barcode generator in vb.net 2005 Figure 12-5: The transaction log in Software

Creating QR Code in Software Figure 12-5: The transaction log

Figure 12-5: The transaction log
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Here is how the transaction log works, in simplified form. When a user executes a SQL statement that modifies the database, the DBMS automatically writes a record in the transaction log showing two copies of each row affected by the statement. One copy shows the row before the change, and the other copy shows the row after the change. Only after the log is written does the DBMS actually modify the row on the disk. If the user subsequently executes a COMMIT statement, the end-of-transaction is noted in the transaction log. If the user executes a ROLLBACK statement, the DBMS examines the log to find the "before" images of the rows that have been modified since the transaction began. Using these images, the DBMS restores the rows to their earlier state, effectively backing out all changes to the database that were made during the transaction. If a system failure occurs, the system operator typically recovers the database by running a special recovery utility supplied with the DBMS. The recovery utility examines the end of the transaction log, looking for transactions that were not committed before the failure. The utility rolls back each of these incomplete transactions, so that only committed transactions are reflected in the database; transactions in process at the time of the failure have been rolled back. The use of a transaction log obviously imposes an overhead on updates to the database. In practice, the mainstream commercial DBMS products use much more sophisticated logging techniques than the simple scheme described here to minimize this overhead. In addition, the transaction log is usually stored on a fast disk drive, different from the one that
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stores the database, to minimize disk access contention. Some personal computer DBMS brands allow you to disable transaction logging to increase the performance of the DBMS. This may also be an acceptable alternative in specialized production databases, for example, where the database contents are replicated on a duplicate computer system. In most common production databases, however, a logging scheme and its overhead are an integral part of the database operation.
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Transactions and Multi-User Processing
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When two or more users concurrently access a database, transaction processing takes on a new dimension. Now the DBMS must not only recover properly from system failures or errors, it must also ensure that the users' actions do not interfere with one another. Ideally, each user should be able to access the database as if he or she had exclusive access to it, without worrying about the actions of other users. The SQL transaction model allows a SQL-based DBMS to insulate users from one another in this way. The best way to understand how SQL handles concurrent transactions is to look at the problems that result if transactions are not handled properly. Although they can show up in many different ways, four fundamental problems can occur. The next four sections give a simple example of each problem.
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The Lost Update Problem
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Figure 12-6 shows a simple application where two users accept telephone orders from customers. The order entry program checks the PRODUCTS file for adequate inventory before accepting the customer's order. In the figure, Joe starts entering an order for 100 ACI-41004 Widgets from his customer. At the same time, Mary starts entering her customer's order for 125 ACI-41004 Widgets. Each order entry program does a query on the PRODUCTS file, and each finds that 139 Widgets are in stock more than enough to cover the customer's request. Joe asks his customer to confirm the order, and his copy of the order entry program updates the PRODUCTS file to show (139 100) = 39 Widgets remaining for sale and inserts a new order for 100 Widgets into the ORDERS table. A few seconds later, Mary asks her customer to confirm their order. Her copy of the order entry program updates the PRODUCTS file to show (139 125) = 14 Widgets remaining in stock and inserts a new order for 125 Widgets into the ORDERS table.
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