code: protected function read(Mapper\Map $map, $identifier)
      {
          $url = $this->buildUri($map->getRemoteName() . '/' . $identifier);
          if (!file_exists($url) || !is_file($url)) {
              return;
          }
          $data = file_get_contents($url, false, $this->context ? : null);
          return $this->descriptor->unserialize($map, $data);
      }
docs: @param Mapper\Map $map
  @param            $identifier
  
  @return mixed