code: public function init()
      {
          if (is_string($this->redis)) {
              $this->redis = Yii::$app->get($this->redis);
          } elseif (is_array($this->redis)) {
              if (!isset($this->redis['class'])) {
                  $this->redis['class'] = Connection::className();
              }
              $this->redis = Yii::createObject($this->redis);
          }
          if (!$this->redis instanceof Connection) {
              throw new InvalidConfigException("Session::redis must be either a Redis connection instance or the application component ID of a Redis connection.");
          }
          if ($this->keyPrefix === null) {
              $this->keyPrefix = substr(md5(Yii::$app->id), 0, 5);
          }
          parent::init();
      }
docs: Initializes the redis Session component.
  This method will initialize the [[redis]] property to make sure it refers to a valid redis connection.
  @throws InvalidConfigException if [[redis]] is invalid.