.net upc-a CREATE DATABASE TestDB; GO USE TestDB; GO CREATE SCHEMA Test; GO in .NET framework

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CREATE DATABASE TestDB; GO USE TestDB; GO CREATE SCHEMA Test; GO
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Lesson 2: Designing Transactions and Optimizing Locking
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CREATE TABLE Test.Accounts ( AccountNumber INT PRIMARY KEY ); CREATE TABLE Test.AccountTransactions ( TransactionID INT IDENTITY PRIMARY KEY ,AccountNumber INT NOT NULL REFERENCES Test.Accounts ,CreatedDateTime DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ,Amount DECIMAL(19, 5) NOT NULL ); GO CREATE PROC Test.spAccountReset AS BEGIN SET NOCOUNT ON; DELETE Test.AccountTransactions; DELETE Test.Accounts; INSERT INSERT VALUES INSERT VALUES INSERT VALUES Test.Accounts (AccountNumber) VALUES (1001); Test.AccountTransactions (AccountNumber, Amount) (1001, 100); Test.AccountTransactions (AccountNumber, Amount) (1001, 500); Test.AccountTransactions (AccountNumber, Amount) (1001, 1400);
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SELECT AccountNumber, SUM(Amount) AS Balance FROM Test.AccountTransactions GROUP BY AccountNumber; END
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3. Open another query window, and type and execute the following SQL statements to create the Test.spAccountWithdraw stored procedure:
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USE TestDB; GO CREATE PROC Test.spAccountWithdraw @AccountNumber INT ,@AmountToWithdraw DECIMAL(19, 5) AS BEGIN SET TRANSACTION ISOLATION LEVEL READ COMMITTED; BEGIN TRY IF(@AmountToWithdraw <= 0) RAISERROR('@AmountToWithdraw must be > 0.', 16, 1); BEGIN TRAN;
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Designing Transactions and Transaction Isolation
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Verify that the account exists and LOCK the account from access by other queries that will write to the account or its transactions. Note that SELECT statements against the account will still be allowed. NOT EXISTS( SELECT * FROM Test.Accounts WITH (UPDLOCK) WHERE AccountNumber = @AccountNumber ) RAISERROR('Account not found.', 16, 1);
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-- Verify that the account will not be overdrawn... IF (@AmountToWithdraw > ( SELECT SUM(Amount) FROM Test.AccountTransactions /* NO LOCKING HINT */ WHERE AccountNumber = @AccountNumber) ) RAISERROR('Not enough funds in account.', 16, 1); -- ** USED TO TEST CONCURRENCY PROBLEMS ** RAISERROR('Pausing procedure for 10 seconds...', 10, 1) WITH NOWAIT; WAITFOR DELAY '00:00:30'; RAISERROR('Procedure continues...', 10, 1) WITH NOWAIT; -- Make the withdrawal... INSERT Test.AccountTransactions (AccountNumber, Amount) VALUES (@AccountNumber, -@AmountToWithdraw); -- Return the new balance of the account: SELECT SUM(Amount) AS BalanceAfterWithdrawal FROM Test.AccountTransactions WHERE AccountNumber = @AccountNumber; COMMIT TRAN; END TRY BEGIN CATCH DECLARE @ErrorMessage NVARCHAR(2047); SET @ErrorMessage = ERROR_MESSAGE(); RAISERROR(@ErrorMessage, 16, 1); -- Should also use ERROR_SEVERITY() and ERROR_STATE()... IF(XACT_STATE() <> 0) ROLLBACK TRAN; END CATCH END
Lesson 2: Designing Transactions and Optimizing Locking
4. Open another query window, which will be referred to as Connection 1, and type and execute the following SQL statement to prepare the connection:
-- Connection 1 /* Leave the above line to easily see that this query window belongs to Connection 1. */ USE TestDB; GO -- Reset/generate the account data: EXEC Test.spAccountReset;
5. Open another query window, which will be referred to as Connection 2, and type and execute the following SQL statement to prepare the connection:
-- Connection 2 /* Leave the above line to easily see that this query window belongs to Connection 2. */ USE TestDB; GO
6. In this step, you will execute two batches at the same time to try to test for concurrency problems. In both the Connection 1 and Connection 2 query windows, type the following SQL statements without executing them yet. The statements will first retrieve the current account balance and will then attempt to empty the account.
SELECT SUM(Amount) AS BalanceBeforeWithdrawal FROM Test.AccountTransactions WHERE AccountNumber = 1001; GO EXEC Test.spAccountWithdraw @AccountNumber = 1001, @AmountToWithdraw = 2000;
To get a better view of what will happen, press Ctrl+T in SQL Server Management Studio to set results to be returned as text instead of grids. Do this for both query windows. Now, start the execution in both query windows simultaneously and wait for both batches to finish execution. (This should take approximately 30 seconds because of the WAITFOR DELAY statement in the Test.spAccountWithdraw stored procedure.) Both connections batches should return two result sets; the first result set will contain the current account balance (which should be 2,000 for both batches), and the second result set will contain the account balance after the withdrawal. What was the result of the two withdrawals Was the account overdrawn What kind of concurrency problem occurred (if any) Was there any other problem with this implementation
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