a single string parameter, and returns a single string greeting. For example, if you pass the parameter world, the service returns the greeting, "Hello, world!"

Example 6-1: HelloService.wsdl

<?xml version="1.0" encoding="UTF-8"?>
<definitions name="HelloService"
   targetNamespace="http://www.ecerami.com/wsdl/HelloService.wsdl"
   xmlns="http://schemas.xmlsoap.org/wsdl/"
   xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
   xmlns:tns="http://www.ecerami.com/wsdl/HelloService.wsdl"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
   <message name="SayHelloRequest">
      <part name="firstName" type="xsd:string"/>
   </message>
   <message name="SayHelloResponse">
      <part name="greeting" type="xsd:string"/>
   </message>
 
   <portType name="Hello_PortType">
      <operation name="sayHello">
         <input message="tns:SayHelloRequest"/>
         <output message="tns:SayHelloResponse"/>
      </operation>
   </portType>
   
   <binding name="Hello_Binding" type="tns:Hello_PortType">
      <soap:binding style="rpc" 
         transport="http://schemas.xmlsoap.org/soap/http"/>
      <operation name="sayHello">
         <soap:operation soapAction="sayHello"/>
         <input>
            <soap:body
               encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"
               namespace="urn:examples:helloservice"
               use="encoded"/>
         </input>
         <output>
            <soap:body
               encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"
               namespace="urn:examples:helloservice"
               use="encoded"/>
         </output>
      </operation>
   </binding>
 
   <service name="Hello_Service">
      <documentation>WSDL File for HelloService</documentation>
      <port binding="tns:Hello_Binding" name="Hello_Port">
         <soap:address 
            location="http://localhost:8080/soap/servlet/rpcrouter"/>
      </port>
   </service>
</definitions>

The WSDL elements are discussed in the next section of this chapter. As you examine each element in detail, you may want to refer to Figure 6-2, which summarizes the most important aspects of Example 6-1.

Figure 6-2. A bird's-eye view of HelloService.wsdl

definitions

The definitions element specifies that this document is the HelloService. It also specifies numerous namespaces that will be used throughout the remainder of the document:

<definitions name="HelloService"
   targetNamespace="http://www.ecerami.com/wsdl/HelloService.wsdl"
   xmlns="http://schemas.xmlsoap.org/wsdl/"
   xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
   xmlns:tns="http://www.ecerami.com/wsdl/HelloService.wsdl"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema">

The use of namespaces is important for differentiating elements, and it enables the document to reference multiple external specifications, including the WSDL specification, the SOAP specification, and the XML Schema specification.

The definitions element also specifies a targetNamespace attribute. The targetNamespace is a convention of XML Schema that enables the WSDL document to refer to itself. In Example 6-1, we specified a targetNamespace of http://www.ecerami.com/wsdl/HelloService.wsdl. Note, however, that the namespace specification does not require that the document actually exist at this location; the important point is that you specify a value that is unique, different from all other namespaces that are defined.

Finally, the definitions element specifies a default namespace: xmlns=http://schemas.xmlsoap.org/wsdl/. All elements without a namespace prefix, such as message or portType, are therefore assumed to be part of the default WSDL namespace.

message

Two message elements are defined. The first represents a request message, SayHelloRequest, and the second represents a response message, SayHelloResponse:

<message name="SayHelloRequest">
   <part name="firstName" type="xsd:string"/>
</message>
<message name="SayHelloResponse">
   <part name="greeting" type="xsd:string"/>
</message>

Each of these messages contains a single part element. For the request, the part specifies the function parameters; in this case, we specify a single firstName parameter. For the response, the part specifies the function return values; in this case, we specify a single greeting return value.

The part element's type attribute specifies an XML Schema data type. The value of the type attribute must be specified as an XML Schema QName--this means that the value of the attribute must be namespace-qualified. For example, the firstName type attribute is set to xsd:string; the xsd prefix references the namespace for XML Schema, defined earlier within the definitions element.

