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

Updategrams have been designed to provide three levels of protection against this kindof conflict, as follows:• Blind updates You specify only the primary key of the record in the block. In this case, the change is persisted without firstchecking whether the current status of the record is consistent with theexpected one.• Partial conflict detection The block contains the primary keyas well as any other field you plan to update. When the Updategramexecutes, the change is applied only if the specified fields haven't beenchanged in the meantime.• Total conflict detection All the columns in the row are checked, andthe change fails if any of them has been modified. You can obtain thisform of protection either by listing all the fields in the block orby using the table timestamp column, if one exists. A timestamp columnwill be updated whenever a user writes something to the row.Updategrams and DiffGramsIf you're familiar with ADO.NET, you'll no doubt notice a close similarity, bothconceptual and physical, between Updategrams and DiffGrams. Although ADO.NETDiffGrams are a newer format—and perhaps the format of the future—currently, SQLServer 2000 natively supports only Updategrams.In the section "SQLXML Managed Classes," on page 386, we'll take a quick tour of themanaged classes in SQLXML 3.0. You'll notice that some of these classes apparentlyenable you to send DiffGrams to SQL Server. Although this is possible, the actualimplementation is not particularly effective. The source DiffGram is in fact internallytransformed into an Updategram and then processed by SQL Server.Apart from the patent similarity in their schemas, Updategrams and DiffGrams haveslightly different goals. Updategrams have been designed to update SQL Server;DiffGrams are mostly a stateful way to persist the contents of a DataSet object.(ADO.NET DiffGrams are covered in Chapter 10.) Converting DiffGrams toUpdategrams is certainly possible at the schema level, but Updategrams areunquestionably more powerful objects. Together with SQLXML 3.0 and the SQL ServerXML extensions, Updategrams let you control concurrency, control the order ofupdates, perform transactional updates, and specify parameters.On the other hand, there is not yet a .NET Framework class that works like anUpdategram. (And SQLXML 3.0 is still a hybrid, half COM and half managed code.)Most of the batch update features you find in Updategrams can be implemented inADO.NET using the DataSet object's Update method and the provider-specific dataadapter object. Nothing comes for free, though, and you must write a lot of code toemulate Updategrams in the .NET Framework.XML Batch UpdateIn ADO.NET, as well as in ADO, you can persist the changes made to a set of recordsstored in memory using a procedure called a batch update. This procedure consists ofa loop that looks up for changed records in the DataSet object (or the Recordsetobject in ADO) and issues a command to the back-end database. From theprogrammer's perspective, a batch update is ideal for working in disconnectedscenarios and in ADO.NET—although it is not yet perfect, it has been significantlyimproved and made applicable to real-world usage.Thanks to the ADO.NET XML serialization mechanism (see Chapter 9), you can loada DataSet object from XML data, enter the needed changes, and then proceed withthe batch update. The ADO.NET DiffGram is one of the possible XML representationsfor a DataSet object. Although, all in all, the Updategram is a more powerful and richer313