code: public function persist($userId = null, $callId = null)
      {
          if ($userId && $callId) {
              $keys = [$this->getCacheKey($userId, $callId)];
          } else {
              $keys = array_keys($this->cache);
          }
  
          $success = true;
          foreach ($keys as $key) {
              if (!$this->persistCacheEntry($key)) {
                  $success = false;
              }
          }
          return $success;
      }
docs: Sends the changes to the storage.
  
  @param string $userId
  @param string $callId
  @return bool