What's New in WSDL 2.0
The W3C's Web Services Description Working Group, part of the Web Services Activity, has defined a language for describing web services and the possible ways to interact with them. The WG published its WSDL 2.0 working drafts on 26 March 2004. This is a significant milestone in the progress of WSDL. In this article, I discuss the changes that were made to the WSDL 1.1 specification and other major improvements to the service description language.
W3C has published the following core working drafts as part of its working group deliverables:
Web Services Description Language (WSDL) Version 2.0 Part 1: Core Language
Web Services Description Language (WSDL) Version 2.0 Part 2: Message Patterns
Web Services Description Language (WSDL) Version 1.2 Part 3: Bindings
Other related working drafts include requirements and usage scenarios.
The W3C XML Schema definition for WSDL 2.0 specification can be found at http://www.w3.org/2003/11/wsdl/.
The editor's copies of these documents provide updated information about the progress of these specifications.
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Related Reading 
Web Services Essentials |
WSDL 1.2 was renamed WSDL 2.0 because of its substantial differences from WSDL 1.1. Some of these changes include:
Adding further semantics to the description language. This is one
of the reasons for making targetNamespace a required attribute
of the definitions element in WSDL 2.0.
Removal of message constructs. These are specified using the XML
schema type system in the types element.
No support for operator overloading.
PortTypes renamed to interfaces. Support for
interface inheritance is achieved by using the extends
attribute in the interface element.
Ports renamed to endpoints.
The description of a web service can be modeled in two parts. In
the abstract part, WSDL describes a web service in terms of messages
it sends and receives through a type system, typically W3C XML
Schema. Message exchange patterns define the sequence and cardinality
of messages. An operation associates message exchange patterns
with one or more messages. An interface groups these operations
in a transport and wire independent manner.
In the concrete part of the description, bindings specify
the transport and wire format for interfaces. A service
endpoint associates network address with a binding. Finally,
a service groups the endpoints that implement a common
interface. Figure 1 shows the conceptual WSDL component model.
Figure 1: WSDL Conceptual model
WSDL provides a set of components and their associated properties for describing web services. Listing 2 shows the skeleton of the WSDL 2.0 description. The following section gives a brief overview about each of these components.
<definitions targetNamespace="xs:anyURI">
<documentation /> ?
[<import /> | <include /> ] *
<types /> ?
[<interface /> | <binding /> | <service /> ] *
</definitions>Listing 2: WSDL 2.0 Skeleton
The definitions element is the root of any WSDL document.
It serves as a container which holds all the necessary information
about the service and its attributes. Figure 2 shows the schema for
the definitions element. The
targetNamespace attribute of the definitions element is
a required attribute of type
anyURI. The namespace can directly or indirectly define the
semantics of the WSDL. Also, the
definitions element can have other optional attributes which
correspond to various namespaces that may be used with in the WSDL
document.
Figure 2: definitions schema
The include element helps to modularize the web service
descriptions so that separation of various service definition
components from the same target namespace can be allowed to exist in
another WSDL document which can be used or shared across web service
descriptions. The location attribute is mandatory and specifies
the location of these WSDL documents. The actual value of the target
namespace of the included WSDL must match the target namespace of
the definitions element in the including WSDL. Figure 3 shows
the XML schema for the include element.
Figure 3: include schema
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The concept behind the import element is very similar to
that of include element, except that the imported WSDL can
be in different target namespaces. The namespace attribute
for the import element is mandatory while the
location attribute is optional. Figure 4 shows the XML
schema for the import element.
Figure 4: import schema
The types element defines the data types used by the
exchanged messages. WSDL uses W3C XML Schema as its preferred
schema language. It also supports other systems such as DTDs and
RELAX NG. Schemas may be used by importing or embedding within the
types element of the WSDL document. Importing can be done
using the construct xs:import while embedding uses the
construct xs:schema. The imported or embedded schema
components are available to WSDL for reference by QName. Figure 5
shows the XML schema for the types element.
