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Controlling Database Transactions

Databases define transactions as logical units of work that can be committed or rolled back as a single change, even if they operate on multiple tables. All queries to a database are executed within the context of a transaction, even if the database driver manages them implicitly. This is called auto-commit mode, in which the database driver creates a transaction for every statement you execute, and commits that transaction after your SQL statement has been executed. By default, all Zend_Db Adapter classes operate in auto-commit mode.

Alternatively, you can specify the beginning and resolution of a transaction, and thus control how many SQL queries are included in a single group that is committed (or rolled back) as a single operation. Use the beginTransaction() method to initiate a transaction. Subsequent SQL statements are executed in the context of the same transaction until you resolve it explicitly.

To resolve the transaction, use either the commit() or rollBack() methods. The commit() method marks changes made during your transaction as committed, which means the effects of these changes are shown in queries run in other transactions.

The rollBack() method does the opposite: it discards the changes made during your transaction. The changes are effectively undone, and the state of the data returns to how it was before you began your transaction. However, rolling back your transaction has no effect on changes made by other transactions running concurrently.

After you resolve this transaction, Zend_Db_Adapter returns to auto-commit mode until you call beginTransaction() again.

Example 216. Managing a Transaction to Ensure Consistency

// Start a transaction explicitly.

try {
// Attempt to execute one or more queries:

// If all succeed, commit the transaction and all changes
    // are committed at once.

} catch (
Exception $e) {
// If any of the queries failed and threw an exception,
    // we want to roll back the whole transaction, reversing
    // changes made in the transaction, even those that succeeded.
    // Thus all changes are committed together, or none are.

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