Hotel GreatH (a fictional hotel) is located in a remote island. It has been relying on fax and phone to provide room reservations. Even though the facilities and prices at GreatH are better than what its competitor offers, GreatH notices that its competitor is getting more customers than GreatH. After research, GreatH realizes that this is because the competitor offers a Web service that permits travel agent reservation systems to reserve rooms directly over the Internet. GreatH then hires us to build a reservation Web service with the following functionality:

CheckAvailability. To check availability, the client must specify a check-in date, a check-out date, and room type. The Web service will return a room rate (a floating point number in USD) if such a room is available, or a zero room rate if not. If any input data is invalid, the service should return an error. Thus, the service will accept a checkAvailability message and return a checkAvailabilityResponse or invalidDataFault message.

The next several sections proceed step-by-step through the process of developing a WSDL 2.0 document that describes the desired Web service. However, for those who can't wait to see a complete example, here is the WSDL 2.0 document that we'll be creating.

Before writing our WSDL 2.0 document, we need to decide on a WSDL 2.0 target namespace URI for it. The WSDL 2.0 target namespace is analogous to an XML Schema target namespace. Interface, binding and service names that we define in our WSDL 2.0 document will be associated with the WSDL 2.0 target namespace, and thus will be distinguishable from similar names in a different WSDL 2.0 target namespace. (This will become important if using WSDL 2.0's import or interface inheritance mechanisms.)

The value of the WSDL 2.0 target namespace must be an absolute URI. Furthermore, it should be dereferenceable to a WSDL 2.0 document that describes the Web service that the WSDL 2.0 target namespace is used to describe. For example, the GreatH owners should make the WSDL 2.0 document available from this URI. (And if a WSDL 2.0 description is split into multiple documents, then the WSDL 2.0 target namespace should resolve to a master document that includes all the WSDL 2.0 documents needed for that service description.) However, there is no absolute requirement for this URI to be dereferenceable, so a WSDL 2.0 processor must not depend on it being dereferenceable.

This recommendation may sound circular, but bear in mind that the client might have obtained the WSDL 2.0 document from anywhere -- not necessarily an authoritative source. But by dereferencing the WSDL 2.0 target namespace URI, a user should be able to obtain an authoritative version. Since GreatH will be the owner of the service, the WSDL 2.0 target namespace URI should refer to a location on the GreatH Web site or otherwise within its control.

Once we have decided on a WSDL 2.0 target namespace URI, we can begin our WSDL 2.0 document as the following empty shell.

We know that the GreatH service will be sending and receiving messages, so a good starting point in describing the service is to define the message types that the service will use. We'll use XML Schema to do so, because WSDL 2.0 processors are likely to support XML Schema at a minimum. However, WSDL 2.0 does not prohibit the use of some other schema definition language.

WSDL 2.0 allows message types to be defined directly within the WSDL 2.0 document, inside the types element, which is a child of the description element. (Later we'll see how we can provide the type definitions in a separate document, using XML Schema's import mechanism.) The following schema defines checkAvailability, checkAvailabilityResponse and invalidDataError message types that we'll need.

In WSDL 2.0, all normal and fault message types must be defined as single elements at the topmost level (though of course each element may have any amount of substructure inside it). Thus, a message type must not directly consist of a sequence of elements or other complex type.

WSDL 2.0 enables one to separate the description of a Web service's abstract functionality from the concrete details of how and where that functionality is offered. This separation facilitates different levels of reusability and distribution of work in the lifecycle of a Web service and the WSDL 2.0 document that describes it.

A WSDL 2.0 interface defines the abstract interface of a Web service as a set of abstract operations, each operation representing a simple interaction between the client and the service. Each operation specifies the types of messages that the service can send or receive as part of that operation. Each operation also specifies a message exchange pattern that indicates the sequence in which the associated messages are to be transmitted between the parties. For example, the in-out pattern (see WSDL 2.0 Predefined Extensions section 2.2.3 In-Out) indicates that if the client sends a message in to the service, the service will either send a reply message back out to the client (in the normal case) or it will send a fault message back to the client (in the case of an error). We will explain more about message exchange patterns in

For the GreatH service, we will (initially) define an interface containing a single operation, opCheckAvailability, using the checkAvailability and checkAvailabilityResponse message types that we defined in the types section. We'll use the in-out pattern for this operation, because this is the most natural way to represent a simple request-response interaction. We could have instead (for example) defined two separate operations using the in-only and out-only patterns (see WSDL 2.0 Predefined Extensions section 2.2.1 In-Only and section 2.2.5 Out-Only), but that would just complicate matters for the client, because we would then have to separately indicate to the client developer that the two operations should be used together as a request-response pair.

