ACC2000: DBEngine.Idle dbRefreshCache Refresh Is Not Immediate (202097)

MORE INFORMATION

With Microsoft Jet database engine 4.0, issuing DBEngine.Idle dbRefreshCache only flushes the cache for Database objects opened from Workspace objects that are in the DBEngine.Workspaces collection. If the Workspace is not in the collection, then its cached records will not be flushed.

NOTE: In addition to refreshing the read cache, the program writing the data must flush the lazy-write cache in order to prevent delays. This can result in a slow-down in data access if done in areas where it is not required. To flush the lazy-write cache, wrap the code in a transaction, and commit using the dbForceOSFlush flag. For example:

DBEngine(0).BeginTrans
' Update operation here.
DBEngine(0).CommitTrans dbForceOSFlush
				

Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific requirements. The example provided below uses a Visual Basic 5.0 out of process ActiveX server to provide the second instance of Jet while at the same time being able to be precisely synchronized.

Create the Out of Process Server

1. Create a new ActiveX EXE project in Visual Basic 5.0.
2. Add a Project Reference for Microsoft DAO 3.6 Object Library.
3. Add the following code to the class module:

   Option Explicit
   
   Dim db As Database, rs As Recordset
   
   Private Sub Class_Initialize()
     Set db = DBEngine(0).OpenDatabase("nwind.mdb")
     Set rs = db.OpenRecordset("Customers")
   End Sub
   
   Private Sub Class_Terminate()
     rs.Close
     db.Close
   End Sub
   
   Public Sub UpdateTitle(ByVal NewTitle As String)
     DBEngine(0).BeginTrans
     rs.Edit
     rs!ContactTitle = NewTitle
     rs.Update
     DBEngine(0).CommitTrans dbForceOSFlush
   End Sub
   					

4. Save the Project files.
5. Run the project. While the project runs, start a second instance of Visual Basic.

Create the Test Application

1. Create a new Standard EXE project in the second instance of Visual Basic and add a form with a command button (Command1).
2. Change the caption of the button to Operator.
3. Change the project name to Project2.
4. Add a Project Reference for Project1 and the Microsoft DAO 3.6 Object Library.
5. Add the following code:

   Option Explicit
   
   Dim ws As Workspace, db As Database, obj As Project1.Class1
   
   Private Sub Command1_Click()
      Dim rs As Recordset, Temp As String
      Dim fDone As Boolean, Counter As Long
      If Me!Command1.Caption = "Operator" Then
         Temp = "Programmer"
      Else
         Temp = "Operator"
      End If
      obj.UpdateTitle Temp
      fDone = False
      Counter = 0
      Do
         Counter = Counter + 1
         ' DBEngine.Idle dbRefreshCache
         Set rs = db.OpenRecordset("Customers")
         If rs!ContactTitle = Temp Then fDone = True
            rs.Close
      Loop Until fDone
      Me!Command1.Caption = Temp
      MsgBox "Took " & Counter & " retries to get the updated data."
   End Sub
   
   Private Sub Form_Load()
      Set ws = DBEngine.CreateWorkspace("", "Admin", "")
      ' Set ws = DBEngine.CreateWorkspace("JetTest", "Admin", "")
      ' DBEngine.Workspaces.Append ws
      Set db = ws.OpenDatabase("nwind")
      Set obj = New Project1.Class1
   End Sub
   
   Private Sub Form_Unload(Cancel As Integer)
      Set obj = Nothing
      db.Close
      ws.Close
   End Sub
   					

6. Run the project and click the button on the form. It will take many retries to see the new data.
7. End execution of the project, and then uncomment the "DBEngine.Idle dbRefreshCache" line in the Command1_Click procedure and try again. It will still take many tries to see the new data.
8. End execution of the project, and then comment the "Set ws =" line in the Form_Load procedure and uncomment the two lines following it. This time, the new data will be read on the first try.

NOTE: You have to give the Workspace object a name in order to be able to append it to the Workspaces collection.