Emil's Wicked Cool Blog

With IIS 7.5, you can now auto-start applications and have them continuously run.  However, implementing System.Web.Hosting.IProcessHostPreloadClient means having a Process method with some significant restrictions.  In a Spring.Net environment, the IOC Container's context is not yet started, and it will fail in rather spectacular ways if you try to crank it up with a hack.  Even if you manage to do this, Quartz does not start up, so your jobs still will fail to run.

I spent some time on this problem and discovered that I do have access to System.Web.HttpRuntime.AppDomainAppVirtualPath, which allows us to automatically fire an initial request at the app automatically, if we're careful.  This will crank up Application_Start and the rest of the Spring (and Quartz) machinery, allowing us to keep an app running 100% of the time.  This initial request, however, will be thrown away, even if fired asynchronously, if initiated during the Preload() method.  I ended up using a thread to get around this problem, allowing the requests to fire 500ms after the Preload method runs.

In certain cases, we can't just use http://localhost/yourvirtualpathhere.  You might have multiple sites listening to different host headers, or you might be running SSL and don't want to hit the application using localhost.  To cover those cases, I devised a web.config appSettings scheme where additional URLs can be applied to Preload (space delimited).  You can also specify an IP address or machine name, so the web.config can stay static but different URLs can be applied as you move through environments.  Here is the code:

public class Preloader : System.Web.Hosting.IProcessHostPreloadClient
    {
        public void Preload(string[] parameters)
        {
            var uris = System.Configuration.ConfigurationManager.AppSettings["AdditionalStartupUris"];
            StartupApplication(AllUris(uris));
        }

        public void StartupApplication(IEnumerable<Uri> uris)
        {
            new System.Threading.Thread(o =>
            {
                System.Threading.Thread.Sleep(500);
                foreach (var uri in (IEnumerable<Uri>)o) {
                    var client = new System.Net.WebClient();
                    client.DownloadStringAsync(uris.First());
                }
            }).Start(uris);
        }

        public IEnumerable<Uri> AllUris(string userConfiguration)
        {
            if (userConfiguration == null)
                return GuessedUris();
            return AllUris(userConfiguration.Split(' ')).Union(GuessedUris());
        }

        private IEnumerable<Uri> GuessedUris()
        {
            string path = System.Web.HttpRuntime.AppDomainAppVirtualPath;
            if (path != null)
                yield return new Uri("http://localhost" + path);
        }

        private IEnumerable<uri> AllUris(params string[] configurationParts)
        {
            return configurationParts
                .Select(p => ParseConfiguration(p))
                .Where(p => p.Item1)
                .Select(p => ToUri(p.Item2))
                .Where(u => u != null);
        }

        private Uri ToUri(string value)
        {
            try {
                return new Uri(value);
            }
            catch (UriFormatException) {
                return null;
            }
        }

        private Tuple<bool string> ParseConfiguration(string part)
        {
            return new Tuple<bool, string>(IsRelevant(part), ParsePart(part));
        }

        private string ParsePart(string part)
        {
            // We expect IPv4 or MachineName followed by |
            var portions = part.Split('|');
            return portions.Last();
        }

        private bool IsRelevant(string part)
        {
            var portions = part.Split('|');
            return
                portions.Count() == 1 ||
                portions[0] == System.Environment.MachineName ||
                HostIpAddresses().Any(a => a == portions[0]);
        }

        private IEnumerable<string> HostIpAddresses()
        {
            var adaptors = System.Net.NetworkInformation.NetworkInterface.GetAllNetworkInterfaces();
            return adaptors
                    .Where(a => a.OperationalStatus == System.Net.NetworkInformation.OperationalStatus.Up)
                    .SelectMany(a => a.GetIPProperties().UnicastAddresses)
                    .Where(a => a.Address.AddressFamily == System.Net.Sockets.AddressFamily.InterNetwork)
                    .Select(a => a.Address.ToString());
        }
    }