Programming .NET Windows Applications

Multiuser Updates

In the previous section, you read data from the database into a DataSet, updated the data in the DataSet, and then wrote the changes back to the database. In a real-world application, it would be possible for other people to read the same data into DataSets of their own, edit their data, and write their changes back to the database.

You can easily imagine that this possibility could possibly cause tremendous data corruption. Imagine, for example, that a quality assurance person downloads the current open bugs with an eye toward updating some of the information. Meanwhile, across the office (or across town) a developer has downloaded and is reviewing a few open bugs. Both of them are reading bug 17, which looks like this:

BugID 17
Reporter: John Galt
Severity: High
Status: Assigned
Owner: Jesse Liberty

The QA person decides to change the severity to Medium and reassign the bug to Dan Hurwitz. Meanwhile, the developer is updating the DataSet to change the action taken on the bug. The QA person writes back the changed DataSet, and the database now thinks the Owner is Dan and the Severity is Medium. The record now appears as follows:

BugID 17
Reporter: John Galt
Severity: Medium
Status: Assigned
Owner: Dan Hurwitz

Then the developer writes back his DataSet, in which the Owner was Jesse and the Severity was High. These earlier values are written over the values updated by QA, and the QA edits are lost. The technical term for this is bad.

To prevent this kind of problem, ...