98

Designing a Service's Interaction Layer

One question remains: How does the client get the final result of its request?

The service may make the result of the client's request available in one of two

ways:

  The client that invoked the service periodically checks the status of the request 

using the correlation identifier that was provided at the time the request was 

submitted. This is also known as polling, and it appears as Option 1 in Figure 

3.8.

  Or, if the client itself is a Web service peer, the service calls back the client's 

service with the result. The client may use the correlation identifier to relate the 

response with the original request (Option 2 in Figure 3.8).

Often this is decided by the nature of the service itself. For example, if the

service runs a business process workflow, the workflow requires the service to

take appropriate action after processing the request.

3.4.4 Formulating Responses

After you delegate the request to the business logic portion of the application, and

the business logic completes its processing, you are ready for the next step: to form

the response to the request.

E

You should perform response generation, which is simply constructing the 

method call return values and output parameters, on the interaction layer, as 

close as possible to the service endpoint.

This permits having a common location for response assembly and XML doc 

ument transformations, particularly if the document you return to the caller must

conform to a different schema from the internal schema. Keeping this functional 

ity near the endpoint lets you implement data caching and avoid extra trips to the

processing layer. (See Figure 3.9.). 

Consider response generation from the weather information service's point 

of view. The weather information service may be used by a variety of client types,

from browsers to rich clients to handheld devices. A well designed weather infor 

mation service would render its responses in formats suitable for these different

client types.