Microsoft KB Archive/129647

= Using ROLLBACK and END TRANSACTION in Nested Transactions =

Article ID: 129647

Article Last Modified on 8/25/1999

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APPLIES TO


 * Microsoft Visual FoxPro 3.0 Standard Edition

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This article was previously published under Q129647



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:

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

Keywords: kbcode KB129647

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