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.
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.
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 Microsoft Advisory Services. For more information, visit these Microsoft Web sites:
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- 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
- Open the KeyStore table and add a record. Set NextValue to the amountfrom which you want your counter to start. For example: 3.
- Save the record and close Microsoft Access.
- Open a new Visual Basic or Visual Basic for Applications project with a Form (Form1) and a Module (Module1).
- 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.
- Add the following code to the Module:
Option ExplicitPrivate Const MAX_RETRIES = 10Function NextKeyValue(cn As ADODB.Connection, _ ByVal TableName As String, _ Optional Increment As Long = 1) As LongDim OldCommitMode As Long, rs As Recordset, ErrorCount As LongDim 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 = TrueNKV_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 FunctionNKV_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 SelectEnd Function
- Add a command button (Command1) and a text box (Text1) to the form.
- Add the following code to the form:
Option ExplicitDim cn As ADODB.ConnectionPrivate 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: ResumeEnd SubPrivate Sub Form_Load() Randomize Set cn = New ADODB.Connection cn.Open "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=\\moonbase\public\northwind40.mdb"End SubPrivate Sub Form_Unload(Cancel As Integer) cn.CloseEnd Sub
- 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.
- The KeyStore table does not require an index. You will get betterperformance 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 anegative effect on the ability to lock and update data quickly. If youhave 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.