If the function expects multiple arguments or returns multiple values, you can specify multiple part elements.

portType

The portType element defines a single operation, called sayHello. The operation itself consists of a single input message (SayHelloRequest) and a single output message (SayHelloResponse):

<portType name="Hello_PortType">
   <operation name="sayHello">
      <input message="tns:SayHelloRequest"/>
      <output message="tns:SayHelloResponse"/>
   </operation>
</portType>

Much like the type attribute defined earlier, the message attribute must be specified as an XML Schema QName. This means that the value of the attribute must be namespace-qualified. For example, the input element specifies a message attribute of tns:SayHelloRequest; the tns prefix references the targetNamespace defined earlier within the definitions element.

WSDL supports four basic patterns of operation:

One-way: The service receives a message. The operation therefore has a single input element.
Request-response: The service receives a message and sends a response. The operation therefore has one input element, followed by one output element (illustrated previously in Example 6-1). To encapsulate errors, an optional fault element can also be specified.
Solicit-response: The service sends a message and receives a response. The operation therefore has one output element, followed by one input element. To encapsulate errors, an optional fault element can also be specified.
Notification: The service sends a message. The operation therefore has a single output element.

These patterns of operation are also shown in Figure 6-3. The request-response pattern is most commonly used in SOAP services.

Figure 6-3. Operation patterns supported by WSDL 1.1

binding

The binding element provides specific details on how a portType operation will actually be transmitted over the wire. Bindings can be made available via multiple transports, including HTTP GET, HTTP POST, or SOAP. In fact, you can specify multiple bindings for a single portType.

The binding element itself specifies name and type attributes:

<binding name="Hello_Binding" type="tns:Hello_PortType">

The type attribute references the portType defined earlier in the document. In our case, the binding element therefore references tns:Hello_PortType, defined earlier in the document. The binding element is therefore saying, "I will provide specific details on how the sayHello operation will be transported over the Internet."

SOAP binding

WSDL 1.1 includes built-in extensions for SOAP 1.1. This enables you to specify SOAP-specific details, including SOAP headers, SOAP encoding styles, and the SOAPAction HTTP header. The SOAP extension elements include:

soap:binding: This element indicates that the binding will be made available via SOAP. The style attribute indicates the overall style of the SOAP message format. A style value of rpc specifies an RPC format. This means that the body of the SOAP request will include a wrapper XML element indicating the function name. Function parameters are then embedded inside the wrapper element. Likewise, the body of the SOAP response will include a wrapper XML element that mirrors the function request. Return values are then embedded inside the response wrapper element.; A style value of document specifies an XML document call format. This means that the request and response messages will consist simply of XML documents. The document style is flatter than the rpc style and does not require the use of wrapper elements. (See the upcoming note for additional details.); The transport attribute indicates the transport of the SOAP messages. The valuehttp://schemas.xmlsoap.org/soap/http indicates the SOAP HTTP transport, whereas http://schemas.xmlsoap.org/soap/smtp indicates the SOAP SMTP transport.
soap:operation: This element indicates the binding of a specific operation to a specific SOAP implementation. The soapAction attribute specifies that the SOAPAction HTTP header be used for identifying the service. You can ignore this attribute without much harm.
soap:body: This element enables you to specify the details of the input and output messages. In the case of HelloWorld, the body element specifies the SOAP encoding style and the namespace URN associated with the specified service.

TIP: The choice between the rpc style and the document style is controversial. The topic has been hotly debated on the WSDL newsgroup (http://groups.yahoo.com/group/wsdl). The debate is further complicated because not all WSDL-aware tools even differentiate between the two styles. Because the rpc style is more in line with the SOAP examples from previous chapters, I have chosen to stick with the rpc style for all the examples within this chapter. Note, however, that most Microsoft .NET WSDL files use the document style.

service

The service element specifies the location of the service. Because this is a SOAP service, we use the soap:address element, and specify the local host address for the Apache SOAP rpcrouter servlet: http://localhost:8080/soap/servlet/rpcrouter.

