protected function configuration_definition() {
        global $CFG;
        $form = $this->_form;
        $form->addElement('text', 'prefix', get_string('prefix', 'cachestore_apcu'),
            array('maxlength' => 5, 'size' => 5));
        $form->addHelpButton('prefix', 'prefix', 'cachestore_apcu');
        $form->setType('prefix', PARAM_TEXT); // We set to text but we have a rule to limit to alphanumext.
        $form->setDefault('prefix', $CFG->prefix);
        $form->addRule('prefix', get_string('prefixinvalid', 'cachestore_apcu'), 'regex', '#^[a-zA-Z0-9\-_]+$#');
        $form->addElement('header', 'apc_notice', get_string('notice', 'cachestore_apcu'));
        $form->setExpanded('apc_notice');
        $link = get_docs_url('Caching#APC');
        $form->addElement('html', nl2br(get_string('clusternotice', 'cachestore_apcu', $link)));
    }