In addition to the normal input and output messages, we also need to specify the fault message that we wish to use in the event of an error. WSDL 2.0 permits fault messages to be declared within the interface element in order to facilitate reuse of faults across operations. If a fault occurs, it terminates whatever message sequence was indicated by the message exchange pattern of the operation.

Let's add these to our WSDL 2.0 document.

Although we have specified what abstract messages can be exchanged with the GreatH Web service, we have not yet specified how those messages can be exchanged. This is the purpose of a binding. A binding specifies concrete message format and transmission protocol details for an interface, and must supply such details for every operation and fault in the interface.

In the general case, binding details for each operation and fault are specified using operation and fault elements inside a binding element, as shown in the example below. However, in some cases it is possible to use defaulting rules to supply the information. The WSDL 2.0 SOAP binding extension, for example, defines some defaulting rules for operations. (See Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts , Default Binding Rules.)

In order to accommodate new kinds of message formats and transmission protocols, bindings are defined using extensions to the WSDL 2.0 language, via WSDL 2.0's open content model. (See for more on extensibility.) WSDL 2.0 Part 2 defines binding extensions for SOAP 1.2 and HTTP 1.1 as predefined extensions, so that SOAP 1.2 or HTTP 1.1 bindings can be easily defined in WSDL 2.0 documents. However, other specifications could define new binding extensions that could also be used to define bindings. (As with any extension, other WSDL 2.0 processors would have to know about the new constructs in order to make use of them.)

For the GreatH service, we will use SOAP 1.2 as our concrete message format and HTTP as our underlying transmission protocol, as shown below.

Now that our binding has specified how messages will be transmitted, we are ready to specify where the service can be accessed, by use of the service element.

A WSDL 2.0 service specifies a single interface that the service will support, and a list of endpoint locations where that service can be accessed. Each endpoint must also reference a previously defined binding to indicate what protocols and transmission formats are to be used at that endpoint. A service is only permitted to have one interface. (See for further discussion of this limitation.)

Here is a definition for our GreatH service.

As we have seen, a WSDL 2.0 document is inherently only a partial description of a service. Although it captures the basic mechanics of interacting with the service -- the message types, transmission protocols, service location, etc. -- in general, additional documentation will need to explain other application-level requirements for its use. For example, such documentation should explain the purpose and use of the service, the meanings of all messages, constraints on their use, and the sequence in which operations should be invoked.

The documentation element allows the WSDL 2.0 author to include some human-readable documentation inside a WSDL 2.0 document. It is also a convenient place to reference any additional external documentation that a client developer may need in order to use the service. It can appear in a number of places in a WSDL 2.0 document (see ), though in this example we have only demonstrated its use at the beginning.

The following diagram gives an overview of the XML Infoset for a WSDL 2.0 document.

The WSDL 2.0 specification supplies a normative WSDL 2.0 schema, defined in , which can be used as an aid in validating WSDL 2.0 documents. We say "as an aid" here because WSDL 2.0 specification often provides further constraints to the WSDL 2.0 schema. In addition to being valid with the normative schema, a WSDL 2.0 document must also follow all the constraints defined by the WSDL 2.0 specification.

The WSDL 2.0 Infoset model above illustrates the required structure of a WSDL 2.0 document, using the XML Infoset. However, the WSDL 2.0 language also imposes many semantic constraints over and above structural conformance to this XML Infoset. In order to precisely describe these constraints, and as an aid in precisely defining the meaning of each WSDL 2.0 document, the WSDL 2.0 specification defines a component model as an additional layer of abstraction above the XML Infoset. Constraints and meaning are defined in terms of this component model, and the definition of each component includes a mapping that specifies how values in the component model are derived from corresponding items in the XML Infoset. The following diagram gives an overview of the WSDL 2.0 components and their containment hierarchy.