Note that the service element includes a documentation element to provide human-readable documentation.

WSDL Invocation Tools

Given the WSDL file in Example 6-1, you could manually create a SOAP client to invoke the service. A better alternative is to automatically invoke the service via a WSDL invocation tool. (See Figure 6-4.)

Figure 6-4. WSDL invocation tools

Many WSDL invocation tools already exist. This section provides a brief overview of three invocation tools.

GLUE

The Mind Electric provides a complete web service platform called GLUE (available at http://www.themindelectric.com/). The platform itself provides extensive support for SOAP, WSDL, and UDDI. Some of its advanced functionality, including support for complex data types, will be explored later in this chapter.

For now, you can try out the GLUE invoke command-line tool. Here is the command-line usage:

usage: invoke URL method arg1 arg2 arg3...

For example, to invoke the HelloService, make sure that your Apache Tomcat server is running, and place the HelloService.wsdl file within a publicly available directory. Then, issue the following command:

invoke http://localhost:8080/wsdl/HelloService.wsdl sayHello World

Once invoked, GLUE will immediately download the specified WSDL file, invoke the sayHello method, and pass World as a parameter. GLUE will then automatically display the server response:

Output:  result = Hello, World!

That's all there is to it!

GLUE also supports an excellent logging facility that enables you to easily view all SOAP messages. To activate the logging facility, set the electric.logging system property. The easiest option is to modify the invoke.bat file. The original file looks like this:

call java electric.glue.tools.Invoke %1 %2 %3 %4 %5 %6 %7 %8 %9

Modify the file to include the logging property via the -D option to the Java interpreter:

call java -Delectric.logging="SOAP" electric.glue.tools.Invoke %1 %2 %3 %4
    %5 %6 %7 %8 %9

When you invoke the HelloService, GLUE now generates the following output:

LOG.SOAP: request to http://207.237.201.187:8080/soap/servlet/rpcrouter
<?xml version='1.0' encoding='UTF-8'?>
<soap:Envelope 
   xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
   xmlns:xsd='http://www.w3.org/2001/XMLSchema'
   xmlns:soap='http://schemas.xmlsoap.org/soap/
   envelope/' xmlns:soapenc='http://schemas.xmlsoap.org/soap/encoding/' 
   soap:encodingStyle='http://schemas.xmlsoap.org/soap/encoding/'>
   <soap:Body>
      <n:sayHello xmlns:n='urn:examples:helloservice'>
         <firstName xsi:type='xsd:string'>World</firstName>
      </n:sayHello>
   </soap:Body>
</soap:Envelope>
 
LOG.SOAP: response from http://207.237.201.187:8080/soap/servlet/rpcrouter
<?xml version='1.0' encoding='UTF-8'?>
<SOAP-ENV:Envelope 
   xmlns:SOAP-ENV='http://schemas.xmlsoap.org/soap/envelope/'
   xmlns:xsi='http://www.w3.org/1999/XMLSchema-instance'
   xmlns:xsd='http://www.w3.org/1999/XMLSchema'>
   <SOAP-ENV:Body>
      <ns1:sayHelloResponse 
         xmlns:ns1='urn:examples:helloservice' 
         SOAP-ENV:encodingStyle=
            'http://schemas.xmlsoap.org/soap/encoding/'>
         <return xsi:type='xsd:string'>Hello, World!</return>
      </ns1:sayHelloResponse>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>
 
result = Hello, World!

To view additional HTTP information, just set electric.logging to SOAP,HTTP.

SOAP::Lite for Perl

SOAP::Lite for Perl, written by Paul Kulchenko, also provides limited support for WSDL. The package is available at http://www.soaplite.com.

Example 6-2 provides a complete Perl program for invoking the HelloService.