Figure 5: types schema
An interface element encloses a named set of abstract
operations and the abstract messages. It can optionally extend one
or more other interfaces. Interfaces are referred to by QName in
other components such as bindings. The interface operation
element has name and pattern as required attributes, while style is
an optional attribute. Figure 6 shows the schema for the
interface element. Features defines functionalities
associated with the message exchanges between communicating
parties, which might include reliability, security, correlation,
and routing. Property is used to control the behavior of a feature.
It has a set of possible and permissible values specified by
references to a schema description. These values can be shared
among features.
Figure 6: interface schema
The binding element defines the underlying transport and
wire format for messages. Each binding in the WSDL references to an
interface. All operations defined within the interface must be bound
in the binding. An endpoint in the service component references a
binding. Both endpoints and bindings are modeled to support
flexibility and location transparency. Multiple endpoints with
different network address can still share the same protocol
binding. WSDL 2.0 Bindings specification defines binding extensions
for protocols and message formats such as SOAP, HTTP and MIME. Figure
7 shows the XML schema for the binding element.
Figure 7: binding schema
A service element describes a set of endpoints which
refer to a single network address for a binding. All other protocol
specific information is contained in the binding. Service can be
referred by QName. service element has a name and
interface which are required attributes. Figure 8 shows the
XML schema for the service element.
Figure 8: service schema
Message exchange patterns define the sequence and cardinality of messages within an operation. Several types of message patterns are defined in the “Part 2: Message Patterns†of the WSDL 2.0 specifications. WSDL message exchange patterns uses fault generation rules to indicate the occurrence of faults. Message exchange may be terminated if fault generation happens regardless of standard rule sets. The following standard rule set outlines the behavior of fault generation.
Fault Replaces Messages
Message Triggers Fault
No Faults
Figure 9 shows the various message exchange patterns with their fault reference rules.
Figure 9: WSDL Message Exchange Patterns
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In this section, you can have a preview of a simple stock quote service which is described using WSDL 1.1 and WSDL 2.0. Listing 2 shows the XML schema types that are used in the describing stock quote service. Listing 3 and 4 shows the stock quote service interface definition in WSDL 1.1 and WSDL 2.0 respectively. Listing 5 and 6 shows the stock quote service implementation definition in WSDL 1.1 and WSDL 2.0 respectively.
http://example.com/stockquote/stockquoteV11.xsd
<?xml version="1.0"?>
<schema targetNamespace="http://example.com/stockquote/schemas"
xmlns="http://www.w3.org/2000/10/XMLSchema">
<element name="TradePriceRequest">
<complexType>
<all>
<element name="tickerSymbol" type="string"/>
</all>
</complexType>
</element>
<element name="TradePrice">
<complexType>
<all>
<element name="price" type="float"/>
</all>
</complexType>
</element>
</schema>http://example.com/stockquote/stockquoteV20.xsd
<?xml version="1.0"?>
<schema targetNamespace="http://example.com/stockquote/schemas"
xmlns="http://www.w3.org/2001/XMLSchema">
<element name="TradePriceRequest">
<complexType>
<all>
<element name="tickerSymbol" type="string"/>
</all>
</complexType>
</element>
<element name="TradePrice">
<complexType>
<all>
<element name="price" type="float"/>
</all>
</complexType>
</element>
</schema>Listing 2: XML Schema definition for Stock Quote Service
http://example.com/stockquote/stockquoteV11.wsdl
<?xml version="1.0"?>
<definitions name="StockQuote"
targetNamespace="http://example.com/stockquote/definitions"
xmlns:tns="http://example.com/stockquote/definitions"
xmlns:xsd1="http://example.com/stockquote/schemas"
xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<import namespace="http://example.com/stockquote/schemas"
location="http://example.com/stockquote/stockquoteV11.xsd"/>
<message name="GetLastTradePriceInput">
