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ASP .Net MapRequestHandler slow

We have rolled out a classic ASP.Net WebService application with large traffic. Though our database is running quite well (<10 ms response times), most of the time spent in WebServer is in the MapRequestHandler stage.
The issue seems to be deep in the ASP .Net stack and without any information available on net, I am clueless as to how to go about improving it.
We use XML payloads for request/response (if that would help in providing a solution).

Please post you handler code and your config files.
MapRequestHandler - The MapRequestHandler event is used by the ASP.NET infrastructure to determine
request handler for the current request based on the file-name extension of the requested resource. MapRequestHandler is an event that the handlers need to implement, I suspect its stuck at some delegate that maps some custom file.
I suspect its 1) Not finding, looping around to find that custom file handler, you may not 2) using the async handler
You have to chase down the various delegate that use this event and setup a break point
Make sure they are Async & Registered for e.g.
<add verb="*" path="*.aspx" type="System.Web.UI.PageHandlerFactory" />
<add verb="*" path="*.config" type="System.Web.HttpForbiddenHandler" />
<add verb="*" path="*.asmx" type="System.Web.Services.Protocols.WebServiceHandlerFactory" />
Then under the handlers verify
<httpHandlers>
<add verb="*" path="*.MyCustomaspx" type="MyCustomHTTPhandler"/>
</httpHandlers>
In your implementation use the async version base handler
// dervie from Async HttpTaskAsyncHandler
public class MyCustomHTTPhandler: HttpTaskAsyncHandler {
public override Task ProcessRequestAsync(HttpContext context)
{
//throw new NotImplementedException();
//blah blah.. some code
}
}
Last resort, not recommended - from SO here, if your handler/page doesnt modify session variables, you can skip the session lock.
<% #Page EnableSessionState="ReadOnly" %>
If your page does not read any session variables, you can opt out of this lock entirely, for that page.
<% #Page EnableSessionState="False" %>
If none of your pages use session variables, just turn off session state in the web.config.
<sessionState mode="Off" />
Based on this if you want to customize session state just based on your specific page/handler
using System;
using System.Web;
public class CustomSessionStateModule : IHttpModule
{
public void Dispose(){ //.. }
public void Init(HttpApplication context){
context.BeginRequest += new EventHandler(context_BeginRequest);
}
void context_BeginRequest(object sender, EventArgs e){
HttpContext currentContext = (sender as HttpApplication).Context;
// here you can filter and turn off/on the session state
if (!currentContext.Request.Url.ToString().Contains("My Custom Handler or Page Value")){
// for e.g. change it to read only
currentContext.SetSessionStateBehavior(
System.Web.SessionState.SessionStateBehavior.ReadOnly);
}
else {
//set it back to default
currentContext.SetSessionStateBehavior(
System.Web.SessionState.SessionStateBehavior.Default);
}
}
}

How do I restrict POST access to a HttpHandler?

I have a few http handlers (IHttpHandler) in my asp.net web project. Now I want to restrict access to these handlers. For Handler1 I want to allow only POST requests, and for Handler2 I want to allow only GET requests.
In my web.config I modified the <httpHandlers> section as shown below, but both handlers still process all verb types. Is there something I've missed? I'm testing it using IIS Express.
<httpHandlers>
<add verb="POST" path="Handler1.ashx" type="MyNamesapce.Handler1, MyAssembly"/>
<add verb="GET" path="Handler2.ashx" type="MyNamesapce.Handler2, MyAssembly"/>
</httpHandlers>

