Q129647: Using ROLLBACK and END TRANSACTION in Nested Transactions

Article: Q129647
Product(s): Microsoft FoxPro
Version(s): 3.00
Operating System(s): 
Keyword(s): kbcode
Last Modified: 25-AUG-1999

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0 
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SUMMARY
=======

Visual FoxPro allows transactions to be nested up to five levels deep. This
article explains the results of using ROLLBACK and END TRANSACTION within nested
transactions.

MORE INFORMATION
================

When you issue a BEGIN TRANSACTION command, the transaction level is incremented
by one.

When you issue an END TRANSACTION, changes to the same record are written
starting at the first transaction level and ending with the fifth or last
transaction level.

When you issue a ROLLBACK, changes to the same record are discarded beginning
with the highest (fifth or last) transaction level and ending with the first
transaction level.

Code Sample One
---------------

To test the behavior of nested transactions, follow these steps:

1. Create a new database named NESTED that holds one table named TEST. Add one
  Character field named cLevel to the table.

2. Create and run a new program containing the following code:

     OPEN DATABASE nested
     USE test
     APPEND BLANK
     BEGIN TRANSACTION
        REPLACE cLevel WITH "One"
        =tranlevel("Begin transaction")
        BEGIN TRANSACTION
           REPLACE cLevel WITH "Two"
           =tranlevel("Begin transaction")
           BEGIN TRANSACTION
              REPLACE cLevel WITH "Three"
              =tranlevel("Begin transaction")
           END TRANSACTION
           =tranlevel("End transaction")
        END TRANSACTION
        =tranlevel("End transaction")
     END TRANSACTION
     =tranlevel("End transaction")

     PROCEDURE tranlevel
     PARAMETER lcAction

     =MESSAGEBOX("Transaction level:  " + STR(TXNLEVEL()) + ;
                 CHR(13) + ;
                 "Action performed:   " + lcAction + ;
                 CHR(13) + ;
                 "cLevel value:      " + cLevel)

     *** End of code example

When you run the program, you should see the level name change from "One" to
"Two" to "Three" as the transaction level increases. As each transaction nesting
level is committed with the END TRANSACTION command, you should see the
transaction level decrease, but the value contained in the field cLevel name
should remain constant at "Three."

Code Sample Two
---------------

Modify the program so that it reads as follows (the inner two END TRANSACTION
commands have been changed to ROLLBACK commands). Rerun the program, and notice
that the results are much different.

     OPEN DATABASE nested
     USE test
     APPEND BLANK
     BEGIN TRANSACTION
        REPLACE cLevel WITH "One"
        =tranlevel("Begin transaction")
        BEGIN TRANSACTION
           REPLACE cLevel WITH "Two"
           =tranlevel("Begin transaction")
           BEGIN TRANSACTION
              REPLACE cLevel WITH "Three"
              =tranlevel("Begin transaction")
           ROLLBACK
           =tranlevel("Rollback")
        ROLLBACK
        =tranlevel("Rollback")
     END TRANSACTION
     =tranlevel("End transaction")

     * PROCEDURE tranlevel DOES NOT CHANGE
     *** END OF CODE EXAMPLE

The final value of the field cLevel should be "One."

Additional query words: VFoxWin

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Keywords          : kbcode 
Technology        : kbVFPsearch kbAudDeveloper kbVFP300
Version           : 3.00

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