Design data integrity. in Visual Basic .NET

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Design data integrity.
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Reconcile data conflicts. Make implicit constraints explicit. Assign data types to control characteristics of data stored in a column.
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Lessons in this chapter:
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Lesson 1: Reconciling Data Conflicts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 Lesson 2: Making Implicit Constraints Explicit . . . . . . . . . . . . . . . . . . . . . . . . . 428 Lesson 3: Assigning Data Types to Control Characteristics of Data Stored in a Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
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Design Data Integrity
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Before You Begin
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To complete the lessons in this chapter, you must have completed the following tasks:
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Configured a Microsoft Windows Server 2003 R2 computer with SQL Server 2005 Enterprise Edition SP1 as detailed in the Appendix. Installed an updated copy of the AdventureWorks sample database as detailed in the Appendix.
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No additional configuration is required for this chapter.
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Lesson 1: Reconciling Data Conflicts
Lesson 1: Reconciling Data Conflicts
Data conflicts are inevitable when it is possible to modify data in replicated databases at multiple locations. This inevitability stems from the latency inherent in replication between a publisher and its subscribers. The possibility always exists that during that replication window two separate people at two separate locations might alter the same piece of information. This lesson discusses the processes that you can configure within SQL Server to resolve this type of conflict.
NOTE
Replication
Although 10, Replication, more explicitly details the intricacies of the SQL Server 2005 replication process, understanding this lesson requires only that you grasp the concepts of Publisher, Subscriber, and Article, which are key components of this process. A Publisher is a database instance that makes data available to other locations through replication. A Subscriber is a database instance that receives replicated data from one or more Publishers. Subscribers are able to forward changes back to Publishers or republish data to other Subscribers. An Article is a database object, such as a table, included in a publication.
After this lesson, you will be able to:
Understand how SQL Server 2005 detects data conflicts. Understand the different conflict resolution policies available in SQL Server 2005. Configure a conflict resolution policy.
Estimated lesson time: 30 minutes
Detecting Conflicts
When you create a publication, the replication process adds a Uniqueidentifier column to all tables included in the Article, as shown in Figure 8-1. Whenever published data is modified at either the Publisher or the Subscriber, SQL Server inserts a new globally unique identifier (GUID) in the Uniqueidentifier rows where the modified data resides. When synchronization between the Publisher and Subscriber occurs, the Queue Reader Agent compares the previous and current values of these GUIDs to determine whether a conflict exists. During synchronization, a transaction in the queue maintains both the old and new rows of GUIDs. The two GUIDs in the transaction and the GUID in the relevant table on the publication are compared when the transaction is applied at the Publisher. If the old GUID stored within the transaction is identical to the GUID in the publication, SQL Server updates the publication. When this update occurs, the appropriate table
8
Design Data Integrity
row s Uniqueidentifier column is assigned the GUID that was generated by the Subscriber when the change was made. The old GUID matching the publication s GUID indicates that the publication s data for that row has not changed since the data at the Subscriber was last synchronized.
Figure 8-1
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