Chapter 3 Service Endpoint Design

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vices, the content of the request parameters initiates and determines the process 

ing workflow the steps to fulfill the request for the Web service. Often,

fulfilling a request requires multiple workflow steps. 

The travel agency service is a good example of an asynchronous interaction

between a client and a service. A client requests arrangements for a particular trip

by sending the travel service all pertinent information (most likely in an XML

document). Based on the document's content, the service performs such steps as

verifying the user's account, checking and getting authorization for the credit

card, checking accommodations and transportation availability, building an itiner 

ary, purchasing tickets, and so forth. Since the travel service must perform a series

of often time consuming steps in its normal workflow, the client cannot afford to

pause and wait for these steps to complete.

Figure 3.7 shows one recommended approach for asynchronously delegating

these types of Web service requests to the processing layer. In this architecture,

the client sends a request to the service endpoint. The service endpoint validates

the incoming request in the interaction layer and then delegates the client's request

to the appropriate processing layer of the service. It does so by sending the request

as a JMS message to a JMS queue or topic specifically designated for this type of

request. 

E

Delegating a request to the processing layer through JMS 

before

 validating the 

request should be avoided.

Validation ensures that a request is correct. Delegating the request before val 

idation may result in passing an invalid request to the processing layer, making

error tracking and error handling overly complex. After the request is successfully

delegated to the processing layer, the service endpoint may return a correlation

identifier to the client. This correlation identifier is for the client's future reference

and may help the client associate a response that corresponds to its previous

request. If the business logic is implemented using enterprise beans, message 

driven beans in the EJB tier read the request and initiate processing so that a

response can ultimately be formulated.