Vadim's Weblog

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Detect when another site is unavailable. (404 detection)

Posted by Vadim on December 25, 2010

A long time ago we used to store our code on punched cards.  Since that time, our development tools improved a lot.  Thanks to more powerful languages and tools, it became easier for us to write code, and as a result, our applications became more and more complex.  In 1986 Fred Brooks predicted exactly that in his article “No Silver Bullet”.  Today, any modern application uses third party libraries or components.  Our web apps need to integrate with other sites to use their services. But what do you do if the third party site is down?

At first, the problem doesn’t seem that difficult, but then I realized that it’s impossible to detect with JavaScript when another site is unavailable. (Please let me know if I’m wrong.)  If it would be possible to do this with client scripting, it would be huge security hole.  Since the release of Netscape Navigator 2.0 in 1996, browsers prevent access to most methods and properties across pages on different sites.  Today, this policy is known as Same Origin Policy.

The solution I came up with is to ping the third party host on the server side before JavaScript is loaded.  If I get a good response, then I proceed with my JavaScript; otherwise, it’s redirected to my error page.

Here’s a sample how you can check if the host available with ASP.NET:

protected void Page_Load(object sender, EventArgs e)
{
    HttpWebResponse response;
    try
    {
        var request = 
            (HttpWebRequest)WebRequest.Create("http://www.someSite.com/camdsa");
        response = request.GetResponse() as HttpWebResponse;
    }
    catch (WebException webException)
    {
        Response.Redirect("ErrorPage.aspx?status=" + webException.Status);
        return;
    }
}

If GetResponse() throws a WebException, it means that something went wrong with our request.  We can figure out what’s wrong by analyzing the WebException Status and Response properties.

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One Response to “Detect when another site is unavailable. (404 detection)”

  1. stephen webb said

    want is this for

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