Drupal如何处理系统变量？

Drupal的系统变量都保存在数据库variable表中：

然后，开发人员可以通过下面的API函数操作这些系统变量：

function variable_get($name, $default = NULL) {
  global $conf;
  return isset($conf[$name]) ? $conf[$name] : $default;
}

function variable_set($name, $value) {
  global $conf;
  db_merge('variable')->key(array('name' => $name))->fields(array('value' => serialize($value)))->execute();
  cache_clear_all('variables', 'cache_bootstrap');
  $conf[$name] = $value;
}

function variable_del($name) {
  global $conf;
  db_delete('variable')
    ->condition('name', $name)
    ->execute();
  cache_clear_all('variables', 'cache_bootstrap');
  unset($conf[$name]);
}

从这些API函数可以知道，系统变量从variable表读取出来后，都放在了全局变量$conf里面。那全局变量$conf是如何初始化的呢？这实际上是通过在Bootstrap过程中调用variable_initialize()函数完成的:

function variable_initialize($conf = array()) {
  if ($cached = cache_get('variables', 'cache_bootstrap')) {
    $variables = $cached->data;
  }
  else {
    // Cache miss. Avoid a stampede.
    $name = 'variable_init';
    if (!lock_acquire($name, 1)) {
      // Another request is building the variable cache.
      // Wait, then re-run this function.
      lock_wait($name);
      return variable_initialize($conf);
    }
    else {
      // Proceed with variable rebuild.
      $variables = array_map('unserialize', db_query('SELECT name, value FROM {variable}')->fetchAllKeyed());
      cache_set('variables', $variables, 'cache_bootstrap');
      lock_release($name);
    }
  }

  foreach ($conf as $name => $value) {
    $variables[$name] = $value;
  }

  return $variables;
}