TN068: Performing Transactions with the Microsoft Access 7 ODBC Driver

This note describes how to perform transactions when using the MFC ODBC database classes and the Microsoft Access 7.0 ODBC driver included in the Microsoft ODBC Desktop Driver Pack version 3.0.

Overview

If your database application performs transactions, you must be careful to call CDatabase::BeginTrans and CRecordset::Open in the correct sequence in your application. The Microsoft Access 7.0 driver uses the Microsoft Jet database engine, and Jet requires that your application not begin a transaction on any database that has an open cursor. For the MFC ODBC database classes, an open cursor equates to an open CRecordset object.

If you open a recordset before calling BeginTrans, you may not see any error messages. However, any recordset updates your application makes become permanent after calling CRecordset::Update, and the updates will not be rolled back by calling Rollback. To avoid this problem, you must call BeginTrans first and then open the recordset.

MFC checks the driver functionality for cursor commit and rollback behavior. Class CDatabase provides two member functions, GetCursorCommitBehavior and GetCursorRollbackBehavior, to determine the effect of any transaction on your open CRecordset object. For the Microsoft Access 7.0 ODBC driver, these member functions return SQL_CB_CLOSE because the Access driver does not support cursor preservation. Therefore, you must call CRecordset::Requery following a CommitTrans or Rollback operation.

When you need to perform multiple transactions one after another, you cannot call Requery after the first transaction and then start the next one. You must close the recordset before the next call to BeginTrans in order to satisfy Jet's requirement. This technical note describes two methods of handling this situation:

• Closing the recordset after each CommitTrans or Rollback operation.

• Using the ODBC API function SQLFreeStmt.

Closing the Recordset after each CommitTrans or Rollback Operation

Before starting a transaction, make sure the recordset object is closed. After calling BeginTrans, call the recordset's Open member function. Close the recordset immediately after calling CommitTrans or Rollback. Note that repeatedly opening and closing the recordset can slow an application's performance.

Using SQLFreeStmt

You can also use the ODBC API function SQLFreeStmt to explicitly close the cursor after ending a transaction. To start another transaction, call BeginTrans followed by CRecordset::Requery. When calling SQLFreeStmt, you must specify the recordset's HSTMT as the first parameter and SQL_CLOSE as the second parameter. This method is faster than closing and opening the recordset at the start of every transaction. The following code demonstrates how to implement this technique:

CMyDatabase db;
db.Open( "MYDATASOURCE" );
CMyRecordset rs( &db );

// start transaction 1 and 
// open the recordset
db.BeginTrans( );
rs.Open( );

// manipulate data

// end transaction 1
db.CommitTrans( );  // or Rollback( )

// close the cursor
::SQLFreeStmt( rs.m_hstmt, SQL_CLOSE );

// start transaction 2
db.BeginTrans( );

// now get the result set
rs.Requery( );

// manipulate data

// end transaction 2
db.CommitTrans( );

rs.Close( );
db.Close( );

Another way to implement this technique is to write a new function, RequeryWithBeginTrans, which you can call to start the next transaction after you commit or rollback the first one. To write such a function, do the following steps:

1. Copy the code for CRecordset::Requery( ) to the new function.

2. Add the following line immediately after the call to SQLFreeStmt:

   m_pDatabase->BeginTrans( );

Now you can call this function between each pair of transactions:

// start transaction 1 and 
// open the recordset
db.BeginTrans( );
rs.Open( );

// manipulate data

// end transaction 1
db.CommitTrans( );  // or Rollback( )

// close the cursor, start new transaction,
// and get the result set
rs.RequeryWithBeginTrans( );

// manipulate data

// end transaction 2
db.CommitTrans( );  // or Rollback( )

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