The reason this isn't working for you is that you've conflated two slightly different "flavours" of something that implements IHttpHandler.
There are two ways that you can implement an IHttpHandler with asp.net:
Create a class that implements IHttpHandler, e.g. MyCustomHandler.cs. This type of handler won't respond to any requests without being configured in your web.config file.
Create an .ashx file (which it looks like you've done), e.g. MyOtherHandler.ashx. This type of handler will respond to any requests to its URL, e.g. http://localhost/MyOtherHandler.ashx
The first type requires entries in the web.config file to work, the second doesn't. This is why you're seeing your .ashx handlers responding to all HTTP verbs, because they're being handled by the part of the asp.net framework that responds to requests for .ashx files, rather than being triggered by your web.config file. If you're using IIS Express, you can see this configured in the file %USERPROFILE%\Documents\IISExpress\config\applicationhost.config. Search for ".ashx" and you'll find a line similar to the below in the <system.webServer><handlers> section:
<add name="SimpleHandlerFactory-Integrated-4.0" path="*.ashx"
verb="GET,HEAD,POST,DEBUG" type="System.Web.UI.SimpleHandlerFactory"
preCondition="integratedMode,runtimeVersionv4.0" />
This is equivalent to what you've been adding to your web.config, but is responsible for telling IIS/asp.net "respond to any URLs that end in .ashx with any of the listed verbs by having the code in the type System.Web.UI.SimpleHandlerFactory deal with it. This code then loads your .ashx file.
To create a handler that can respond to any address you choose, you need (in short) a .cs file containing something similar to:
using System.Web;
namespace HttpHandlers
{
public class Handler4 : IHttpHandler
{
public bool IsReusable
{
get { return true; }
}
public void ProcessRequest(HttpContext context)
{
context.Response.Write("Hello World from Handler4.cs");
}
}
}
You can then wire it into your web.config file with:
<add name="Handler4" verb="POST" path="Handler4.ashx" type="HttpHandlers.Handler4, HttpHandlers" />
NOTE: My project that I created to test this is called "HttpHandlers", hence the type declaration that I've specified in that web.config snippet.

How to redirect user while an error happens during OnApplicationStarted?

I am doing a system version comparison during OnApplicationStarted method in Global.asax which does not allow the system boot up if the database version and system version are not matching.
So it looks like this:
protected override void OnApplicationStarted()
{
try{
if(systemVersion != dbVersion)
throw new Exception("They are not same!");
}
catch{
//do some other things
//Response.Redirect("~/VersionError.html");
}
}
but it says "Response is not available in this context." I tried to catch the error in Application_Error but I got the same error.
My question is that how I can redirect users to an error page from within these methods?
Edit:
I know there is no Response at this time but I was asking how to get around this problem. And also one of the reasons why I can hit methods after OnApplicationStarted is that we don't want to load many things if this exception occurs.

Since you don't have access to the Response when the application starts but want all incoming requests to go to an error page if a condition is met, you need to set up something farther up the pipeline. Think IIS modules. Install a module via the following:
<system.webServer>
<modules>
<add name="VersionsNotSameHandler"
type="Company.Exceptions.VersionsNotSameHandler, Company.Exceptions"
preCondition="managedHandler" />
</modules>
</system.webServer>
Now for the module code:
namespace Company.Exceptions
{
public class VersionsNotSameHandler: IHttpModule
{
public void Dispose() { }
public void Init(HttpApplication application)
{
context.PreRequestHandlerExecute += newEventHandler(OnPreRequestHandlerExecute)
}
public void OnPreRequestHandlerExecute(Object source, EventArgs e){
HttpApplication app = (HttpApplication)source;
HttpRequest request = app.Context.Request;
// Check for invalid versioning
app.Response.Redirect('.html page displaying error', true);
}
}
}
The OnPreRequest occurs before every request, making sure that the versions are always checked...you may want to cache if you notice things slowing down.

There is no response at this point you are in application start! This is when the application is starting not when a response is being made.
You could set something in here to direct every url to an error page if this condition is met by setting up a default route which will catch all requests. This would kind of give you the functionality you want as future requests would then get an error page.

Can you just throw an HTTP error:
throw new HttpException(403, "Forbidden");
And then in your web.config:
<customErrors mode="On">
<error statusCode="403" redirect="~/VersionError.html"/>
</customErrors>

