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!
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。同时发送计时器(对应于
sendTimeout
)请求在队列中时正在运行。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(我们在服务器端看不到有关此请求的任何信息,并且无法用fiddler…捕获它)。
在更实际的情况下,如果我们有大约12个并发调用和默认的1分钟发送超时,并且如果服务调用平均处理时间超过10秒,那么我们可以很容易地获得最后两个调用的超时异常(12/2*10秒=60秒),因为它们将等待所有其他调用。
解决方案是:
-
最小化并发服务调用的数目。
-
增加
发送超时
价值在
客户机
配置。
-
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。
下面是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
}
}
下面是用法:
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。
