Worker process and application pool

Question

Hi everyone I went through a couple of articles over the net to understand worker process and application pools but somehow i am having troubles understanding them. Can someone help me understand in simple words what is an application pool and how the app pools contain worker process. does our .net application run in a worker process ? When a client requests for a asp.net resource the request goes to the worker process which in turn provides the response ? Thanks

Sam Hobbs · Answer

First note that the term application can be confusing. An application is typically a process; typically there is a one-to-one correspondence of application and a process / address space. See my An Introduction to Virtual Memory in Windows for more about that. The term application has a different meaning for IIS. Since a server can have hundreds of sessions it would be too innefficient for each one to have a separate process / address space. So in IIS ASP applications share processes and address spaces with others. The problem with that is that since they are in the same address space the sessions can address memory for other sessions and that is a potential security problem. So Windows has a way to keep the various sessions separate from each other but I forget what that is called. You use the term worker process. I also don't know what Microsoft calls them. Whatever they are called each one is called an application in the traditional definition of application. In my article I explain that there is one and only one application for each process / address space. In the case of an IIS worker process each one can have very many IIS applications.

Vulpes · Answer

Actually I'm forgetting about 'web gardens' which do allow you to assign more than one worker process to an application pool. It's possible with these to have multiple instances of the same application running in the same application pool each of which is served by a different worker process. With regard to having different instances of the same application running in different application pools this won't necessarily give the client a 'seemless' experience if the current instance fails because the other instances will probably be in a different state at the time.

Vipul Kelkar · Answer

ok so lets say I have only one pure .net application running on a server Can i create multiple application pools and each one have a worker process so that if one fails the other would keep the application up and running ?

Vulpes · Answer

Yes that's my understanding of it Vipul.

Vipul Kelkar · Answer

Thanks for your answer vulpes So does this mean that we have multiple application pools and we have multiple instances of the worker process running on a server. An app pool can have only ONE worker process running inside it ?? And this worker process may be running either a single application or multiple application. correct me i may be getting it wrong

Vulpes · Answer

An application pool is essentially a group of applications which run under the same IIS worker process and is a mechanism which was first introduced in IIS version 6.0. A group can consist of a single application. What might be confusing you is that you can have multiple instances of the IIS worker process w3wp.exe running simultaneously with each instance serving a different application pool. Because there are separate instances (i.e. processes) applications in one pool can't interfere with applications in another pool. So if one instance of w3wp.exe fails for some reason it won't affect the other instances because they are separated by process boundaries. IIS itself deals with HTTP requests from the client for static web pages. Requests for dynamic pages are passed to the appropriate application pool where the worker process processes them and sends the resulting HTML back to IIS for transmission as an HTTP response to the client.