code: public static function getSession(ServerRequestInterface $request):SessionDataHolder
      {
          $session = $request->getAttribute(static::REQ_ATTR);
          if (!$session instanceof SessionDataHolder) {
              throw new SessionMiddlewareException(
                  "No session object is available in the request attributes"
              );
          }
          return $session;
      }
docs: Returns the session object attached to a request.
  
  @param ServerRequestInterface $request
  @return SessionDataHolder
  @throws SessionMiddlewareException