在C＃中调整HttpWebRequest连接超时

Question

I believe after lengthy research and searching, I have discovered that what I want to do is probably better served by setting up an asynchronous connection and terminating it after the desired timeout... But I will go ahead and ask anyway!

Quick snippet of code:

HttpWebRequest webReq = (HttpWebRequest)HttpWebRequest.Create(url);
webReq.Timeout = 5000;
HttpWebResponse response = (HttpWebResponse)webReq.GetResponse(); 
// this takes ~20+ sec on servers that aren't on the proper port, etc.

I have an HttpWebRequest method that is in a multi-threaded application, in which I am connecting to a large number of company web servers. In cases where the server is not responding, the HttpWebRequest.GetResponse() is taking about 20 seconds to time out, even though I have specified a timeout of only 5 seconds. In the interest of getting through the servers on a regular interval, I want to skip those taking longer than 5 seconds to connect to.

So the question is: "Is there a simple way to specify/decrease a connection timeout for a WebRequest or HttpWebRequest?"

Answer 1

I believe that the problem is that the WebRequest measures the time only after the request is actually made. If you submit multiple requests to the same address then the ServicePointManager will throttle your requests and only actually submit as many concurrent connections as the value of the corresponding ServicePoint.ConnectionLimit which by default gets the value from ServicePointManager.DefaultConnectionLimit . Application CLR host sets this to 2, ASP host to 10. So if you have a multithreaded application that submits multiple requests to the same host only two are actually placed on the wire, the rest are queued up.

I have not researched this to a conclusive evidence whether this is what really happens, but on a similar project I had things were horrible until I removed the ServicePoint limitation.

Another factor to consider is the DNS lookup time. Again, is my belief not backed by hard evidence, but I think the WebRequest does not count the DNS lookup time against the request timeout. DNS lookup time can show up as very big time factor on some deployments.

And yes, you must code your app around the WebRequest.BeginGetRequestStream (for POST s with content) and WebRequest.BeginGetResponse (for GET s and POSTS s). Synchronous calls will not scale (I won't enter into details why, but that I do have hard evidence for). Anyway, the ServicePoint issue is orthogonal to this: the queueing behavior happens with async calls too.

Answer 2

Sorry for tacking on to an old thread, but I think something that was said above may be incorrect/misleading.

From what I can tell .Timeout is NOT the connection time, it is the TOTAL time allowed for the entire life of the HttpWebRequest and response. Proof:

I Set:

.Timeout=5000
.ReadWriteTimeout=32000

The connect and post time for the HttpWebRequest took 26ms

but the subsequent call HttpWebRequest.GetResponse() timed out in 4974ms thus proving that the 5000ms was the time limit for the whole send request/get response set of calls.

I didn't verify if the DNS name resolution was measured as part of the time as this is irrelevant to me since none of this works the way I really need it to work--my intention was to time out quicker when connecting to systems that weren't accepting connections as shown by them failing during the connect phase of the request.

For example: I'm willing to wait 30 seconds on a connection request that has a chance of returning a result, but I only want to burn 10 seconds waiting to send a request to a host that is misbehaving.

Answer 3

Something I found later which helped, is the .ReadWriteTimeout property. This, in addition to the .Timeout property seemed to finally cut down on the time threads would spend trying to download from a problematic server. The default time for .ReadWriteTimeout is 5 minutes, which for my application was far too long.

So, it seems to me:

.Timeout = time spent trying to establish a connection (not including lookup time) .ReadWriteTimeout = time spent trying to read or write data after connection established

More info: HttpWebRequest.ReadWriteTimeout Property

Edit:

Per @KyleM's comment, the Timeout property is for the entire connection attempt, and reading up on it at MSDN shows:

Timeout is the number of milliseconds that a subsequent synchronous request made with the GetResponse method waits for a response, and the GetRequestStream method waits for a stream. The Timeout applies to the entire request and response, not individually to the GetRequestStream and GetResponse method calls. If the resource is not returned within the time-out period, the request throws a WebException with the Status property set to WebExceptionStatus.Timeout.

(Emphasis mine.)

Answer 4

From the documentation of the HttpWebRequest.Timeout property:

A Domain Name System (DNS) query may take up to 15 seconds to return or time out. If your request contains a host name that requires resolution and you set Timeout to a value less than 15 seconds, it may take 15 seconds or more before a WebException is thrown to indicate a timeout on your request.

Is it possible that your DNS query is the cause of the timeout?

Answer 5

No matter what we tried we couldn't manage to get the timeout below 21 seconds when the server we were checking was down.

To work around this we combined a TcpClient check to see if the domain was alive followed by a separate check to see if the URL was active

public static bool IsUrlAlive(string aUrl, int aTimeoutSeconds)
{
    try
    {
        //check the domain first
        if (IsDomainAlive(new Uri(aUrl).Host, aTimeoutSeconds))
        {
            //only now check the url itself
            var request = System.Net.WebRequest.Create(aUrl);
            request.Method = "HEAD";
            request.Timeout = aTimeoutSeconds * 1000;
            var response = (HttpWebResponse)request.GetResponse();
            return response.StatusCode == HttpStatusCode.OK;
        }
    }
    catch
    {
    }
    return false;

}

private static bool IsDomainAlive(string aDomain, int aTimeoutSeconds)
{
    try
    {
        using (TcpClient client = new TcpClient())
        {
            var result = client.BeginConnect(aDomain, 80, null, null);

            var success = result.AsyncWaitHandle.WaitOne(TimeSpan.FromSeconds(aTimeoutSeconds));

            if (!success)
            {
                return false;
            }

            // we have connected
            client.EndConnect(result);
            return true;
        }
    }
    catch
    {
    }
    return false;
}