code: public function set($userId, $callId, $data)
      {
          $key = $this->getCacheKey($userId, $callId);
          $cache = $this->getFromCache($key);
          if (is_null($cache) || $cache != $data) {
              $this->putInCache($key, $userId, $callId, $data, self::STATE_PENDING_WRITE);
          }
          return true;
      }
docs: Applies a dataset to be stored.
  
  @param string $userId
  @param string $callId
  @param string $data
  @return boolean