C++ tcp socket连接重试方法

Question

在开发了一个可以交换一些数据的示例客户端服务器应用程序之后，我试图在其中实现重试机制。目前我的应用程序遵循以下协议(protocol):

1. 客户端连接到服务器(非阻塞模式)，超时 3 秒，重试 2 次。
2. 开始从客户端发送固定长度的数据。发送有一些错误检查它是否正在发送完整的数据。
3. 从服务器接收响应(超时:3 秒)并验证。如果收到不正确的响应，则重新发送数据并等待响应。如果失败，请重复此操作两次。

上面的实现代码部分如下所示:

1. connect() 和 select() 用于打开连接
2. select() 和 send() 用于数据发送
3. select() 和 recv() 用于数据接收

现在我根据套接字函数的返回类型进行重试，如果 send() 或 recv() 失败，我将重试相同的方法。但不记得 connect()。

我通过在数据传输之间重新启动服务器来测试这个东西，结果客户端无法与服务器通信并在几次重试后退出，我相信这是因为没有 connect() 调用重试方法。

有什么建议吗？

接收socket数据的示例代码

bool CTCPCommunication::ReceiveSocketData(char* pchBuff, int iBuffLen)
{
  bool bReturn = true;

  //check whether the socket is ready to receive
  fd_set stRead;
  FD_ZERO(&stRead);
  FD_SET(m_hSocket, &stRead);
  int iRet = select(0, &stRead, NULL, NULL, &m_stTimeout);

  //if socket is not ready this line will be hit after 3 sec timeout and go to the end
  //if it is ready control will go inside the read loop and reads data until data ends or
  //socket error is getting triggered continuously for more than 3 secs.
  if ((iRet > 0) && (FD_ISSET(m_hSocket, &stRead)))
  {
    DWORD dwStartTime = GetTickCount();
    DWORD dwCurrentTime = 0;

    while ((iBuffLen-1) > 0)
    {
      int iRcvLen = recv(m_hSocket, pchBuff, iBuffLen-1, 0);
      dwCurrentTime = GetTickCount();

      //receive failed due to socket error
      if (iRcvLen == SOCKET_ERROR)
      {
        if((dwCurrentTime - dwStartTime) >= SOCK_TIMEOUT_SECONDS * 1000)
        {
          WRITELOG("Call to socket API 'recv' failed after 3 secs continuous retries, error: %d", WSAGetLastError());
          bReturn = false;
          break;
        }
      }
      //connection closed by remote host
      else if (iRcvLen == 0)
      {
        WRITELOG("recv() returned zero - time to do something: %d", WSAGetLastError());
        break;
      }

      pchBuff  += iRcvLen;
      iBuffLen -= iRcvLen;
    }
  }
  else
  {
    WRITELOG("Call to API 'select' failed inside 'ReceiveSocketData', error: %d", WSAGetLastError());
    bReturn = false;
  }

  return bReturn;
}

Answer 1

Currently my application is following below protocol:

1. Client connects to server (non blocking mode) with 3 secs timeout and with 2 retries.

您无法重试连接。您必须关闭连接尝试失败的套接字，创建一个新套接字，然后再次调用 connect()。

2. Start sending data from client with fixed length. Send has some error checking whether it is sending the complete data or not.

这在阻塞模式下不是必需的:POSIX 标准保证阻塞模式 send() 将发送所有数据，否则会因错误而失败。

3. Receive response (timeout: 3secs) from server and verify that. If incorrect response received, re-send the data and wait for response. Repeat this for two times if failed.

这是个坏主意。很可能所有数据都会到达，包括所有重试，或者没有。如果您使用此技术，您需要确保您的交易是幂等的。您还需要密切注意实际的超时时间。 3 秒通常是不够的。起点是预期服务时间的两倍。

For the above implementation code sections look likes something below:

   connect() and select() for opening connection
   select() and send() for data send
   select() and recv() for data receiving

在阻塞模式下您不需要 select()。您可以使用 SO_RCVTIMEO 设置读取超时。

Now I'm making the retries based on return types of the socket functions, and if send() or recv() fails I'm retrying the same methods. But not recalling connect().

I tested the thing by restarting the server in between the data transfer, and as a result client fails to communicate with the server and it quits after several retries, I believe this is happening as because there is no connect() call on retry methods.

如果那是真的，你会得到这样的错误。