Applied XML Programming for Microsoft .NET.pdf - Csbdu.in

a DiffGram. To make DiffGrams really usable, the availability of schema information isvital.A Save-and-Resume ApplicationAs a stateful data format, a DiffGram is particularly useful for building save-and-resumeapplications. In this context, a save-and-resume application is a desktop or Webapplication that can work both on line and off line. For such applications, the connectionto the rest of the back-end system is optional and is not guaranteed to be up all thetime. From the connectivity standpoint, a save-and-resume application is intermittentand must be able to get its core data either remotely (for example, from the centralsystem) or locally (for example, from data persisted to files).In this section, we'll build a Windows Forms application that connects to a database,downloads some data, and disconnects. From this point on, the application worksdisconnected, the data it needs is stored locally, and the application can be usedanywhere and shut down and resumed any number of times. All the changes made tothe local data are correctly tracked and reported as insertions, deletions, and updates.At a later time, the application reconnects to the system and submits its changes.In this description, common words such as connection, back-end system, data,database, and updates are treated as blanket terms that each application canimplement as needed. For example, a simple query executed on a SQL Server table inthe sample application can easily become a call to a middle-tier object. Similarly, asimple connection to SQL Server in the sample application could be viewed as a loginin a distributed application.NoteWhile looking at the sample application discussed here, keep inmind that it is just a sample. Focus on the technologies involved andtheir interactions rather than on the implementation details. Theoverall context of the sample application, while representative of acommon type of application, is certainly not a real-world scenario!Setting Up the ApplicationThe key functions of a save-and-resume application can be summarized in threecategories. First, the application must be able to work disconnected, thus transparentlyusing a local copy of the back-end database. Next, the application must allow you toreview, filter, and reject changes. Finally, the application must allow you to reconnectand submit changes at any time. Figure 10-7 shows the key elements of thearchitecture.378