Enabling Enterprise-Class Web Services

This chapter is from the book

J2EE Web Services on BEA WebLogic

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This chapter is from the book

This chapter is from the book 

J2EE Web Services on BEA WebLogic

Learn More Buy

Publishing Enduring Web Services Contracts

A web service contract, usually expressed via a WSDL, describes the relationship between the web services that you publish and the manner in which an external client can interact with them. These touch upon what XML messages to expect, what business operations you can perform, and what XML messages you generate. You can do so without letting the other party know about the technical details of your application and how they change over time. By the same token, you can use other developers' public contracts to consume their applications without needing to know the details or even the language and platform they use to implement their web service.

These contracts can be made more durable by following certain principles, such as loose coupling, XML strategy selection, and versioning.

Integrating Through Loose Coupling

An important aspect of web services design is to ensure that the web services are loosely coupled. Web services are loosely coupled when you separate the internal implementation from the interface exposed to the user of your service. The web service has an interface defined by the WSDL that is exposed to the outside world. The internal implementation of a web service is the specific application server that it runs on or the EJBs that do the business logic. If you need to change your application server, the user of the service is not affected because there is a change only to the internal implementation. When this is ensured, the user of the web service does not need to get involved in testing changes unless the interface changes as defined by the WSDL. You might still want to test with the client even if the WSDL doesn't change, to make sure that SLAs and performance requirements can still be met.

Using the WSDL, however, is not enough to guarantee loose coupling. If your WSDL is automatically generated from the code and you change the code, the WSDL changes, thus making it tightly coupled to the code. WebLogic Workshop provides XQuery Maps which is a simple, declarative way of describing how your Java code relates to the WSDL of your web service. If your Java code changes over time, you can change your XQuery Map so that your WSDL stays intact. XQuery Map maps the Java code to your WSDL. When the Java code changes, the mapping can be alerted to keep the WSDL the same. This enables you to truly realize the promise of loose coupling. WebLogic also provides the facility of modeling the WSDL in XMLSpy and then importing it into WebLogic Workshop. With this WSDL, you can create a web service in WebLogic Workshop. See Figures 5.2 and 5.3 for an illustration of the modeling and importing of WSDL.

Figure 5.2 Modeling of WSDL

Figure 5.3 Importing of WSDL in WebLogic Workshop

Here is an example of using an XQuery map in WebLogic Workshop. The start and end of the XML map is shown by the annotation:

 * @jws:operation
 * @jws:parameter-xml xml-map::
 *  < order>
 * <item xm:multiple="String name in nameA,
 int amount in amountA, float price in priceA">
 *    <name>{name}</name>
 *    <amount>{amount}</amount>
 *    <price>{price}</price>
 *   </item>
 *  </order>
 * ::
 * @jws:return-xml xml-map::
 *  <totalPrice>{return}</totalPrice>
 * ::
 */
 public float getTotalPrice(String [] nameA,
 int [] amountA, float [] priceA)
 {
   float totalPrice = 0.0f;
   if( nameArr != null )
   {
     // for each item, compute subtotal and add to total
     for (int i = 0; i < nameArr.length; i++)
     {
       totalPrice += amountArr[i] * priceArr[i];
     }
    }
    return totalPrice;
 }

This map specifies that the getTotalPrice Java method receives an XML document that contains an order with multiple line items, each with a name, amount, and price. These fields are extracted from the XML message and mapped into Java arrays. Similarly, the float value that is returned from the method is placed in the context of a simple XML document that has a <totalPrice> tag.

Choosing an XML Strategy

XML is the messaging standard for web services. You have to choose the right strategy for handling XML messages in web services, depending on how the web services you are designing fit into the overall application.

The XML strategy that you will use depends on two criteria. One is whether you have any application logic and whether you are exposing the application logic. The second criterion is whether the interface to the web service you are developing has a predefined XML Schema and whether you want to define your own.

This can lead to four different strategies for handling XML Schemas:

• Defined application logic and predefined XML Schema—In this case, the strategy that you use is mapping between defined incoming messages to fields in your internal data types. XQuery maps in WebLogic Workshop can be used to map the data.

• No internal classes and predefined XML Schema—If schemas are defined in the WSDL file, you should import the WSDL in WebLogic Workshop and let XML Beans accept the schema file and return a set of Java classes that a developer can conveniently leverage to process any XML document that conforms to the original schema file. Having schema definitions at the core of the XML Beans system provides a variety of benefits. For example, when you first receive an XML document for processing, you can validate the data based on the schema definition. Any time you manipulate the XML document via the schema-inspired Java classes, the XML Beans system can always ensure that all changes remain consistent with the prevailing schema definition. This unambiguously disallows the creation of invalid XML documents.

• When internal classes are defined and the schema is not defined, you should define your schema according to your internal classes. If both are not defined, first define the schemas and then derive the Java classes from there. This follows the concept of designing WSDL first, as described in the earlier section.

Versioning New Releases

You need to consider versioning of your web service to facilitate managing multiple versions of a web service. Versioning should minimize code replication and maximize code reuse. It should also put a logical and manageable naming paradigm in place. This allows for upgrades and improvements to be made to existing web services, while continuously supporting previously released versions of that web service.

Two areas should be considered for versioning a web service: the public interface, as described by the WSDL file, and the web service implementation, including its conversational state.

