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Preserving Database Integrity Using Transactions
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Oracle supports only two transaction modes: autocommit and implicit. As with Microsoft SQL Server and Sybase Adaptive Server, support varies when ODBC and JDBC drivers are used, so the driver vendor s documentation should be consulted in those cases. The two transaction modes in Oracle are Autocommit mode As with other DBMSs, in autocommit mode each SQL statement is automatically committed as it completes. However, in Oracle, autocommit is not the default mode. Autocommit mode is toggled on and off using the SET AUTOCOMMIT command, as shown here, and it is off by default:
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SET AUTOCOMMIT ON SET AUTOCOMMIT OFF
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Implicit mode A transaction is implicitly started when the database user connects to the database (that is, when a new database session begins). This is the default transaction mode in Oracle. When a transaction ends with a commit or rollback, a new transaction is automatically started. Unlike Microsoft SQL Server and Sybase Adaptive Server, nested transactions (transactions within transactions) are not permitted. A transaction ends with a commit when any of the following occurs: 1) the database user issues the SQL COMMIT statement; 2) the database session ends normally (that is, the user issues an EXIT command); 3) the database user issues an SQL DDL statement (that is, a CREATE, DROP, or ALTER statement). A transaction ends with a rollback when either of the following occurs: 1) the database user issues the SQL ROLLBACK statement; 2) the database sessions ends abnormally (that is, client connection is canceled or the database crashes or is shut down using one of the shutdown options that aborts client connections instead of waiting for them to complete).
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Transaction support was added to MySQL beginning with version 3.23.0. However, in order to provide upward compatibility from older releases, transaction support was added using new storage engines. Tables created using the default storage engine (ISAM, which is called MyISAM in newer versions of MySQL) do not have transaction support. In order to enable transaction support, either the InnoDB or
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BDB (Berkeley DB) storage engine must be speci ed when the table is created. For example:
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CREATE TABLE LANGUAGE (LANGUAGE_CODE CHAR(2) NOT NULL, LANGUAGE_NAME VARCHAR(40) NOT NULL, PRIMARY KEY (LANGUAGE_CODE)) ENGINE = INNODB;
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There are many implications to using a different storage engine, so you should consult the documentation for the version of MySQL you are running before making a decision. For tables that are created using either the InnoDB or BDB storage engines, transaction support is available in two modes: Autocommit mode Autocommit mode is toggled on and off using a SET statement where a value of 0 turns off autocommit and a value of 1 turns it on:
SET AUTOCOMMIT=0; SET AUTOCOMMIT=1;
Implicit mode Implicit mode takes effect whenever autocommit mode is turned off. The following SQL statements may be used in implicit transaction mode:
START TRANSACTION [WITH CONSISTENT SNAPSHOT]; SAVEPOINT savepoint_identifier; ROLLBACK [TO savepoint_identifier]; COMMIT;
Transaction Support in DB2 UDB
All transactions in DB2 UDB (Universal Database) are implicit. The behavior is quite similar to transaction support in Oracle. The SQL statements used for transaction support are
COMMIT [WORK]; SAVEPOINT savepoint_name [options];
Preserving Database Integrity Using Transactions
RELEASE [TO] SAVEPOINT savepoint_name; ROLLBACK [WORK] [TO SAVEPOINT [savepoint_name]];
Locking and Transaction Deadlock
Although the simultaneous sharing of data among many database users has signi cant bene ts, there also is a serious drawback that can cause updates to be lost. Fortunately, the database vendors have worked out solutions to the problem. This section presents the concurrent update problem and various solutions.
The Concurrent Update Problem
Figure 9-1 illustrates the concurrent update problem that occurs when multiple database sessions are allowed to concurrently update the same data. Recall that a session is created every time a database user connects to the database, which includes the same user connecting to the database multiple times. The concurrent update problem happens most often between two different database users who are unaware that they are making con icting updates to the same data. However, one database user with multiple connections can trip themselves up if they apply updates using more than one of their database sessions. The scenario presented uses a ctitious company that sells products and creates an invoice for each order shipped. Figure 9-1 illustrates User A, a clerk in the shipping department who is preparing an invoice for a customer, which requires updating the customer s data to add to the customer s balance due. At the same time, User B, a clerk in the accounts receivable department, is processing a payment from the very
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