"Down for Maintenance" page for Asp.net MVC web app - asp.net-mvc-3

Question

I have the following requirements
Set a value in web.config and enable maintenance mode
All non-ajax requests should be shown a custom error page, with the http status code set to 503. The url of page should be retained.
All ajax requests should be responded with http status code 503
I should have an opportunity to do some basic logging to a file. Log the url and the user Identity if he happened to be logged into the app
I am using ELMAH to track/log all unhandled exceptions. The mechanism for implementing maintenance mode shouldn't need me to not use ELMAH
I have "runAllManagedModulesForAllRequests" set to true. this was originally done for use with RequestReduce. we no longer use it, but I am hesitant to reset its value to false. I am not sure if any other library needs it.
Once I realized there is nothing built in which supports the above requirements, I felt I had the following two options in front of me (App_offile.html won't work for me).
an HttpModule
an MVC ActionFilter
I dropped the MVC ActionFilter as I couldn't figure out how to guarantee it to run before any authentication/authorization filters. I have a custom authentication filter which hits the db. The idea behind the maintenance mode is the db might be offline, yet the web-app shouldn't show a 500 custom error page, but a 503 custom error page.
I wrote the following httpmodule and added in my web.config. It works for ajax requests. It kinda works for non-ajax requests. All requests get redirected to the 503 error page. The side-effect is all requests for static content also result in a 503. My error page thus is shown unstyled :(
// the http module
public class MaintenanceModeModule : IHttpModule
{
private static bool _isUnderMaintenance;
static MaintenanceModeModule()
{
var valueStr = (ConfigurationManager.AppSettings["UnderMaintenance"] ?? (false).ToString());
bool underMaintenance;
bool.TryParse(valueStr, out underMaintenance);
_isUnderMaintenance = underMaintenance;
}
public void Init(HttpApplication application)
{
application.BeginRequest += OnBeginRequest;
}
private void OnBeginRequest(object sender, EventArgs e)
{
var application = (HttpApplication) sender;
var request = application.Request;
var response = application.Response;
if (_isUnderMaintenance == false)
{
return;
}
application.Context.Items["under_maintenance"] = true; // used later
if (request.Url.PathAndQuery == "/503") // the url of the action that renders the custom error page
{
return;
}
const int statusCode = (int) HttpStatusCode.ServiceUnavailable;
const string statusMessage = "Temporarily down for maintenance";
var requestWrapper = new HttpRequestWrapper(request);
if (requestWrapper.IsAjaxRequest())
{
response.Clear();
response.ClearContent();
response.ClearHeaders();
response.StatusCode = statusCode;
response.TrySkipIisCustomErrors = true;
response.StatusDescription = statusMessage;
response.End();
return;
}
// doesn't work, shows the Yellow Screen of Death (YSoD)
// application.Context.Server.Transfer("~/503", preserveForm: true);
// doesn't work, shows the Yellow Screen of Death (YSoD)
// throw new HttpException(statusCode, statusMessage);
response.Redirect("~/503");
}
public void Dispose()
{
}
}
...
// web.config
// only the relevant portions of each section is shown
<appSettings>
<add key="UnderMaintenance" value="true" />
</appSettings>
<customErrors mode="On"> <!-- Custom errors are on, even then I was seeing YSoDs during my attempts -->
<error statusCode="404" redirect="404" />
<error statusCode="503" redirect="503" />
</customErrors>
<system.webServer>
<httpErrors existingResponse="PassThrough">
</httpErrors>
<modules runAllManagedModulesForAllRequests="true">
<add name="MaintenanceMode" type="WebApp.Code.MvcInfrastructure.MaintenanceModeModule" />
</modules>
</system.webServer>
...
// route config
routes.MapRoute("503Error", "503", new { controller = "Error", action = "UnderMaintenance" });
...
// error controller
// the authentication filter skips authentication if the allowanonymous attribute is present
[AllowAnonymous]
public class ErrorController : CustomBaseController
{
public ErrorController(AppConfig appConfig)
: base(appConfig)
{
}
public ActionResult UnderMaintenance()
{
// behind the scenes reads the value from HttpContext.Items.
// This was set during the execution of the httpmodule
if (AppConfig.UnderMaintenance == false)
{
return new RedirectResult("~/");
}
Response.StatusCode = (int) HttpStatusCode.ServiceUnavailable;
Response.TrySkipIisCustomErrors = true;
// the actual content of the view is not relevant now
return View("Error503");
}
}
The problems with this approach,
Each non-ajax request is responded with a 302 and then a 503
The URL requested by the browser is not retained
It returns a 503 for all static assets as well
The code I wrote and web.config settings I enabled are all cobbled together from various sources. I am not fully sure what those settings do or what the recommended way is. Please feel free to answer with a completely different method, as long as it can meet the requirements stated.