In general, the WSDL 2.0 component model parallels the structure of the required XML Infoset illustrated above. For example, the Description, Interface, Binding, Service and Endpoint components correspond to the description, interface, binding, service, and endpoint element information items, respectively. Since WSDL 2.0 relies heavily on the component model to convey the meaning of the constructs in the WSDL 2.0 language, you can think of the Description component as representing the meaning of the description element information item, and hence, it represents the meaning of the WSDL 2.0 document as a whole.

Furthermore, each of these components has properties whose values are (usually) derived from the element and attribute information item children of those element information items. For example, the Service component corresponds to the service element information item, so the Service component has an {endpoints} property whose value is a set of Endpoint components corresponding to the endpoint element information item children of that service element information item. (Whew!)

We have already seen an example of using inlined schema definitions in section . When XML Schema is inlined directly in a WSDL 2.0 document, it uses the existing top-level xs:schema element defined by XML Schema to do so, as though a schema file had been copied and pasted into the types element. The schema components defined in the inlined schema are then available to the containing WSDL 2.0 description for reference by QName. For instance, in , the input message of the interface operation "opCheckAvailability" is defined by the "ghns:checkAvailability" element in the inlined schema.

XML Schema components can be defined in separate schema files and be made available to a WSDL2.0 description by using xs:import directly under types.

There are many cases where one would prefer having schema definitions in separate schema files. One reason is the reusability of the schema definitions. Inlined schema definitions are only available to the containing WSDL 2.0 description. Although WSDL 2.0 provides a wsdl:import mechanism for importing other WSDL files, schema definitions inlined in an imported WSDL document are NOT automatically made available to the importing WSDL 2.0 document, even though other WSDL 2.0 components (such as Interfaces, Bindings, etc.) do become available. Therefore, if one wishes to share schema definitions across several WSDL 2.0 descriptions, these schema definitions should instead be placed in separate XML Schema documents and imported into each WSDL 2.0 description using xs:import directly under types.

Let's see an example. Assuming the message types in are defined in a separate schema file named "http://greath.example.com/2004/schemas/resSvc.xsd" with a target namespace "http://greath.example.com/2004/schemas/resSvc", the schema definition can then be brought into the WSDL 2.0 description using xs:import. Note that only components in the imported namespace "http://greath.example.com/2004/schemas/resSvc" are available for reference in the WSDL 2.0 document.

It's important to note that xs:import used directly under wsdl:types has been given a different visibility than xs:import used inside an inlined schema. An inlined schema may use native XML schema xs:import to bring in external schema definitions that are in different namespaces; However, though this is the schema importing mechanism recommended for WSDL 1.1 in WS-I Basic Profile, according to XML Schema specification, such enclosed message definitions are only visible to the importing schema (in this case, the inlined schema). They are not visible to the containing WSDL 2.0 description.

If we change to use XML Schema's native xs:import element in an inlined schema, the schema components defined in the namespace http://greath.example.com/2004/schemas/resSvc are not available to our example WSDL 2.0 definition any more.

Of course, an inlined XML schema may also use XML Schema's native xs:include element to refer to schemas defined in separate files when the included schema has no namespace or has the same namespace as the including schema. In this case, according to XML Schema, the included schema components become a part of the including schema as though they had been copied and pasted into the including schema. Hence, the included schema components are also available to the containing WSDL 2.0 description for reference by QName.

The following example has the same effect as :

So far we have briefly covered both WSDL import and include and schema import and include. The following table summarizes the similarities and differences between the WSDL 2.0 and XML Schema include and import mechanisms. We will talk a lot more about importing mechanisms in and

Below is the XML syntax summary of the interface element, simplified by omitting optional <documentation> elements:

The interface element has two optional attributes: styleDefault and extends. The styleDefault attribute can be used to define a default value for the style attributes of all operations under this interface (see WSDL 2.0 Part 1 "styleDefault attribute information item"). The extends attribute is for inheritance, and is explained next.

The optional extends attribute allows an interface to extend or inherit from one or more other interfaces. In such cases the interface contains the operations of the interfaces it extends, along with any operations it defines directly. Two things about extending interfaces deserve some attention.

