public TThrottleQuotaResp showThrottleQuota(TShowThrottleReq req) {
    TThrottleQuotaResp throttleQuotaResp = new TThrottleQuotaResp();
    if (req.getUserName() == null) {
      throttleQuotaResp.setThrottleQuota(quotaInfo.getThrottleQuotaLimit());
    } else {
      Map<String, TThrottleQuota> throttleLimit = new HashMap<>();
      throttleLimit.put(
          req.getUserName(),
          quotaInfo.getThrottleQuotaLimit().get(req.getUserName()) == null
              ? new TThrottleQuota()
              : quotaInfo.getThrottleQuotaLimit().get(req.getUserName()));
      throttleQuotaResp.setThrottleQuota(throttleLimit);
    }
    throttleQuotaResp.setStatus(RpcUtils.getStatus(TSStatusCode.SUCCESS_STATUS));
    return throttleQuotaResp;
  }
If the new quota is smaller than the quota already used, the setting fails.
iotdb-core/confignode/src/main/java/org/apache/iotdb/confignode/manager/ClusterQuotaManager.java