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FIGURE 12-5 The transaction log
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SQL statement sequence 12:01 UPDATE OFFICES
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Transaction log Row location: _ Before: _, _, _, _, _, _, _ After: _, _, _, _, _, _, _ Row location: _ Before: _, _, _, _ After: (Empty)
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12:04 DELETE FROM CUSTOMERS DBMS 12:05 INSERT INTO PRODUCTS
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Row location: _ Before: _, _, _, _ After: (Empty) Row location: _ Before: (Empty) After: _, _, _, _, _
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Transaction committed
much more sophisticated logging techniques than the simple scheme described here. In addition, the transaction log is usually stored on a disk storage system, different from the one that stores the database, to minimize disk access contention. Some DBMS brands allow you to disable transaction logging for specific database objects or for particular database operations such as bulk loading a table, to increase the performance of the DBMS. Specialized databases, such as in-memory databases or cached database copies, may also use this log-free architecture. 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.
Transactions and Multiuser Processing
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, but 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.
12:
Tr a n s a c t i o n P r o c e s s i n g
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.
The Lost Update Problem
Figure 12-6 shows a simple application where two users accept telephone orders from customers. The order-entry program checks the PRODUCTS table for adequate inventory before accepting the customer s order. In the figure, Joe starts entering an order for 100 ACI41004 widgets from his customer. At nearly 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 table, 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 table to show (139 100) = 39 widgets
Joe s Program
PRODUCTS Table 12:00 MFR_ID PRODUCT_ID QTY_ON_HAND ACI
Mary s Program
PART III
139 12:02 SELECT QTY_ON_HAND FROM PRODUCTS... Answer: 139
12:01 SELECT QTY_ON_HAND FROM PRODUCTS... Answer: 139
12:04 MFR_ID PRODUCT_ID Accept order for 100 ACI
QTY_ON_HAND Accept order for 125
39 12:05 UPDATE PRODUCTS SET QTY_ON_HAND = 14... QTY_ON_HAND
12:04 UPDATE PRODUCTS SET QTY_ON_HAND = 39...
12:05 MFR_ID PRODUCT_ID ACI
FIGURE 12-6
The lost update problem
Part III:
Updating Data
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 the order. Her copy of the order-entry program updates the PRODUCTS table to show (139 125) = 14 widgets remaining in stock and inserts a new order for 125 widgets into the ORDERS table. The handling of the two orders has obviously left the database in an inconsistent state. The first of the two updates to the PRODUCTS table has been lost! Both customers orders have been accepted, but not enough widgets are in inventory to satisfy both orders. Further, the database shows that there are still 14 widgets remaining for sale. This example illustrates the lost update problem that can occur whenever two programs read the same data from the database, use the data as the basis for a calculation, and then try to update the data.
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