Versioning the Public Interface

You can version the public interface—specifically, your WSDL—in different ways:

• Use a different endpoint for your service:

<wsdl:service name="Validate_v1_2">
    <wsdl:port name="Validate" binding="tns1:ValidateSoapBinding">
      <wsdlsoap:address location="localhost:7001/ValidateConfigNewWeb/
ValidateConfigNew/validate_v1_2ControlTest.jws"/>
    </wsdl:port>
  </wsdl:service>

In this example, you can change to the next version validate_v1_3 by changing the address of the endpoint for the service as follows:

<wsdlsoap:address location="localhost:7001/ValidateConfigNewWeb
/ValidateConfigNew/validatev_1_3ControlTest.jws"/>

In this approach, the XML Schema does not change and there is no change in your SOAP message. The advantage in this option is that you can insulate your users from changing schemas. The drawback is that there is no reference to the version validate_v1_2 within the SOAP message, so you cannot use management tools that can direct the message to the right version:

<SOAP-ENV:Body>
    <m:inValidateConfig xmlns:m="http://production.psg.hp.com/types">
      ...
  </SOAP-ENV:Body>

• Use a date stamp as part of the target namespace of your XML Schema. This is in compliance with the W3C XML Schema specification. The disadvantage here is that your schema changes every time the version changes. The advantage of this approach is that you can see the version in your SOAP messages, and you can write code or use management tools to direct the web service to the right version, according to the SOAP message:

<SOAP-ENV:Body>
    <m:inValidateConfigv1_2
xmlns:m="http://production.psg.hp.com/types/2004/02/04">
      ...
  </SOAP-ENV:Body>

• Add new operations to the WSDL and support old ones until the user moves to the new operation. You can add new operations to the public interface of a web service to reflect the new versions, keeping the existing operations. The advantage of doing so is that you do not disrupt clients that rely on the web service. By adding new operations and making them known to clients, you can gradually shift clients over to a new set of operations, but you should leave the original operations intact for backward compatibility. For example, the operation section of the WSDL will look as shown here:

<wsdl:operation name="validateConfig">
    <wsdlsoap:operation soapAction="validate_v1_2"/>
    <wsdl:input>
      <wsdlsoap:body use="literal"/>
    </wsdl:input>
    <wsdl:output>
      <wsdlsoap:body use="literal"/>
    </wsdl:output>
  </wsdl:operation>
  <wsdl:operation name="validatePrice">
    <wsdlsoap:operation soapAction=" validate_v1_3 "/>
    <output>
      <wsdlsoap:body use="literal"/>
    </output>
    <input>
      <wsdlsoap:body use="literal"/>
    </input>
  </wsdl:operation>

The types section of the WSDL will look as shown here:

<wsdl:types>
    <xsd:schema targetNamespace="http://production.psg.hp.com/types"
xmlns="http://production.psg.hp.com/types" elementFormDefault="qualified">
      <xsd:include schemaLocation="ValidateConfigv1_2.xsd"/>
      <xsd:include schemaLocation="ValidateConfigv1_3.xsd"/>
    </xsd:schema>
  </wsdl:types>

The SOAP message for each of the operations will look as shown next. Again, you can use management tools to direct messages to different versions:

<SOAP-ENV:Body>
    <m:inValidateConfigv1_2 xmlns:m="http://production.psg.hp.com/types">
      ...
  </SOAP-ENV:Body>
<SOAP-ENV:Body>
    <m:inValidateConfigv1_3 xmlns:m="http://production.psg.hp.com/types">
      ...
  </SOAP-ENV:Body>

• Use UDDI and versioning UDDI in conjunction with WSDL to provide for versioning. The UDDI data model is rich enough that the current best practices can be enhanced to include service versioning. A given service can advertise more than one interface that represents its different versions. Different interfaces have different tModels. The service can reference the tModel for each of the interface in its tModelInstanceDetails collection. tModelInstanceDetails is a class that has tModelKey as a mandatory attribute.

Versioning the Implementation

WebLogic Workshop uses Java serialization to persist the state associated with web service requests and conversations. For this reason, the primary requirement for supporting versioned implementations is maintaining backward serialization compatibility. Specifically, it must be possible to load state into the current version of a class that was stored using any older versions of that class.

Versioning Lifecycle

Understanding the versioning lifecycle will help you to implement the new versions of your web service and deprecate the older versions effectively. First, you need to put together a plan for the lifecycle of the web services you are supporting. The main aspects of the plan of the versioning lifecycle are listed here:

1. Your plan should contain the frequency of the release of your versions. For example, you could release one version per year. Consider how many versions of the web service you want to support in parallel.

2. The plan should also contain the time frame for your users to move to the new version. This would be the same as the time you would support an older version after the new one is released.

3. Consider using a pilot for the new version of the web services with an early release of version.

4. Consider releasing new functionality and conformance to new web service specifications only through this versioning strategy, as with software releases.

5. Communicate the versioning strategy to the users of your web service.

After you lay out the versioning strategy for each version, follow these steps to release each version of your web service:

1. Make changes to the services that you are supporting, as detailed in the previous sections.

2. Do unit and functional testing of the service.

3. Deploy the new service either through new WSDLs for users of the service or to UDDI registries.

4. Notify the consumers of your new service and pilot the new service with one of your consumers.

5. Run the new and old versions in parallel for the time frame you have allocated in your versioning plan.

6. Notify the consumers of the date when the old service is obsoleted.

7. Remove the old service version from descriptions, registries, and so on to keep new consumers from discovering and using the old web service. Remove the functional behavior of the old service; return only an appropriate error message.

8. Retire the old service. Physically remove the old service version.