PRB: Updated Data Delayed by Five (5) Millliseconds in Jet 3.x

• Microsoft Windows NT 4.0
• Microsoft Visual C++, 32-bit Professional Edition, versions 4.0, 4.1

SYMPTOMS

When two users are trying to gain access to the same Access database using Jet 3.x, there is a delay before data modified by one user becomes visible to the other user even if the code re-queries the database.

CAUSE

In order to improve performance, Microsoft Jet version 3.x uses threads to process read-ahead and write-behind caches asynchronously. Jet 3.0 defines a key in the registry that determines the delay before the internal cache is refreshed.

The PageTimeout property specifies the length of time, in milliseconds, between when data is placed in an internal cache and when it is checked to be potentially invalidated. Jet 3.x has a default value for the PageTimeout property of five (5) seconds, which means that the shared data will be refreshed after no more than five (5) milliseconds.

RESOLUTION

To ensure that the data is being written out to the disk, set PageTimeout to a value that is appropriate for your system and wait at least that amount of time before checking for the updated data.

Note that setting the value of PageTimeout too low may adversely affect system performance.

There are two techniques for setting the PageTimeout value:

1. Manually edit the registry. WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD). On Windows NT, run REGEDT32.EXE and select the HKEY_LOCAL_MACHINE hive. On Windows 95, run REGEDIT.EXE. Go to HKEY_LOCAL_MACHINE\Software\Microsoft\Jet\3.0\Engines\Jet and HKEY_LOCAL_MACHINE\Software\Microsoft\Jet\3.5\Engines\Jet3.5 for Jet 3.5. You may need to add the last "Jet" key. Make a REG_DWORD entry into the key called PageTimeout, and set the value to an appropriate value, in milliseconds.



2. Make the changes to the system registry programmatically. The sample code below demonstrates how to achieve this.

STATUS

This behavior is by design.

MORE INFORMATION

There are two factors involved in determining when updated data becomes visible to another user: the data must be written to the database and the updated data must become visible to the other user. The PageTimeout property addresses only the second issue; it specifies when the updated data becomes visible to another user after it has been written to the database.

The simplest way to ensure that the data is written to the database is to wrap the update in a transaction. The issue of when updated data is written to a database is affected by:

• whether explicit transactions are used.



• whether transactions are synchronous.



• the size of the internal cache.



• the delay before asynchronous changes are written out.



• whether the database is opened exclusively.

Q153491 PRB: Jet 3.0 UserCommitSync & ImplicitCommitSync Values Wrong

Sample Code

   #include <winreg.h>


   int SetPageTimeoutInSystemRegistry( )
   {
     HKEY hkey = HKEY_LOCAL_MACHINE;
     HKEY hResult;
     char szSubKey[]=_T( "Software\\Microsoft\\Jet\\3.0\\Engines\\Jet" );
     char szValue[]=_T( "PageTimeout" );
     char szClass[]=_T( "" );
     DWORD TimeOut = 20;  // set to desired timeout value, in milliseconds
     LONG lRetVal;
     DWORD dwDisp;


     // If key doesn't exist, it will be created.
     lRetVal = RegCreateKeyEx( hkey, szSubKey, 0, szClass,
                               REG_OPTION_NON_VOLATILE,
                               KEY_ALL_ACCESS, 0, &hResult, &dwDisp );


     // check the return value
     switch( lRetVal )
     {
       case ERROR_SUCCESS:
       case ERROR_FILE_NOT_FOUND:
         if( ERROR_SUCCESS != RegSetValueEx( hResult, szValue, 0,
             REG_DWORD, (const unsigned char *)&TimeOut, 4 ) )
         {
           ::MessageBox( NULL,  _T("error"),  _T("error"), MB_OK );
           return 0;
         }
       break;


       // default is error condition
       default:
         ::MessageBox( NULL,  _T("error"),  _T("error"), MB_OK );
       break;
     } // end of switch


     // Must close the key.
     if( ERROR_SUCCESS != RegCloseKey ( hResult ) )
     {
       ::MessageBox( NULL,  _T("error"),  _T("error"), MB_OK );
       return 0;
     }


     // Success!
     return 1;
   } 

REFERENCES

Microsoft Jet Database Engine Programmer's Guide.

Additional query words: 4.00 4.10

Keywords          : kbprg kbusage kbDAO kbDatabase kbMFC kbODBC kbVC kbprb 
Version           : winnt:4.0,4.1
Platform          : winnt 
Issue type        : kbprb 

Last Reviewed: July 29, 1999