I realy like the array approach for determining standard classes in the factory, but a cleaner solution to modify the array might be to add:

public static function addFrontendClass($name, $classname);
public static function addBackendClass($name, $classname);

Another approach might be to move the classname resolution to seperate protected functions, so that subclassing Zend_Cache is easier.

this sounds good

I just commited something in trunk SVN.

Can you have a look at it ?

 
    /**
     * Factory
     *
     * @param string $frontend        frontend name
     * @param string $backend         backend name
     * @param array  $frontendOptions associative array of options for the corresponding frontend constructor
     * @param array  $backendOptions  associative array of options for the corresponding backend constructor
     * @param boolean $customFrontendNaming if true, the frontend argument is used as a complete class name ; if false, the frontend argument is used as the end of "Zend_Cache_Frontend_[...]" class name
     * @param boolean $customBackendNaming if true, the backend argument is used as a complete class name ; if false, the backend argument is used as the end of "Zend_Cache_Backend_[...]" class name
     * @param boolean $autoload if true, there will no require_once for backend and frontend (usefull only for custom backends/frontends)
     * @throws Zend_Cache_Exception
     * @return Zend_Cache_Frontend
     */

So, to use a completly custom frontend, you can use something like :

Zend_Cache::factory('MyCustomFrontend','MyCustomBackend',array(...),array(...), true, true, false); (without autoload)

Zend_Cache will do a require_once "My/Custom/Frontend.php" (the include_path must be ok for that)

or :

Zend_Cache::factory('MyCustomFrontend','MyCustomBackend',array(...),array(...), true, true, true); (with autoload)

Zend_Cache won't do any require_once at all (but the autoload has to work for MyCustomFrontend)

In my current project, i had to have my dummy backend. I subclassed Zend_Cache like this:

<?php

/** FIXME: this is a modified version of Zend_Cache, because ZF doesn't have a
 *  clean way to add custom front/backends.
 *  See http://framework.zend.com/issues/browse/ZF-2468
 */

abstract class My_Cache extends Zend_Cache
{
    // FIXME: these arrays could be pre-loaded, but that would break inheritance
    protected static $frontendClasses = null;
    protected static $backendClasses = null;

    protected static function _loadDefaultClasses() {
        if(!is_array(self::$frontendClasses)) {
            foreach(self::$standardFrontends as $name) {
                self::addFrontendClass($name, 'Zend_Cache_' . ($name != 'Core' ? 'Frontend_' : '') . $name);
            }
        }
        if(!is_array(self::$backendClasses)) {
            foreach(self::$standardBackends as $name) {
                self::addBackendClass($name, 'Zend_Cache_Backend_' . $name);
            }
        }
    }
    protected static function _normalizeKey($name) {
        return strtolower(str_replace(array(' ','-','_','.'),'',$name));
    }
    public static function addFrontendClass($name, $classname) {
        self::$frontendClasses[self::_normalizeKey($name)] = $classname;
    }
    public static function addBackendClass($name, $classname) {
        self::$backendClasses[self::_normalizeKey($name)] = $classname;
    }
    /**
     * Factory
     *
     * @param string $frontend frontend name
     * @param string $backend backend name
     * @param array $frontendOptions associative array of options for the corresponding frontend constructor
     * @param array $backendOptions associative array of options for the corresponding backend constructor
     */
    public static function factory($frontend, $backend, $frontendOptions = array(), $backendOptions = array())
    {
        // FIXME: these arrays could be pre-loaded, but that would break inheritance
        self::_loadDefaultClasses();

        // working on the frontend
        if (array_key_exists(self::_normalizeKey($frontend), self::$frontendClasses)) {
            // we use a standard frontend
            $frontendClass = self::$frontendClasses[self::_normalizeKey($frontend)];
        } else {
            // we use a custom frontend
            $frontendClass = $frontend;
        }
        Zend_Loader::loadClass($frontendClass);

        // working on the backend
        if (array_key_exists(self::_normalizeKey($backend), self::$backendClasses)) {
            // we use a standard frontend
            $backendClass = self::$backendClasses[self::_normalizeKey($backend)];
        } else {
            // we use a custom frontend
            $backendClass = $backend;
        }
        Zend_Loader::loadClass($backendClass);

        // Making objects
        $frontendObject = new $frontendClass($frontendOptions);
        $backendObject = new $backendClass($backendOptions);
        $frontendObject->setBackend($backendObject);
        return $frontendObject;
    }
}

Of course I used the occasion to use the Zend_Loader instead of require, although I also use an autoloader. I would like to see this add* interface and only a fifth argument on the factory to enable autoloading. Default classed should be in the assoc. array, instead of using this workaround. This makes loading custom classes transparent and keeps the argumentlist clean.

Updating for the 1.6.0 release.