PRB: Single-User Concurrency Problems With ADO and Jet (216925)

MORE INFORMATION

Microsoft Jet maintains a separate cache for each connection to the engine. By default, the cache times out after five seconds, which means that changes made to an MDB file on one connection may not be visible on another connection for up to five seconds.

This is different behavior than DAO, where all Connection or Database objects default to share the same read cache.

This article outlines some strategies that allow you to share connections between Recordset objects and the ADO Data control.

When opening an ADO Recordset object, it is easy to share a connection. You can open a global connection object and reference it in the rs.Open method. This method can provide application or form-wide connection sharing:

rs1.Open "Employees", cn
rs2.Open "Employees", cn
				

Another more limited method of connection sharing is to reference the ActiveConnection property of an already open Recordset:

rs1.Open "Employees", "Provider=Microsoft.Jet.OLEDB.3.51;Data Source=NWIND.MDB"
rs2.Open "Employees", rs1.ActiveConnection
				

You can open a Recordset on the same connection as an ADO Data control by referencing the ActiveConnection property of the Recordset property:

rs.Open "Employees", ADODC1.Recordset.ActiveConnection
				

If the ADO Data control (ADODC2, ADODC3, and ADODC4 in the example below) is using client-side cursors, then you can make it share an existing connection:

Set ADODC2.Recordset.ActiveConnection = cn
Set ADODC3.Recordset.ActiveConnection = rs.ActiveConnection
Set ADODC4.Recordset.ActiveConnection = ADODC1.Recordset.ActiveConnection
				

The next technique allows ADO Data controls to share connections and use server-side cursors. It involves setting the ADO Data control's Recordset property to an existing Recordset that you create using a shared Connection object.

NOTE: You cannot substitute the connection if using server-side cursors (the default):

rs.Open "Employees", cn
Set ADODC1.Recordset = rs
				

Steps to Reproduce Behavior

1. Open a new Project in Visual Basic 6.0 with a form (Form1).
2. Add a reference to Microsoft ActiveX Data Objects 2.0 Library.
3. Add the ADO Data Control (6.0) to the project.
4. Add the ADO Data control to the form and set the following properties:
   
   

   Property	Value
   Name	ADODC1
   CommandType	adCmdTable
   ConnectionString	Provider=Microsoft.Jet.OLEDB.3.51;Data Source=NWIND.MDB
   CursorLocation	adUseServer
   CursorType	adOpenStatic
   LockType	adLockOptimistic
   RecordSource	Employees

5. Add four textboxes to the form and set the following properties:
   
   

   Property	Value
   Name	FirstName
   DataSource	ADODC1
   DataField	FirstName

   
   

   Property	Value
   Name	LastName
   DataSource	ADODC1
   DataField	LastName

   
   

   Property	Value
   Name	SameConnection

   
   

   Property	Value
   Name	NewConnection

6. Add a CommandButton (Command1) to the form.
7. Add the following code to the form:
   

   Option Explicit
   
   Private Sub Command1_Click()
   Dim rs As ADODB.Recordset
     Set rs = New ADODB.Recordset
   
     ' Save the data if necessary 
     If ADODC1.Recordset.EditMode <> adEditNone Then
       ADODC1.Recordset.Update
     End If
   
     ' Open Recordset on same connection as the data control
     rs.Open "Employees", ADODC1.Recordset.ActiveConnection, , , adCmdTable
     SameConnection.Text = rs!LastName
     rs.Close
   
     ' Open recordset on a new connection
     rs.Open "Employees", ADODC1.ConnectionString, , , adCmdTable
     NewConnection.Text = rs!LastName
     rs.Close
   End Sub
   						

8. Run the project and edit the Employee's last name.
9. Click on the CommandButton to save the data, and then read it. You should see the edited value displayed in the SameConnection textbox and the original value displayed in the NewConnection textbox.
10. Wait a few seconds and click the CommandButton again. The edited value should now also show in the NewConnection text box. This is due to the cache timing out.
    
    NOTE: When using the ADO Data control in the manner described above, avoid using the Refresh method:
    

    ADODC1.Refresh
    						

    because this will result in a new connection. Use the Requery method of the Recordset property instead:

    ADODC1.Recordset.Requery