First, an inheritance loop (or infinite recursion) is prohibited: the interfaces that a given interface extends must NOT themselves extend that interface either directly or indirectly.

Second, we must explain what happens when operations from two different interfaces have the same target namespace and operation name. There are two cases: either the component models of the operations are the same, or they are different. If the component models are the same (per the component comparison algorithm defined in WSDL 2.0 Part 1 " Equivalence of Components ") then they are considered to be the same operation, i.e., they are collapsed into a single operation, and the fact that they were included more than once is not considered an error. (For operations, component equivalence basically means that the two operations have the same set of attributes and descendants.) In the second case, if two operations have the same name in the same WSDL 2.0 target namespace but are not equivalent, then it is an error. For the above reason, it is considered good practice to ensure that all operations within the same target namespace are named uniquely.

Finally, since faults can also be defined as children of the interface element (as described in the following sections), the same name-collision rules apply to those constructs.

Let's say the GreatH hotel wants to maintain a standard message log operation for all received messages. It wants this operation to be reusable across the whole reservation system, so each service will send out, for potential use of a logging service, the content of each message it receives together with a timestamp and the originator of the message. One way to meet such requirement is to define the log operation in an interface which can be inherited by other interfaces. Assuming a messageLog element is already defined in the ghns namespace with the required content, the inheritance use case is illustrated in the following example. As a result of the inheritance, the reservationInterface now contains two operations: opCheckAvailability and opLogMessage

Now let's have a look at the element children of interface, beginning with fault.

The fault element is used to declare faults that may occur during execution of operations of an interface. They are declared directly under interface, and referenced from operations where they apply, in order to permit reuse across multiple operations.

Faults are very similar to messages and can be viewed as a special kind of message. Both faults and messages may carry a payload that is normally described by an element declaration. However, WSDL 2.0 treats faults and messages slightly differently. The messages of an operation directly refer to their element declaration, however the faults of an operation indirectly refer to their element declaration via a fault element that is defined on the interface.

The reason for defining faults at the interface level is to allow their reuse across multiple operations. This design is especially beneficial when bindings are defined, since in binding extensions like SOAP there is additional information that is associated with faults. In the case of SOAP, faults have codes and subcodes in addition to a payload. By defining faults at the interface level, common codes and subcodes can be associated with them, thereby ensuring consistency across all operations that use the faults

The fault element has a required name attribute that must be unique within the parent interface element, and permits it to be referenced from operation declarations. The optional element attribute can be used to indicate a schema for the content or payload of the fault message. Its value should be the QName of a global element defined in the types section. Please note that when other type systems are used to define the schema for a fault message, additional attributes may need to be defined via WSDL 2.0's attribute extension mechanism to allow the schema to be associated with the fault.

As shown earlier, the operation element is used to indicate an operation supported by the containing interface. It associates message schemas with a message exchange pattern (MEP), in order to abstractly describe a simple interaction with a Web service.

Since bindings are specified using extensions to the WSDL 2.0 language (i.e., binding extensions are not in the WSDL 2.0 namespace), the XML for expressing a binding will consist of a mixture of elements and attributes from WSDL 2.0 namespace and from the binding extension's namespace, using WSDL 2.0's open content model.

Here is a syntax summary for binding, simplified by omitting optional documentation elements. Bear in mind that this syntax summary only shows the elements and attributes defined within the WSDL 2.0 namespace. When an actual binding is defined, elements and attributes from the namespace of the desired binding extension will also be intermingled as required by that particular binding extension.

The binding syntax parallels the syntax of interface: each interface construct has a binding counterpart. Despite this syntactic similarity, they are indeed different constructs, since they are in different symbol spaces and are designed for different purposes.

A binding can either be reusable (applicable to any interface) or non-reusable (specified for a particular interface). Non-reusable bindings may be specified at the granularity of the interface (assuming the binding extension provides suitable defaulting rules), or on a per-operation basis if needed. A non-reusable binding was demonstrated in .

