vendor/doctrine/dbal/lib/Doctrine/DBAL/Connections/MasterSlaveConnection.php

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<?php
/*
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * This software consists of voluntary contributions made by many individuals
 * and is licensed under the MIT license. For more information, see
 * <http://www.doctrine-project.org>.
 */

namespace Doctrine\DBAL\Connections;

use Doctrine\DBAL\Connection;
use Doctrine\DBAL\Driver;
use Doctrine\DBAL\Configuration;
use Doctrine\Common\EventManager;
use Doctrine\DBAL\Event\ConnectionEventArgs;
use Doctrine\DBAL\Events;

/**
 * Master-Slave Connection
 *
 * Connection can be used with master-slave setups.
 *
 * Important for the understanding of this connection should be how and when
 * it picks the slave or master.
 *
 * 1. Slave if master was never picked before and ONLY if 'getWrappedConnection'
 *    or 'executeQuery' is used.
 * 2. Master picked when 'exec', 'executeUpdate', 'insert', 'delete', 'update', 'createSavepoint',
 *    'releaseSavepoint', 'beginTransaction', 'rollback', 'commit', 'query' or
 *    'prepare' is called.
 * 3. If master was picked once during the lifetime of the connection it will always get picked afterwards.
 * 4. One slave connection is randomly picked ONCE during a request.
 *
 * ATTENTION: You can write to the slave with this connection if you execute a write query without
 * opening up a transaction. For example:
 *
 *      $conn = DriverManager::getConnection(...);
 *      $conn->executeQuery("DELETE FROM table");
 *
 * Be aware that Connection#executeQuery is a method specifically for READ
 * operations only.
 *
 * This connection is limited to slave operations using the
 * Connection#executeQuery operation only, because it wouldn't be compatible
 * with the ORM or SchemaManager code otherwise. Both use all the other
 * operations in a context where writes could happen to a slave, which makes
 * this restricted approach necessary.
 *
 * You can manually connect to the master at any time by calling: