Using Data Synchronization Services in VS .NET

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Another option for data synchronization is to use a Data Synchronization Service Data Synchronization Services are new to NET Framework 35 They provide more flexibility than Merge Replication, and they do so by
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shiftingmuchofthedevelopmentworkloadfromthedatabaseadministrator to the application developer A discussion of Data Synchronization Services could easily fill an entire chapter, perhaps even an entire book What we will do here is provide some basic background information and then walk through the development of a simple Data Synchronization Service; one that is specific to NET Compact Framework clients
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861 Understanding Data Synchronization Service Requirements A Data Synchronization Service is a Web service Like any Web service, it is accessible via a URL and contains a class of functionality whose methods can be invoked via a Web protocol The standard for Data Synchronization Services, that is, the interface definition specified by the NET Framework, mandates only four methods
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1 ApplyChanges: Receive all changes that have occurred on the client since the last exchange of data (ie, since the last synchronization) and apply them on the server 2 GetChanges: Retrieve all changes that have been made by other uses on the server since the last synchronization and deliver them to the client 3 GetSchema: Retrieve schema information regarding the tables that participate in the synchronization from the server and deliver it to the client 4 GetServerInfo: Retrieve the name and description of the tables that participate in the synchronization from the server and deliver them to the client Normally, the service combines the first two methods in the preceding list into a single method call named Synchronize Although few in number, these methods represent an extensive amount of functionality, for implied in ApplyChanges and GetChanges is concurrency conflict detection That is, the service must work in conjunction with the client application and the server-side data store to detect whether the client is attempting change data that was modified on the server by some other user as it was being modified on the client Furthermore, the service should give the client a choice between the following
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Synchronizing Mobile Data The modification of the row does not occur and the client is so noti-
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fied The modification of the row does occur at the server, regardless of the impact on the changes made by the other user The second option is often referred to as Last updater wins, but Forced update is a more correct phrase For a data store to be able to provide concurrency conflict detection, it must be able to track changes Any data store that is to be used by a Data Synchronization Service must be able to do this SQL Server CE 35 SP1 and SQL Server 2008 have a built-in change tracking capability that was designed with Data Synchronization Services support as a requirement SQL Server 2005, on the other hand, has no built-in change tracking support As will become apparent when we develop our sample application later in this chapter, the Configure Data Synchronization Wizard will generate, execute, and store SQL scripts that will add this capability to selected tables of a SQL Server 2005 database This is a less than ideal solution Moving the application s back-end component to a new server means running the scripts at the new server (and possibly running the undo scripts at the old server) Many of you have probably added some form of change tracking to a SQL Server database in support of an application that you have developed and have encountered similar database maintenance overhead In our upcoming sample, we will use SQL Server 2005 as our server-side data store This will allow us to see the Designer-generated SQL code and the impact that it has on the Northwind database Interestingly,thechangetrackingneededforDataSynchronizationServices does not require that a before and after image of a modified row be maintained; only that the time and user of the modification be tracked The image of a deleted row, however, must be maintained in the database If the client submits, and is determined to force acceptance of, an update of a row that was concurrently deleted at the server, that row first must be undeleted at the server and then the client s update applied This can happen only if the deleted row is still available at the server Rows that have been deleted from one table are normally maintained in a separate table that is referred to as the tombstone version of the original table Given this background information, we can begin development of our own Data Synchronization Service and a NET Compact Framework application that will be a client of that service
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