Example 6-2: Hello_Service.pl

use SOAP::Lite;
 
print "Connecting to Hello Service...\n";
print SOAP::Lite
   -> service('http://localhost:8080/wsdl/HelloService.wsdl')
   -> sayHello ('World');

The program generates the following output:

Connecting to Hello Service...
Hello, World!

IBM Web Services Invocation Framework (WSIF)

Finally, IBM has recently released WSIF. The package is available at http://www.alphaworks.ibm.com/tech/wsif.

Much like GLUE, WSIF provides a simple command-line option for automatically invoking WSDL services. For example, the following command:

java clients.DynamicInvoker http://localhost:8080/wsdl/HelloService.wsdl
    sayHello World

generates the following output:

Reading WSDL document from 'http://localhost:8080/wsdl/HelloService.wsdl'
Preparing WSIF dynamic invocation
Executing operation sayHello
Result:
greeting=Hello, World!
 
Done!

Basic WSDL Example: XMethods eBay Price Watcher Service

Before moving on to more complicated WSDL examples, let's examine another relatively simple one. Example 6-3 provides a WSDL file for the XMethods eBay Price Watcher Service. The service takes an existing eBay auction ID, and returns the value of the current bid.

Example 6-3: eBayWatcherService.wsdl (reprinted with permission of XMethods, Inc.)

<?xml version="1.0"?>
<definitions name="eBayWatcherService" 
   targetNamespace=
      "http://www.xmethods.net/sd/eBayWatcherService.wsdl"
   xmlns:tns="http://www.xmethods.net/sd/eBayWatcherService.wsdl"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema"
   xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
   xmlns="http://schemas.xmlsoap.org/wsdl/">
  
   <message name="getCurrentPriceRequest">
      <part name="auction_id" type = "xsd:string"/>
   </message>
   <message name="getCurrentPriceResponse">
      <part name="return" type = "xsd:float"/>
   </message>
  
   <portType name="eBayWatcherPortType">
      <operation name="getCurrentPrice">
         <input 
            message="tns:getCurrentPriceRequest"
            name="getCurrentPrice"/>
         <output 
            message="tns:getCurrentPriceResponse"
            name="getCurrentPriceResponse"/>
      </operation>
   </portType>
  
   <binding name="eBayWatcherBinding" type="tns:eBayWatcherPortType">
      <soap:binding 
         style="rpc"
         transport="http://schemas.xmlsoap.org/soap/http"/>
      <operation name="getCurrentPrice">
         <soap:operation soapAction=""/>
         <input name="getCurrentPrice">
            <soap:body 
               use="encoded" 
               namespace="urn:xmethods-EbayWatcher"
               encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
         </input>
         <output name="getCurrentPriceResponse">
            <soap:body
               use="encoded"
               namespace="urn:xmethods-EbayWatcher"
               encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
         </output>
      </operation>
   </binding>
  
   <service name="eBayWatcherService">
      <documentation>
         Checks current high bid for an eBay auction
      </documentation>
      <port name="eBayWatcherPort" binding="tns:eBayWatcherBinding">
         <soap:address 
          location="http://services.xmethods.net:80/soap/servlet/rpcrouter"/>
      </port>
   </service>
</definitions>

Here is an overview of the main WSDL elements:

messages: Two messages are defined: getCurrentPriceRequest and getCurrentPriceResponse. The request message contains a single string parameter; the response message contains a single float parameter.
portType: A single operation, getCurrentPrice, is defined. Again, we see the request/response operation pattern.
binding: The binding element specifies HTTP SOAP as the transport. The soapAction attribute is left as an empty string ("").
service: This element specifies that the service is available at http://services.xmethods.net/soap/servlet/rpcrouter.

To access the eBay watcher service, you can use any of the WSDL invocation tools defined earlier. For example, the following call to GLUE:

invoke http://www.xmethods.net/sd/2001/EBayWatcherService.wsdl
    getCurrentPrice 1271062297

retrieves the current bid price for a Handspring Visor Deluxe:

result = 103.5