<part name="body" element="xsd1:TradePriceRequest"/>
</message>
<message name="GetLastTradePriceOutput">
<part name="body" element="xsd1:TradePrice"/>
</message>
<portType name="StockQuotePortType">
<operation name="GetLastTradePrice">
<input message="tns:GetLastTradePriceInput"/>
<output message="tns:GetLastTradePriceOutput"/>
</operation>
</portType>
</definitions>
Listing 3: WSDL 1.1 Interface definition for Stock Quote Service
http://example.com/stockquote/stockquoteV20.wsdl
<?xml version="1.0"?>
<definitions name="StockQuote"
targetNamespace="http://example.com/stockquote/definitions"
xmlns:tns="http://example.com/stockquote/definitions"
xmlns:xsd1="http://example.com/stockquote/schemas"
xmlns:soap="http://www.w3.org/2003/11/wsdl/soap12"
xmlns="http://www.w3.org/2003/11/wsdl">
<import namespace="http://example.com/stockquote/schemas"
location="http://example.com/stockquote/stockquoteV20.xsd"/>
<types>
<schema targetNamespace="http://example.com/stockquote/definitions">
<element name="GetLastTradePriceInput" type="xsd1:TradePriceRequest"/>
<element name="GetLastTradePriceOutput" type="xsd1:TradePrice"/>
</schema>
</types>
<interface name="StockQuoteInterface">
<operation name="GetLastTradePrice" pattern="http://www.w3.org/2003/11/wsdl/in-out">
<input message="tns:GetLastTradePriceInput"/>
<output message="tns:GetLastTradePriceOutput"/>
</operation>
</interface>
</definitions>
Listing 4: WSDL 2.0 Interface definition for Stock Quote Service
http://example.com/stockquote/stockquoteserviceV11.wsdl
<?xml version="1.0"?>
<definitions name="StockQuote"
targetNamespace="http://example.com/stockquote/service"
xmlns:tns="http://example.com/stockquote/service"
xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
xmlns:defs="http://example.com/stockquote/definitions"
xmlns="http://schemas.xmlsoap.org/wsdl/">
<import namespace="http://example.com/stockquote/definitions"
location="http://example.com/stockquote/stockquoteV11.wsdl"/>
<binding name="StockQuoteSoapBinding" type="defs:StockQuotePortType">
<soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http"/>
<operation name="GetLastTradePrice">
<soap:operation soapAction="http://example.com/GetLastTradePrice"/>
<input>
<soap:body use="literal"/>
</input>
<output>
<soap:body use="literal"/>
</output>
</operation>
</binding>
<service name="StockQuoteService">
<documentation>My first service</documentation>
<port name="StockQuotePort" binding="tns:StockQuoteBinding">
<soap:address location="http://example.com/stockquote"/>
</port>
</service>
</definitions>
Listing 5: WSDL 1.1 Implementation definition for Stock Quote Service
http://example.com/stockquote/stockquoteserviceV20.wsdl
<?xml version="1.0"?>
<definitions name="StockQuote"
targetNamespace="http://example.com/stockquote/service"
xmlns:tns="http://example.com/stockquote/service"
xmlns:wsoap="http://www.w3.org/2003/11/wsdl/soap12"
xmlns:defs="http://example.com/stockquote/definitions"
xmlns="http://www.w3.org/2003/11/wsdl">
<import namespace="http://example.com/stockquote/definitions"
location="http://example.com/stockquote/stockquoteV12.wsdl"/>
<binding name="StockQuoteSoapBinding" interface="defs:StockQuoteInterface">
<wsoap:binding protocol="http://www.w3.org/2003/11/wsdl/http"/>
<operation name="GetLastTradePrice">
<wsoap:operation soapAction="http://example.com/GetLastTradePrice"/>
<input>
<wsoap:body/>
</input>
<output>
<wsoap:body/>
</output>
</operation>
</binding>
<service name="StockQuoteService">
<documentation>My stock quote service</documentation>
<endpoint name="StockQuoteEndPoint" binding="tns:StockQuoteSoapBinding">
<wsoap:address location="http://example.com/stockquote"/>
</endpoint>
</service>
</definitions>
Listing 6: WSDL 2.0 Implementation definition for Stock Quote Service
In this article I have explained some details of the working draft WSDL 2.0 specifications. Discussions are happening in this working group for adding more features to the existing specification in order to provide a highly flexible and robust language for describing web services. Some of the features include web service references, versioning, attributes, and compositors. In addition to these new features, further refinements are made to the existing specification. The developer community is anticipating a more stable version of WSDL 2.0 specification in the near future.
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