Transactions -- Performing transactions
Beschreibung
PEAR MDB2 defaults to auto-committing all queries. However using the
beginTransaction() method one can open a new transaction
and with the
commit() and
rollback() methods, a transaction is finished. These
three methods optionally accept a string name of a savepoint to set, release
or rollback to respectively. The method
inTransaction() may be used to check if a transaction is
currently open.
Beispiel 39-1. Doing a transaction
<?php
// Create a valid MDB2 object named $mdb2
// at the beginning of your program...
require_once 'MDB2.php';
$mdb2 =& MDB2::connect('pgsql://usr:pw@localhost/dbnam');
if (PEAR::isError($mdb2)) {
die($mdb2->getMessage());
}
// check if transaction are supported by this driver
if (!$mdb2->supports('transactions')) {
exit();
}
// Open a transaction
$res = $mdb2->beginTransaction();
..
// check if we are inside a transaction and if savepoints are supported
if ($mdb2->inTransaction() && $mdb2->supports('savepoints')) {
// Set a savepoint
$savepoint = 'MYSAVEPOINT';
$res = $mdb2->beginTransaction($savepoint);
..
// determine if the savepoint should be released or to rollback to the savepoint
if ($error_condition) {
$res = $mdb2->rollback($savepoint);
} else {
$res = $mdb2->commit($savepoint);
}
}
..
// determine if the commit or rollback
if ($error_condition) {
$res = $mdb2->rollback();
} else {
$res = $mdb2->commit();
}
?>
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| Warnung |
PEAR MDB2 does not emulate transactions or savepoints. This means that it
depends on the underlying RDBMS (and in the case of MySQL the storage
engines used) if transactions will be available in MDB2. Also note that
some RDBMS implicitly commit transactions when executing DDL statements -
notable exceptions are Oracle and PostgreSQL.
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MDB2 also supports "nested" transactions using the
beginNestedTransaction() method. Actually these are not
true nested transactions as they are natively supported in Interbase for
example. MDB2 maintains a counter of opened nested transactions. The
transaction is finished once that counter is decremented back to 1 with
completeNestedTransaction() calls. If the RDBMS supports
savepoints then MDB2 will automatically set a savepoint on every call of the
beginNestedTransaction() method after the initial call
and will return the name. These savepoints are automatically released by the
completeNestedTransaction() method. The name of these
automatic savepoints are determined by the "savepoint_format" option
and the nested transaction counter. The "savepoint_format" defaults to
'MDB2_SAVEPOINT_%s'.
If, after initially opening a nested transaction, an unexpected PEAR error is raised on the
MDB2 instance the transaction is rolled back, otherwise it is commited at
this point. Using the
getNestedTransactionError() method it is possible to check
if there has been an error inside the transaction. Alternatively a rollback
can be forced using the
failNestedTransaction(). This method optionally accepts
a mixed parameter which will set the error to return if the
getNestedTransactionError() method is called, as well as
a second boolean parameter that optionally forces an immidiate rollback.
Beispiel 39-2. Using emulated nested transactions
<?php
$mdb2->beginNestedTransaction(); # open transaction
$query = "INSERT INTO autoinc (id) VALUES (?)";
$stmt = $mdb2->prepare($query);
$stmt->execute(array(1));
$savepoint = $mdb2->beginNestedTransaction(); # ignored / sets savepoint
..
// never true for this example
if (false) {
// raise an error
$error = $mdb2->raiseError(MDB2_ERROR, null, null, 'kaboom');
$mdb2->failNestedTransaction();
}
if(($error = $mdb2->getNestedTransactionError())) {
die($error->getUserinfo());
}
$mdb2->completeNestedTransaction(); # ignored / releases savepoint
$stmt->execute(array(2));
$mdb2->completeNestedTransaction(); # commit
?>
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Finally MDB2 supports setting the transaction isolation level as per the
SQL 92 standard using the
setTransactionIsolation() method. If a given RDBMS
does not support a given isolation level but supports a higher more strict
isolation level, then MDB2 silently uses that higher transaction level. Some
RDBMS systems support additional options which are silently ignored if they are not
supported.
Beispiel 39-3. Setting the transaction isolation level
<?php
$options = array('wait' => 'WAIT', 'rw' => 'READ WRITE');
$options = array('wait' => 'NO WAIT', 'rw' => 'READ ONLY');
$isolation_level = READ UNCOMMITTED # (allows dirty reads)
$isolation_level = READ COMMITTED # (prevents dirty reads)
$isolation_level = REPEATABLE READ # (prevents nonrepeatable reads)
$isolation_level = SERIALIZABLE # (prevents phantom reads)
$mdb2->setTransactionIsolation($isolation_level, $options);
?>
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Last updated: Sun, 24 Aug 2008 |
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Do you think that something on this page is wrong? Please file a bug report or add a note.
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| User Notes: |
In order for the MySQL driver to realize if transactions are enabled, one needs to call connect() explicitly on the MDB2 connection object. Do note the two options "use_transactions" and "default_table_type" in this context. At any rate MDB2 will not know with what table handler the tables were created, so make sure that transactions are only used on transaction aware table handlers as MySQL will silently ignore transactions for non transaction enabled table handlers
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