To define a reusable binding, the binding element simply omits the interface attribute and omits specifying any operation-specific and fault-specific binding details. Endpoints can later refer to a reusable binding in the same manner as for a non-reusable binding. Thus, a reusable binding becomes associated with a particular interface when it is referenced from an endpoint, because an endpoint is part of a service, and the service specifies a particular interface that it implements. Since a reusable binding does not specify an interface, reusable bindings cannot specify operation-specific details. Therefore, reusable bindings can only be defined using binding extensions that have suitable defaulting rules, such that the binding information only needs to be explicitly supplied at the interface level.

A binding fault associates a concrete message format with an abstract fault of an interface. It describes how faults that occur within a message exchange of an operation will be formatted, since the fault does not occur by itself. Rather, a fault occurs as part of a message exchange specified by an interface operation and its binding counterpart, the binding operation.

A binding fault has one required ref attribute which is a reference, by QName, to an interface fault. It identifies the abstract interface fault for which binding information is being specified. Be aware that the value of ref attribute of all the faults under a binding must be unique. That is, one cannot define multiple bindings for the same interface fault within a given binding.

A binding operation describes a concrete binding of an interface operation to a concrete message format. An interface operation is uniquely identified by the WSDL 2.0 target namespace of the interface and the name of the operation within that interface, via the required ref attribute of binding operation. As with faults, for each operation within a binding, the value of the ref attribute must be unique.

The WSDL 2.0 SOAP Binding Extension (see WSDL 2.0 Part 2 ) was primarily designed to support the features of SOAP 1.2 . However, for backwards compatibility, it also provides some support for SOAP 1.1 .

An example using the WSDL 2.0 SOAP binding extension was already presented in , but some additional points are worth mentioning:

Because the same binding extension is used for both SOAP 1.2 and SOAP 1.1, a wsoap:version attribute is provided to allow you to indicate which version of SOAP you want. If this attribute is not specified, it defaults to SOAP 1.2.

Here is an example that illustrates both a SOAP 1.2 binding (as seen before) and a SOAP 1.1 binding.

In addition to the WSDL 2.0 SOAP binding extension described above, WSDL 2.0 Part 2 defines a binding extension for HTTP 1.1 and HTTPS , so that these protocols can be used natively to send and receive messages, without first encoding them in SOAP.

The HTTP binding extension provides many features to control:

Which HTTP operation will be used. (GET, PUT, POST, DELETE, and other HTTP operations are supported.)

As with the WSDL 2.0 SOAP binding extension, the HTTP binding extension also provides defaulting rules to permit binding information to be specified at the interface level and used by default for each operation in the affected interface, however, defaulting rules are not provided for binding faults.

Here is an example of using the HTTP binding extension to check hotel room availability at GreatH.

When a binding using HTTP is specified for an operation, the WSDL 2.0 author must decide which HTTP method is appropriate to use -- usually a choice between GET and POST. In the context of the Web as a whole (rather than specifically Web services), the W3C Technical Architecture Group (TAG) has addressed the question of when it is appropriate to use GET, versus when to use POST, in a finding entitled URIs, Addressability, and the use of HTTP GET and POST (). From the abstract:

. . . designers should adopt [GET] for safe operations such as simple queries. POST is appropriate for other types of applications where a user request has the potential to change the state of the resource (or of related resources). The finding explains how to choose between HTTP GET and POST for an application taking into account architectural, security, and practical considerations.

Recall that the concept of a safe operation was discussed in . (Briefly, a safe operation is one that does not cause the invoker to incur new obligations.) Although the wsdlx:safe attribute of an interface operation indicates that the abstract operation is safe, it does not automatically cause GET to be used at the HTTP level when the binding is specified. The choice of GET or POST is determined at the binding level:

For example, in the GreatH interface definition shown in , the wsdlx:safe attribute is set to "true". The HTTP binding definition in may take advantage of that and be simplified as below and still have the http method set to GET by default:

In this example, we consider some GreatH Hotel Web services that retrieve and update reservation details. The retrieval Web service is defined in the retrieveDetails.wsdl WSDL 2.0 document, along with a schema for the message format. The updating Web service is defined in the updateDetails.wsdl WSDL 2.0 document which imports the first document and refers to both WSDL 2.0 and schema definitions contained in the imported document.

shows the definition of the retrieval Web service in the http://greath.example.com/2004/services/retrieveDetails namespace. This WSDL 2.0 document also contains an inline schema that describes the reservation detail in the http://greath.example.com/2004/schemas/reservationDetails namespace. This schema is visible to the retrieveDetailsInterface interface definition which refers to it in the retrieve operation's output message.

shows the definition of the updating Web service in the http://greath.example.com/2004/services/updateDetails namespace. The updateDetailsInterface interface extends the retrieveDetailsInterface interface. However, the retrieveDetailsInterface belongs to the http://greath.example.com/2004/services/retrieveDetails namespace, so updateDetails.wsdl must import retrieveDetails.wsdl to make that namespace visible.

