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
- Create a new ActiveX EXE project in Visual Basic 5.0.
- Add a Project Reference for Microsoft DAO 3.6 Object Library.
- 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
- Save the Project files.
- Run the project. While the project runs, start a second instance of
Visual Basic.
Create the Test Application
- Create a new Standard EXE project in the second instance of Visual Basic and add a form with a command button (Command1).
- Change the caption of the button to Operator.
- Change the project name to Project2.
- Add a Project Reference for Project1 and the Microsoft DAO 3.6 Object
Library.
- 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
- Run the project and click the button on the form. It will take many
retries to see the new data.
- 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.
- 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.