Article: Q240317
Product(s): Microsoft Visual Basic for Windows
Version(s): 2.1 SP1,2.1 SP2,2.5,2.6,5.0,6.0
Operating System(s):
Keyword(s): kbClient kbDatabase kbJET kbOLEDB kbGrpDSVBDB kbDSupport kbADO250 kbADO260 kbATM
Last Modified: 13-MAY-2002
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The information in this article applies to:
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0
- ActiveX Data Objects (ADO), versions 2.1 SP1, 2.1 SP2, 2.5, 2.6
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SUMMARY
=======
Because the Microsoft Jet database engine has a read cache and lazy writes, you
can get duplicate values in your custom counter field if two applications add
records in less time than it takes for the cache to refresh and the lazy-write
mechanism to flush to disk. This article presents a method that takes these
factors into account.
MORE INFORMATION
================
The Microsoft Jet database engine provides a Counter (AutoIncrement) field.
Starting with Jet 4.0, you can specify a starting value and an increment value.
This solves most of the need for using a custom counter that users of earlier
versions of Jet had. However, if your counter is a text value or does not
increase in a fixed amount, the steps outlined in this article may benefit your
application.
Microsoft Jet has a read-cache that is updated every PageTimeout milliseconds
(default is 5000ms = 5 seconds). It also has a lazy-write mechanism that
operates on a separate thread to main processing and thus writes changes to disk
asynchronously. These two mechanisms help boost performance, but in certain
situations that require high concurrency, they may create problems.
The Microsoft Jet 4.0 OLEDB provider and the Microsoft Jet and Replication
Objects type library provide two methods to ensure that your application has
current data:
- JetEngine object's RefreshCache method.
- "Jet OLEDB:Transaction Commit Mode" Connection property.
JetEngine object's RefreshCache Method
--------------------------------------
There is a separate read-cache for each connection object. Calling the
RefreshCache method of the JetEngine object, as exposed by the Microsoft Jet and
Replication Objects type library, immediately refreshes the read-cache for the
Connection objects passed as a method argument. The read-cache for all other
Connection objects in the application are not affected.
"Jet OLEDB:Transaction Commit Mode" Connection Property
-------------------------------------------------------
In Microsoft Jet 2.x and earlier, all writes were immediately committed. With
Win32 and multi-threading, Microsoft Jet introduced a lazy-write mechanism.
Setting this property to a value of 1 causes all transaction commits to be
written immediately to disk for that Connection object. All other Connection
objects are unaffected.
These methods are preferable to modifying registry values to get the same effect
because you can precisely control where you need this value and they are more
deterministic in operation. Global registry programs will adversely affect
engine performance in other areas and in other applications.
EXAMPLE
The following example provides a function for generating custom counter numbers
and handling the concurrency and locking issues that result from the process. It
involves the use of a second table to store the next available key value. This
is used for performance reasons and also to avoid adversely affecting users who
would just need to read data.
The main function is NextKeyValue. It accepts three arguments:
- Connection object
- Table name
- Increment value
Increment value is optional and defaults to 1. The function opens the table using
a server-side cursor, a transaction, and pessimistic locking and then reads the
value from the first field of the first record. This is the key value returned.
It then increments the value for the next user and releases the table.
When you set the initial value in the table, this is the first value returned by
the function.
The error handling is designed to handle locking problems opening the table. If
the locks time-out, the function returns -1 as the next key value. If any other
error occurs, the function raises a run-time error that the main application
will need to trap.
Because most people leave their registry settings untouched, Microsoft Jet will
usually have a 100ms delay between lock-retries. If all instances of Jet have
the same delay, this could result in a race situation and cause your application
to time-out more than is necessary. The NextKeyValue function sets the lock
retry to a random interval between 60 and 150 milliseconds to reduce the chance
of a race condition occurring. The test application is responsible for using the
RANDOM statement to seed the random number generator.
The test application adds 100 records to the table. It does not implement any but
the most basic error handling to handle lock timeouts between the program and
others like it.
Microsoft provides programming examples for illustration only, without warranty
either expressed or implied, including, but not limited to, the implied
warranties of merchantability and/or fitness for a particular purpose. This
article assumes that you are familiar with the programming language being
demonstrated and the tools used to create and debug procedures. Microsoft
support professionals 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 needs. If you have
limited programming experience, you may want to contact a Microsoft Certified
Partner or the Microsoft fee-based consulting line at (800) 936-5200. For more
information about Microsoft Certified Partners, please visit the following
Microsoft Web site:
http://www.microsoft.com/partner/referral/
For more information about the support options that are available and about how
to contact Microsoft, visit the following Microsoft Web site:
http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS
Database Setup
--------------
1. In Microsoft Access, open the NWind.MDB file, and create the following
tables:
Table: KeyStore
Field Name: NextValue
Type: Number, Long Integer
Field Name: Dummy
Type: Number, Long Integer
Table: KeyTest
Field Name: ID
Type: Number, Long Integer
Field Name: Description
Type: Text, 50
2. Open the KeyStore table and add a record. Set NextValue to the amount from
which you want your counter to start. For example: 3.