The updateDetailsInterface interface also uses the reservationDetails element definition that is contained in the inline schema of the imported retrieveDetails.wsdl document. However, this schema is not automatically visible within the updateDetails.wsdl document. To make it visible, the updateDetails.wsdl document must import the namespace of the inline schema within the types element using the XML schema import element.

In this example, the schemaLocation attribute of the import element has been omitted. The schemaLocation attribute is a hint to the WSDL 2.0 processor that tells it where to look for the imported schema namespace. However, the WSDL 2.0 processor has already processed the retrieveDetails.wsdl document which contains the imported namespace in an inline schema so it should not need any hints. However, this behavior depends on the implementation of the processor and so cannot be relied on.

Although the WSDL 2.0 document may validly omit the schemaLocation attribute, it is a best practice to either provide a reliable value for it or move the inline schema into a separate document, say reservationDetails.xsd, and directly import it in the types element of both retrieveDetails.wsdl and updateDetails.wsdl. In general, schemas that are expected to be referenced from more than one WSDL 2.0 document should be defined in a separate schema document rather than be inlined.

A WSDL 2.0 document may define multiple inline schemas in its types element. The two or more schemas may have the same target namespace provided that they do not define the same elements or types. It is an error to define the same element or type more than once, even if the definitions are identical.

Each namespace of an inline schema becomes visible to the Web service definitions. However, the namespaces are not automatically visible to the other inline schemas. Each inline schema must explicitly import any other namespace it references. The schemaLocation attribute is not required in this case since the WSDL 2.0 processor knows the location of each schema by virtue of having processed the enclosing WSDL 2.0 document.

To illustrate this, consider which contains two inline schemas. The http://greath.example.com/2004/schemas/reservationItems namespace contains some elements for items that appear in the reservation details. The http://greath.example.com/2004/schemas/reservationDetails namespace contains the reservationDetails element which refers to the item elements. The schema for the http://greath.example.com/2004/schemas/reservationDetails namespace contains an import element that imports the http://greath.example.com/2004/schemas/reservationItems namespace. No schemaLocation attribute is required for this import since the schema is defined inline in the importing document.

In the preceding examples, schemas were defined inline in WSDL 2.0 documents. This section discusses the correct way to specify a schemaLocation attribute on a schema import element to provide a processor with a hint for locating these schemas.

shows how one WSDL 2.0 document imports a schema defined in another, i.e. . Similarly, shows how one schema in a WSDL 2.0 document imports another schema defined in the same document. In both of these examples, the schemaLocation attribute was omitted since the WSDL 2.0 processor was assumed to know how to locate the imported schemas because they were part of the WSDL 2.0 documents being processed. The schemaLocation attribute can be used to give the processor a URI reference that explicitly locates the schemas. A URI reference is a URI plus an optional fragment identifier that indicates part of the resource. For schemas, the fragment should identify the schema element. The simplest way to accomplish this is to use the id attribute, however XPointer (see ) can also be used.

Extensions can either be required or optional.

An optional extension is one that the client may either engage or ignore, entirely at its discretion, and is signaled by wsdl:required="false" or the absence of the wsdl:required attribute (because it defaults to false). Thus, a WSDL 2.0 processor, acting on behalf of the client, that encounters an unknown optional extension can safely ignore it and continue to process the WSDL 2.0 document. However, it is important to stress that optional extensions are only optional to the client -- not the service. A service must support all optional and required extensions that it advertises in its WSDL 2.0 document.

