free barcode generator using vb.net 12: Transaction Processing in Software

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12: Transaction Processing
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Database updates are usually triggered by real-world events, such as the receipt of a new order from a customer. In fact, receiving a new order would generate not just one, but this series of four updates to the sample database: Add the new order to the ORDERS table. Update the sales total for the salesperson who took the order. Update the sales total for the salesperson's office. Update the quantity-on-hand total for the ordered product. To leave the database in a self-consistent state, all four updates must occur as a unit. If a system failure or another error creates a situation where some of the updates are processed and others are not, the integrity of the database will be lost. Similarly, if another user calculates totals or ratios part way through the sequence of updates, the calculations will be incorrect. The sequence of updates must thus be an "all-or-nothing" proposition in the database. SQL provides precisely this capability through its transaction processing features, which are described in this chapter.
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What Is a Transaction
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A transaction is a sequence of one or more SQL statements that together form a logical unit of work. The SQL statements that form the transaction are typically closely related and perform interdependent actions. Each statement in the transaction performs some part of a task, but all of them are required to complete the task. Grouping the statements as a single transaction tells the DBMS that the entire statement sequence should be
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executed atomically all of the statements must be completed for the database to be in a consistent state. Here are some examples of typical transactions for the sample database, along with the SQL statement sequence that comprises each transaction: Add-an-order. To accept a customer's order, the order entry program should (a) query the PRODUCTS table to ensure that the product is in stock, (b) insert the order into the ORDERS table, (c) update the PRODUCTS table, subtracting the quantity ordered from the quantity-on-hand of the product, (d) update the SALESREPS table, adding the order amount to the total sales of the salesperson who took the order, and (e) update the OFFICES table, adding the order amount to the total sales of the office where the salesperson works. Cancel-an-order. To cancel a customer's order, the program should (a) delete the order from the ORDERS table, (b) update the PRODUCTS table, adjusting the quantityon-hand total for the product, (c) update the SALESREPS table, subtracting the order amount from the salesperson's total sales, and (d) update the OFFICES table, subtracting the order amount from the office's total sales. Reassign-a-customer. When a customer is reassigned from one salesperson to another, the program should (a) update the CUSTOMERS table to reflect the change, (b) update the ORDERS table to show the new salesperson for all orders placed by the customer, (c) update the SALESREPS table, reducing the quota for the salesperson losing the customer, and (d) update the SALESREPS table, raising the quota for the salesperson gaining the customer. In each of these cases a sequence of four or five actions, where each action consists of a separate SQL statement, is required to handle the single "logical" transaction. The transaction concept is critical for programs that update a database because it ensures the integrity of the database. A SQL-based DBMS makes this commitment about the statements in a transaction: The statements in a transaction will be executed as an atomic unit of work in the database. Either all of the statements will be executed successfully, or none of the statements will be executed. The DBMS is responsible for keeping this commitment even if the application program aborts or a hardware failure occurs in the middle of the transaction, as shown in Figure 12-1. In each case, the DBMS must make sure that when failure recovery is complete, the database never reflects a "partial transaction."
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