Or, between when a user loads a page and when they click the Delete button, another user may have modified the contents of that record.There are three concurrency control strategies available: All of our tutorials thus far have used the default concurrency resolution strategy - namely, we've let the last write win.For a web application that allows multiple users to edit data, there is the risk that two users may be editing the same data at the same time.
Therefore, in situations where pessimistic concurrency is used, there's typically a timeout that, if reached, cancels the lock.
Ticket sales websites, which lock a particular seating location for short period while the user completes the order process, is an example of pessimistic concurrency control.
Once this groundwork has been laid, we'll be ready to create the ASP. To create a new Typed Data Set, right-click on the .
As we saw in the first tutorial, doing so will add a new Table Adapter to the Typed Data Set, automatically launching the Table Adapter Configuration Wizard.
Enabling optimistic concurrency for a Table Adapter in the Typed Data Set augments the Table Adapter's statement, for example, updates the name and price of a product only if the current database values are equal to the values that were originally retrieved when updating the record in the Grid View.
The , that DB direct methods were those that accepts a list of scalar values as input parameters (rather than a strongly-typed Data Row or Data Table instance).
Since we used ad-hoc SQL queries in our original DAL, use this option here as well.
Figure 4: Specify the Data to Retrieve Using an Ad-Hoc SQL Statement (Click to view full-size image) On the following screen, enter the SQL query to use to retrieve the product information. Discontinued = discontinued End Sub _ Public Function Update Product( By Val product Name As String, By Val supplier ID As Nullable(Of Integer), _ By Val category ID As Nullable(Of Integer), By Val quantity Per Unit As String, _ By Val unit Price As Nullable(Of Decimal), By Val units In Stock As Nullable(Of Short), _ By Val units On Order As Nullable(Of Short), By Val reorder Level As Nullable(Of Short), _ By Val discontinued As Boolean, By Val product ID As Integer, _ _ By Val original_product Name As String, _ By Val original_supplier ID As Nullable(Of Integer), _ By Val original_category ID As Nullable(Of Integer), _ By Val original_quantity Per Unit As String, _ By Val original_unit Price As Nullable(Of Decimal), _ By Val original_units In Stock As Nullable(Of Short), _ By Val original_units On Order As Nullable(Of Short), _ By Val original_reorder Level As Nullable(Of Short), _ By Val original_discontinued As Boolean, _ By Val original_product ID As Integer) _ As Boolean ' STEP 1: Read in the current database product information Dim products As _ Northwind Optimistic Concurrency.
Figure 1 graphically depicts this series of events.
Figure 1: When Two Users Simultaneously Update a Record There s Potential for One User 's Changes to Overwrite the Other 's (Click to view full-size image) Similarly, when two users are visiting a page, one user might be in the midst of updating a record when it is deleted by another user.
This results in an to the corresponding Beverages category ID, and so on.