Error is not logged with Application_Error

I have an Asp.Net 4.0 Web Forms application that throws following error certain times
“Server Error in ‘/MySiteDev’ Application” .
This error comes only at times. And this error is not firing the Application_Error event which is handled in Global.asax.
Since this is not firing Application_Error, what all are the other possible places that will have a log of this error event? Anything other than event viewer available?
Any way to find out the exceptions handled by the ASP.Net framework?
Note: customErrors mode="Off". Also runAllManagedModulesForAllRequests="true"
UPDATE
Reference from How to: Handle Application-Level Errors
An error handler that is defined in the Global.asax file will only catch errors that occur during processing of requests by the ASP.NET runtime. For example, it will catch the error if a user requests an .aspx file that does not occur in your application. However, it does not catch the error if a user requests a nonexistent .htm file. For non-ASP.NET errors, you can create a custom handler in Internet Information Services (IIS). The custom handler will also not be called for server-level errors.
You cannot directly output error information for requests from the Global.asax file; you must transfer control to another page, typically a Web Forms page. When transferring control to another page, use Transfer method. This preserves the current context so that you can get error information from the GetLastError method.
After handling an error, you must clear it by calling the ClearError method of the Server object (HttpServerUtility class).
CODE
protected void Application_Error(object sender, EventArgs e)
{
//Get the exception object
Exception exception = Server.GetLastError().GetBaseException();
//Get the location of the exception
string location = Request.Url.ToString();
if (!String.IsNullOrEmpty(location))
{
string[] partsOfLocation = location.Split('/');
if (partsOfLocation != null)
{
if (partsOfLocation.Length > 0)
{
location = partsOfLocation[partsOfLocation.Length - 1];
}
}
//Maximum allowed length for location is 255
if (location.Length > 255)
{
location = location.Substring(0, 254);
}
}
string connectionString = ConfigurationManager.ConnectionStrings[UIConstants.PayrollSQLConnection].ConnectionString;
ExceptionBL exceptionBL = new ExceptionBL(connectionString);
exceptionBL.SubmitException(exception.Message, location);
Log.Logger.Error(exception.Message);
}
CONFIG
<system.web>
<compilation debug="true" targetFramework="4.0" />
<pages validateRequest="false"></pages>
<httpRuntime requestValidationMode="2.0" />
<customErrors mode="Off"/>
<authentication mode="Windows"></authentication>
<identity impersonate="true" userName="domain\xxxx" password="xxxx"/>
</system.web>
<system.webServer>
<modules runAllManagedModulesForAllRequests="true"/>
<httpErrors errorMode="Detailed" />
</system.webServer>
UPDATED REFERENCES
Application_Error not firing
Global.asax event Application_Error is not firing
Application_Error does not fire?
How to: Handle Application-Level Errors

Examine logs in the Event Viewer, which should log both server-level and application-level errors.
The application error handler probably isn't processing the event because it's happening before your application is successfully started and a context created. So, there would seem to be an application configuration or server configuration ceasing processing the request.
Or, the application is encountering a problem early enough in the request lifecycle that even after starting, it's 'hung' until the server decides to kill the process (for instance, perhaps in the form of a StackOverflowException mentioned by #MikeSmithDev).

This is a load balanced environment with A and B boxes.
The team who deployed the web application confirmed that in one of the boxes, the config file as not copied properly.
I think, the application worked well when it was hitting A box and failing in B box. I think, since the config is not there, it was not possible to call Application_Error.
Please tell if you have different opinion.
Note: The issue is not there when they re-deployed

ASMX web-services and HTTP GET

I'm trying create a ASMX webservice that can perform a HTTP GET request. I have the following simple snippet of code to illustrate what I've already done.
using System.Web.Script.Services;
...
[WebMethod]
[ScriptMethod(UseHttpGet = true)]
public string HelloWorld(HttpContext context)
{
return context.Request.Params.Get("userId").ToString();
}
In addition to this, I've also added the following nodes in my Web.config file
<webServices>
<protocols>
<add name="HttpGet"/>
<add name="HttpPost"/>
</protocols>
</webServices>
The problem that I'm facing is that I'm constantly getting the dreaded "System.Web.HttpContext cannot be serialized because it does not have a parameterless constructor" error message whenever I try to debug this webservice. I have no idea what the problem is, and I would really appreciate any assistance that is offered to get me out of this quandary. I realize that HTTP GET requests are supposed to be very simple, but I'm really uncertain of what the cause of my frustrations are.

I think you want
[WebMethod]
[ScriptMethod(UseHttpGet = true)]
public string HelloWorld(int userId)
{
return userId.ToString();
}
You can specify parameters in the function signature and you can access the HttpContext as Context (a property on the base class WebService) if you need it.