A required extension is one that must be supported and engaged by the client in order for the interaction to proceed properly, and is signaled by wsdl:required="true". If a WSDL 2.0 processor, acting on behalf of the client, encounters a required extension that it does not recognize or does not support, then it cannot safely continue to process the WSDL 2.0 document. In most practical cases, this is likely to mean that the processor will require manual intervention to deal with the extension. For example, a client developer might manually provide an implementation for the required extension to the WSDL 2.0 processor.

This pattern consists of two or more messages in order as follows:

This pattern uses the rule Message Triggers Fault.

An operation using this message exchange pattern has a pattern property with the value http://www.example.com/webservices/meps/confirmed-challenge.

Once the MEP had been defined (and the email binding specification appropriately modified to indicate that this was a supported MEP), the service was redefined and redeployed. Only the changed operations are shown in the excerpt below.

Note: in the second example, the input and output examples are not in the sequence in which they occur in the pattern; this illustrates that the sequence is not significant. Note, however, that for this pattern, the messageLabel attribute is required on every input and output element.

In compatible evolution, designers are expected to limit changes to those that are either backward or forward compatible, or both:

Since Web services and their clients both send and receive messages, these concepts can apply to both parties. However, since WSDL 2.0 is service-centric, we will focus on the case of service evolution.

There are three critical areas in which a service described in WSDL 2.0 my evolve:

The big bang approach to versioning is the simplest to currently represent in WSDL 2.0. In this approach, any change to a WSDL 2.0 document implies a change to the document's namespace, a change to the interface implies a new interface namespace and a change to the message contents is communicated using a new message namespace. This approach has particular benefits where an agent may quickly tell if a service has changed by simply comparing the namespace value.

Compatible changes are far more easily managed than incompatible ones:

It is feasible to combine the "compatible evolution" and "big bang" approaches in a variety of different ways. For example, the namespace could be changed when message descriptions are changed, but the namespace could stay the same when new operations are added.

While the big bang approach is currently the easiest to implement in WSDL 2.0, it can lead to a large number of cloned interfaces that become difficult to manage, thus making the compatible approach preferable to many for widely distributed systems. In the end, the choice of a versioning strategy for Web services described in WSDL 2.0 is left as an exercise to the reader.

When designing a Web application it is natural to give each important concept a URI. In the GreatH hotel reservation system, the important concepts are reservations, so we begin our design by assigning a URI to each reservation. Since each reservation has a unique confirmation number, e.g OMX736, we create a URI for each reservation by appending the confirmation number to a base URI, e.g. http://greath.example.com/2004/reservation/OMX736. This URI will be the endpoint address for a Reservation Details Web service that can retrieve and update the state of a reservation. shows the format of the reservation detail.

The Reservation Details Web service provides operations for retrieving and updating the detail for a reservation. shows the description for this Web service. Note that there is no service element in this description since the set of reservations is dynamic. Instead, the endpoints for the reservations will be returned by querying the Reservation List Web service.

shows the XML schema elements that are used in this Web service.

This XML schema contains the usual definitions for the elements that appear in the messages of the Web service. For example, the reservationDetails element is used in the messages of the retrieve and update operations. In addition, the schema defines the simple type reservationDetailsSOAPEndpointType which is based on xs:anyURI and has the annotation wsdlx:binding = "wdetails:reservationDetailsSOAPBinding" which means that the URI is the address of a Reservation Details Web service endpoint that implements the wdetails:reservationDetailsSOAPBinding binding. Note that the wsdli:wsdlLocation attribute is used to define the location of the WSDL 2.0 document that defines the wdetails:reservationDetailsSOAPBinding binding. This annotated simple type is used to define the reservationDetailsSOAPEndpoint element which will be used in the Reservation List service.

Since the set of reservations changes as reservations are made and cancelled, the Reservation Detail endpoints are not described in a fixed WSDL 2.0 document. Instead they are returned as references to endpoints in response to requests made on a Reservation List Web service. The endpoint address for the Reservation List service will be http://greath.example.com/2004/reservationList.

shows the format of the response from the Reservation List service.

Here, the <details:reservationDetailsSOAPEndpoint> elements contain references to the Reservation Details Web service endpoints for the reservations HSG635, OMX736, and WUH663.

shows the description of the Reservation List Web service. Note that it contains operations to retrieve the entire list and to query for a list of reservations by confirmation number, check-in date, and check-out date. In each case, the operation returns a list of reservations.

shows the schema for the messages used in the Reservation List Web service.

