free barcode generator using vb.net Figure 12-3: Committed and rolled back transactions in Software

Maker QR Code ISO/IEC18004 in Software Figure 12-3: Committed and rolled back transactions

Figure 12-3: Committed and rolled back transactions
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Recall that the ANSI/ISO SQL standard is primarily focused on a programmatic SQL language for use in application programs. Transactions play an important role in programmatic SQL, because even a simple application program often needs to carry out a sequence of two or three SQL statements to accomplish its task. Because users can change their minds and other conditions can occur (such as being out of stock on a product that a customer wants to order), an application program must be able to proceed part way through a transaction and then choose to abort or continue. The COMMIT and ROLLBACK statements provide precisely this capability. The COMMIT and ROLLBACK statements can also be used in interactive SQL, but in practice they are rarely seen in this context. Interactive SQL is generally used for database queries; updates are less common, and multi-statement updates are almost never performed by typing the statements into an interactive SQL facility. As a result, transactions are typically a minor concern in interactive SQL. In fact, many interactive SQL products default to an "auto-commit" mode, where a COMMIT statement is automatically executed after each SQL statement typed by the user. This effectively makes each interactive SQL statement its own transaction.
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Other Transaction Models
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A few commercial SQL products depart from the ANSI/ISO and DB2 transaction model to provide additional transaction processing capability for their users. The Sybase DBMS, which is designed for online transaction processing applications, is one example. SQL Server, which was derived from the Sybase product, also uses the Sybase transaction model. The Transact-SQL dialect used by Sybase includes four transaction processing statements: The BEGIN TRANSACTION statement signals the beginning of a transaction. Unlike the ANSI/ISO transaction model, which implicitly begins a new transaction when the previous one ends, Sybase requires an explicit statement to start a transaction. The COMMIT TRANSACTION statement signals the successful end of a transaction. As
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in the ANSI/ISO model, all changes made to the database during the transaction become permanent. However, a new transaction is not automatically started. The SAVE TRANSACTION statement establishes a savepoint in the middle of a transaction. Sybase saves the state of the database at the current point in the transaction and assigns the saved state a savepoint name, specified in the statement. The ROLLBACK TRANSACTION statement has two roles. If a savepoint is named in the ROLLBACK statement, Sybase backs out the database changes made since the savepoint, effectively rolling the transaction back to the point where the SAVE TRANSACTION statement was executed. If no savepoint is named, the ROLLBACK statement backs out all database changes made since the BEGIN TRANSACTION statement. The Sybase savepoint mechanism is especially useful in complex transactions involving many statements, as shown in Figure 12-4. The application program in the figure periodically saves its status as the transaction progresses, establishing two named savepoints. If problems develop later during the transaction, the application program does not have to abort the entire transaction. Instead, it can roll the transaction back to any of its savepoints and proceed from there. All of the statements executed before the savepoint remain in effect; those executed since the savepoint are backed out by the rollback operation.
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Figure 12-4: An alternative (explicit) transaction model used by Sybase
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Note that the entire transaction is still the logical unit of work for Sybase, as it is for the ANSI/ISO model. If a system or hardware failure occurs in the middle of a transaction, for example, the entire transaction is backed out of the database. Thus, savepoints are a convenience for the application program, but not a fundamental change to the ANSI/ISO transaction model. The explicit use of a BEGIN TRANSACTION statement is, however, a significant departure from the ANSI/ISO model. SQL statements that are executed "outside a transaction" (that is, statements that do not appear between a BEGIN/COMMIT or a BEGIN/ROLLBACK statement pair) are effectively handled in "auto-commit" mode. Each statement is committed as it is executed; there is no way to roll back the statement once it has succeeded. Some DBMS brands that use a Sybase-style transaction model prohibit statements that alter the structure of a database or its security from occurring within a transaction (such as CREATE TABLE, ALTER TABLE, and DROP TABLE, discussed in 13, and GRANT
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and REVOKE, discussed in 15). These statements must be executed outside a transaction. This restriction makes the transaction model easier to implement, because it ensures that the structure of the database cannot change during a transaction. In contrast, the structure of a database can be altered significantly during an ANSI/ISO-style transaction (tables can be dropped, created, and populated, for example), and the DBMS must be able to undo all the alterations if the user later decides to roll back the transaction. In practice, the Sybase prohibitions do not affect the usefulness of the DBMS. Because these prohibitions probably contribute to faster transaction performance, most users gladly make this trade-off.
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Transactions: Behind the Scenes *
The "all-or-nothing" commitment that a DBMS makes for the statements in a transaction seems almost like magic to a new SQL user. How can the DBMS possibly back out the changes made to a database, especially if a system failure occurs during the middle of a transaction The actual techniques used by brands of DBMS vary, but almost all of them are based on a transaction log, as shown in Figure 12-5.
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