互联网停机时,请继续尝试与服务器通信

Question

所以我的应用程序正在与服务器交换请求/响应(没有问题),直到互联网连接死了几秒钟,然后回来.然后是这样的代码:

response = (HttpWebResponse)request.GetResponse();

将抛出一个异常,与状态一样ReceiveFailure,ConnectFailure,KeepAliveFailure等.

现在,非常重要的是,如果互联网连接回来,我能够继续与服务器通信,否则我将不得不从头开始,这将需要很长时间.

当互联网回来时,您将如何恢复此通信？

目前,我一直在检查是否有可能与服务器通信,直到可能(至少理论上).我的代码尝试看起来像这样:

try
{
    response = (HttpWebResponse)request.GetResponse();
}
catch (WebException ex)
{
    // We have a problem receiving stuff from the server. 
    // We'll keep on trying for a while
    if (ex.Status == WebExceptionStatus.ReceiveFailure || 
        ex.Status == WebExceptionStatus.ConnectFailure || 
        ex.Status == WebExceptionStatus.KeepAliveFailure)
    {
        bool stillNoInternet = true;

        // keep trying to talk to the server
        while (stillNoInternet)
        {
            try
            {
                response = (HttpWebResponse)request.GetResponse();
                stillNoInternet = false;
            }
            catch
            {
                stillNoInternet = true;
            }
        }
    }
}

但问题是,即使互联网恢复,第二个try-catch语句仍然会抛出异常.

我究竟做错了什么？还有另一种方法来解决这个问题吗？

谢谢!

Answer 1

您应该每次都重新创建请求,并且应该在循环中执行重试,并在每次重试之间等待.每次失败时,等待时间应逐渐增加.

例如

ExecuteWithRetry (delegate {
    // retry the whole connection attempt each time
    HttpWebRequest request = ...;
    response = request.GetResponse();
    ...
});

private void ExecuteWithRetry (Action action) {
    // Use a maximum count, we don't want to loop forever
    // Alternativly, you could use a time based limit (eg, try for up to 30 minutes)
    const int maxRetries = 5;

    bool done = false;
    int attempts = 0;

    while (!done) {
        attempts++;
        try {
            action ();
            done = true;
        } catch (WebException ex) {
            if (!IsRetryable (ex)) {
                throw;
            }

            if (attempts >= maxRetries) {
                throw;
            }

            // Back-off and retry a bit later, don't just repeatedly hammer the connection
            Thread.Sleep (SleepTime (attempts));
        }
    }
}

private int SleepTime (int retryCount) {
    // I just made these times up, chose correct values depending on your needs.
    // Progressivly increase the wait time as the number of attempts increase.
    switch (retryCount) {
        case 0: return 0;
        case 1: return 1000;
        case 2: return 5000;
        case 3: return 10000;
        default: return 30000;
    }
}

private bool IsRetryable (WebException ex) {
    return
        ex.Status == WebExceptionStatus.ReceiveFailure ||
        ex.Status == WebExceptionStatus.ConnectFailure ||
        ex.Status == WebExceptionStatus.KeepAliveFailure;
}