In the preceding example, there was a single endpoint associated with each Reservation Detail Web service. Suppose GreatH hotel decided to provide a second, secure endpoint. In this case, references to services would be used to collect together the endpoints for each reservation. The reservationDetails.xsd schema defines the reservationDetailsService element for this purpose. It contains the nested elements soap and secure-soap which are each of type reservationDetailsSOAPEndpointType and therefore contain the address of an endpoint that implements the wdetails:reservationDetailsSOAPBinding binding.

shows an example of a message that contains a reference to the service for reservation HGS635. Note that the service contains two endpoints, one of which provides secure access to the Reservation Details Web service.

This section presents a variation on the example in . It illustrates the use of HTTP transfer operations, GET and PUT, to retrieve and update GreatH hotel reservation details using the Representational State Transfer (REST) architectural style described by Roy Fielding . REST is a distillation of the architectural properties that Dr. Fielding identified as being vital to the Web's robustness and enormous scalability.

Since each reservation in our example will have a distinct URI, the Reservation Details Web service can be offered using HTTP GET and HTTP PUT. The binding would be modified as follows:

As with the example in , service and endpoint elements are not provided because the Reservation List Web service provides the endpoints.

This section continues the REST-style example of by modifying the example of to use HTTP GET.

The SOAP version of the Reservation List Web service above offers four different search operations. These can also be expressed as various parameters in a URI used by HTTP GET:

A retrieval by Confirmation Number URI would look like: http://greath.example.com/2004/reservationList/ConfirmationNumber/HSG635 .

Alternatively, a single query type may be provided. This query type is a sequence of optional items. Any items in the sequence are serialized into the URI query string. A query sequence for any of ConfirmationNumber, checkInDate, checkOutDate would look like this:

The WSDL 2.0 service that offers this type serialized as a parameter would look like this:

Various URIs would be: http://greath.example.com/2004/reservationList/ReservationQuery?confirmationNumber=HSG635 http://greath.example.com/2004/reservationList/ReservationQuery?checkInDate=06-06-05 .

It is important to observe that using the URI serialization can result in very flexible queries and few operations. The previous discrete SOAP operations are collapsed into one "parameterized" operation.

WSDL 2.0: Mapping to RDF describes how WSDL 2.0 constructs can be expressed in RDF using classes of resources (described with an ontology expressed in OWL) and assertions over individual resources. As RDF represents knowledge using resources and relationships between them, we need to turn WSDL 2.0 concepts into this model. This is done as follows.

It is a common misperception to equate either the target namespace of an XML Schema or the value of the xmlns attribute in XML instances with the location of the corresponding schema. Even though namespaces are URIs, and URIs may be locations, and it may be possible to retrieve a schema from such a location, this does not mean that the retrieved schema is the only schema that is associated with that namespace. There can be multiple schemas associated with a particular namespace, and it is up to a processor of XML to determine which one to use in a particular processing context. The WSDL 2.0 specification provides the processing context here via the import mechanism, which is based on XML Schema's term for the similar concept.

Throughout this document there are fully qualified URIs used in WSDL 2.0 and XSD examples. In some cases, fully qualified URIs were used simply to illustrate the referencing concepts. However, the use of relative URIs is allowed and warranted in many cases. For information on processing relative URIs, see RFC3986.

In general, when a WSDL 2.0 document is published for use by others, it should only contain URIs that are globally unique. This is usually done by allocating them under a domain name that is controlled by the issuer. For example, the W3C allocates namespace URIs under its base domain name, w3.org.

However, it is sometimes desirable to make up a temporary URI for an entity, for use during development, but not make the URI globally unique for all time and have it "mean" that version of the entity (schema, WSDL 2.0 document, etc.). Reserved Top Level DNS Names specifies some URI base names that are reserved for use for this type of behavior. For example, the base URI http://example.org/ can be used to construct a temporary URI without any unique association to an entity. This means that two people or programs could choose to simultaneously use the temporary URI http://example.org/userSchema for two completely different schemas. As long as the scope of use of these URIs does not intersect, then they would be unique enough. However, it is not recommended that http://example.org/ be used as a base for stable, fixed entities.