问题在于 Internet Explorer 7/6 中单个服务器的最大并发连接数。只有2个！ http://msdn.microsoft.com/en-us/library/cc304129(VS.85).aspx
如果我们有 3 个（例如）并发服务调用，其中两个将立即发送到服务器，但第三个将在队列中等待。当请求在队列中时，发送计时器（对应于sendTimeout）也在运行。如果前两个服务请求将运行很长时间，那么第三个服务请求将生成 TimeoutException，尽管它没有发送到服务器（我们不会在服务器端看到有关此请求的任何信息，也无法使用 Fiddler 捕获它...)。
在更真实的情况下，如果我们有大约 12 个并发调用和默认 1 分钟的发送超时，并且如果服务调用平均处理超过 10 秒，那么我们很容易在最后两个调用（12 / 2 * 10 sec = 60 秒），因为他们会等待所有其他人。
解决办法是：
- 尽量减少并发服务调用的数量。
- 增加客户端配置中的
sendTimeout值。
- 为关键服务实现自动重试功能。
- 实现请求队列以管理它们。
就我而言，我已经做了 1-3 件事，这就足够了。
这是我对自动重试功能的实现：
public static class ClientBaseExtender
{
/// <summary>
/// Tries to execute async service call. If <see cref="TimeoutException"/> occured retries again.
/// </summary>
/// <typeparam name="TChannel">ServiceClient class.</typeparam>
/// <typeparam name="TArgs">Type of service client method return argument.</typeparam>
/// <param name="client">ServiceClient instance.</param>
/// <param name="tryExecute">Delegate that execute starting of service call.</param>
/// <param name="onCompletedSubcribe">Delegate that subcribes an event handler to the OnCompleted event of the service client method.</param>
/// <param name="onCompleted">Delegate that executes when service call is succeeded.</param>
/// <param name="onError">Delegate that executes when service call fails.</param>
/// <param name="maxAttempts">Maximum attempts to execute service call before error if <see cref="TimeoutException"/> occured (by default 5).</param>
public static void ExecuteAsyncRepeatedly<TChannel, TArgs>(this ClientBase<TChannel> client, Action tryExecute,
Action<EventHandler<TArgs>> onCompletedSubcribe, EventHandler<TArgs> onCompleted,
EventHandler<TArgs> onError, int maxAttempts)
where TChannel : class
where TArgs : AsyncCompletedEventArgs
{
int attempts = 0;
var serviceName = client.GetType().Name;
onCompletedSubcribe((s, e) =>
{
if (e.Error == null) // Everything is OK
{
if (onCompleted != null)
onCompleted(s, e);
((ICommunicationObject)client).Close();
Debug.WriteLine("[{1}] Service '{0}' closed.", serviceName, DateTime.Now);
}
else if (e.Error is TimeoutException)
{
attempts++;
if (attempts >= maxAttempts) // Final timeout after n attempts
{
Debug.WriteLine("[{2}], Final Timeout occured in '{0}' service after {1} attempts.", serviceName, attempts, DateTime.Now);
if (onError != null)
onError(s, e);
client.Abort();
Debug.WriteLine("[{1}] Service '{0}' aborted.", serviceName, DateTime.Now);
return;
}
// Local timeout
Debug.WriteLine("[{2}] Timeout occured in '{0}' service (attempt #{1}).", serviceName, attempts, DateTime.Now);
Debug.WriteLine("[{2}] Attempt #{0} to execute call to '{1}' service.", attempts + 1, serviceName, DateTime.Now);
tryExecute(); // Try again.
}
else
{
if (onError != null)
onError(s, e);
client.Abort();
Debug.WriteLine("[{1}] Service '{0}' aborted.", serviceName, DateTime.Now);
}
});
Debug.WriteLine("[{2}] Attempt #{0} to execute call to '{1}' service.", attempts + 1, serviceName, DateTime.Now);
tryExecute(); // First attempt to execute
}
}
这是一个用法：
var client = new MyServiceClient();
client.ExecuteAsyncRepeatedly(() => client.MyOperationAsync(...),
(EventHandler<MyOperationCompletedEventArgs> handler) => client.MyOperationCompleted += handler,
(s, e) => // OnCompleted
{
Do(e.Result);
},
(s, e) => // OnError
{
HandleError(e.Error);
}
);
希望这会有所帮助。