Chapter 5 Client Design

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the client and endpoint makes them tightly coupled. As such, developers should

use this approach only when a tightly coupled situation is acceptable or required,

or when the client and service endpoint responsibilities are clearly documented.

5.4.1.2

Synchronizing Shared State among Clients

There are times when multiple Web service clients share state among themselves.

Such shared state may be read only or it may be state that is updated. Read only

state does not require synchronization since clients are retrieving data only, they

may poll the service at intervals and obtain current data. However, it is possible for

other Web clients to concurrently update shared data. Data consistency is an issue

when one or more clients may be manipulating the same data at the same time. 

An example use case that illustrates sharing modifiable state among Web

service clients might be purchase order management client application. The appli 

cation accesses a set of purchase orders that require manual approval. After the

user manually processes these orders offline, the application uploads them to the

service. It is conceivable that two such client applications may each download

some of the same purchase orders, and some of these orders may be changed inde 

pendent of each other. After the first client application uploads its changed data to

the service, there is a data consistency problem if the second application tries to

upload its modified data.

To prevent such inconsistencies, the client developer should keep certain con 

siderations in mind. The developer can detect data inconsistency using techniques

such as time stamp checking or checksum analysis before uploading modified

data, and this may require cooperation from the service endpoint. Data locking is

another technique to avoid simultaneous changes to the same data. A service end 

point could throw a service specific exception indicating that data has been modi 

fied by another client and might be out of sync. The client application can be

designed to overwrite the service endpoint copy, or it can ask the user to re enter

the data. Both client and service endpoint developers need to ensure that their

respective applications handle such inconsistencies properly.

5.4.2 Enhancing User Experience

The dynamics of the user interface play a large role in determining the quality of a

user's experience. J2SE clients have the advantage of drawing on a rich set of APIs,

in particular the Swing APIs, to make the user experience the highest quality possi 

ble. These APIs give J2SE clients the ability to query a Web service in the back