3. Save the record and close Microsoft Access.
Test Program
------------
1. Open a new Visual Basic or Visual Basic for Applications project with a Form
(Form1) and a Module (Module1).
2. From the Project menu, choose References and add a reference to the Microsoft
ActiveX Data Objects 2.1 (or later) Library and the Microsoft Jet and
Replication Objects 2.1 (or later) Library.
3. Add the following code to the Module:
Option Explicit
Private Const MAX_RETRIES = 10
Function NextKeyValue(cn As ADODB.Connection, _
ByVal TableName As String, _
Optional Increment As Long = 1) As Long
Dim OldCommitMode As Long, rs As Recordset, ErrorCount As Long
Dim TempKeyValue As Long, Jet As JRO.JetEngine, InTrans As Boolean
NextKeyValue = -1 ' returns this if the routine times out
OldCommitMode = cn.Properties("Jet OLEDB:Transaction Commit Mode")
cn.Properties("Jet OLEDB:Transaction Commit Mode") = 1
cn.Properties("Jet OLEDB:Lock Delay") = 90 + Int(Rnd * 60)
Set rs = New ADODB.Recordset
Set Jet = New JRO.JetEngine
On Error GoTo NKV_Error
rs.Open TableName, cn, adOpenDynamic, adLockPessimistic, adCmdTableDirect
cn.BeginTrans
InTrans = True
NKV_InnerRetry:
If rs.EditMode Then rs.CancelUpdate
rs!Dummy = 0 ' lock the record
Jet.RefreshCache cn ' get current data
TempKeyValue = rs(0) ' .UnderlyingValue gives an error
rs(0) = TempKeyValue + Increment
rs.Update
InTrans = False
cn.CommitTrans
rs.Close
cn.Properties("Jet OLEDB:Transaction Commit Mode") = OldCommitMode
NextKeyValue = TempKeyValue
Exit Function
NKV_Abort:
On Error Resume Next
If rs.EditMode Then rs.CancelUpdate
cn.RollbackTrans
rs.Close
cn.Properties("Jet OLEDB:Transaction Commit Mode") = OldCommitMode
Exit Function
NKV_Error:
Select Case Err.Number
Case &H80004005
' Various locking errors (above)
ErrorCount = ErrorCount + 1
If ErrorCount < MAX_RETRIES Then
Resume NKV_Abort
ElseIf InTrans Then
Resume NKV_InnerRetry
Else
Resume
End If
Case Else ' unhandled errors
Err.Raise Err.Number, Err.Source, Err.Description
End Select
End Function
4. Add a command button (Command1) and a text box (Text1) to the form.
5. Add the following code to the form:
Option Explicit
Dim cn As ADODB.Connection
Private Sub Command1_Click()
Dim SQL As String, I As Long
For I = 1 To 100
SQL = "INSERT INTO KeyTest VALUES (" & _
NextKeyValue(cn, "KeyStore", 10) & _
",'Test Record " & Me.hWnd & " " & Time & "')"
On Error GoTo C1_Error
cn.Execute SQL, , adExecuteNoRecords
On Error GoTo 0
Text1 = CStr(I)
Me.Refresh
Next I
Exit Sub
C1_Error:
Resume
End Sub
Private Sub Form_Load()
Randomize
Set cn = New ADODB.Connection
cn.Open "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=\\moonbase\public\northwind40.mdb"
End Sub
Private Sub Form_Unload(Cancel As Integer)
cn.Close
End Sub
6. In Visual Basic, compile the application to an EXE. Run the application from
multiple computers and click Command1 at the same time. It will run to
completion (barring any unhandled errors) and then add records without
duplicate ID numbers.
NOTES
-----
- The KeyStore table does not require an index. You will get better performance
and have fewer problems if you do not add (or allow Microsoft Access to add)
and index or Primary Key to the table.
- The fewer indexes on the KeyText table, the better. Indices have a negative
effect on the ability to lock and update data quickly. If you have problems
in your application, try reducing the number of indices.
- Unlike Data Access Objects (DAO) or Remote Data Objects (RDO), ADO does not
have an Edit method. In order to turn on pessimistic locking, the program
modifies the "Dummy" field in the record to get the same effect. This field
has been added to the table solely for this purpose and does not contain any
useful information.
REFERENCES
==========
For more information about Jet and ADO, please see:
Migrating from DAO to ADO and Using ADO with the Jet Provider
For information on implementing a mutli-user custom counter using Microsoft Data
Access Objects (DAO):
Q191253 HOWTO: Implement Multi-user Custom Counters in DAO 3.5
Additional query words: count autonumber custom multiuser kbVBp500 kbVBp600 kbdse kbDSupport kbVBp
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Keywords : kbClient kbDatabase kbJET kbOLEDB kbGrpDSVBDB kbDSupport kbADO250 kbADO260 kbATM
Technology : kbVBSearch kbAudDeveloper kbADOsearch kbADO210sp1 kbADO210sp2 kbADO250 kbADO260 kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600
Version : :2.1 SP1,2.1 SP2,2.5,2.6,5.0,6.0
Issue type : kbhowto
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