Resources and Database Connections

The role of a resource in Magento is to manage database connections. Resources are defined under the global XML tag of any config.xml file. To make a new database connection you would add XML like the following to any config.xml file. Each resource has a name of the form module/name and each connection has a name. Connection names are generally of the pattern

• module_read,
• module_write,
• or module_setup.

<resources>
<default_setup>
<connection>
<host>localhost</host>
<username></username>
<password></password>

<dbname>magento</dbname>
<model>mysql4</model>
<initStatements>SET NAMES utf8</initStatements>
<type>pdo_mysql</type>
<active>1</active>
</connection>
</default_setup>
….
The format should be mostly self explanatory, most of these values end up being passed to the Zend_Db_Adapter_Abstract class. The initStatements tag is executed upon every connection to the database server. The model tag seems unnecessary, perhaps it was part of an idea that didn’t pan-out completely. The type tag refers to one of two connection types defined in the global app/etc/config.xml file. Only pdo_mysql and mysqli are implemented at the moment.

To retrieve a Magento database connection we must first get a resource. If you require a generic database handle, you can use the name core/resource for the resource  name, and core_write for the connection name. The Mage::getSingleton method keeps track of all classes loaded through it as singletons and returns a previously initialized object if the name matches. Each module may have its own database settings – usernames and passwords – or it might even be connecting to a separate database server, so be sure to use the most appropriate resource name when you can. In a default setup, all of Magento’s connections use the settings of default_setup, default_write, or default_read.

$write = Mage::getSingleton(’core/resource’)
->getConnection(’core_write’);
if ($write instanceof Zend_Db_Adapter_Abstract) {
echo get_class($write);
}
//outputs: Varien_Db_Adapter_Pdo_Mysql