How are threads tied to requests through Http.sys, IIS and ASP.NET

Question

I'm currently reading a lot about node.js. There is a frequent comparison between servers using a traditional thread per request model (Apache), and servers that use an event loop (Nginx, node, Tornado).

I would like to learn in detail about how a request is processed in ASP.NET - from the point it is received in http.sys all the way up to it being processed in ASP.NET itself. I've found the MSDN documentation on http.sys and IIS a little lacking, but perhaps my google-fu is weak today. So far, the best resource I have found is a post on Thomas Marquardt's Blog.

Could anyone shed more light on the topic, or point me to any other resources?

(For the purposes of this question I'm only interested in IIS7 with a typical integrated pipeline)

Saporous

My Google-fu turns up west-wind.com/presentations/howaspnetworks/howaspnetworks.asp which might answer your question – Bregma

This is a great question, and something I have wondered myself. – Salomie

Answer 1

From my research so far, its my understanding that when a request comes in it gets put into a kernel-mode request queue. According to this, this avoids many of the problems with context switching when there are massive amounts of requests (or processes or threads...), providing similar benefits to evented IO. Quoted from the article:

"Each request queue corresponds to one application pool. An application pool corresponds to one request queue within HTTP.sys and one or more worker processes."

So according to that, every request queue may have more than one "Worker Process." (Google cache) More on worker processes

From my understanding:

• IIS Opens creates a request queue (see the http.sys api below)
• A "Web Site" configured in IIS corresponds to one Worker Process
• A Web Site/Worker Process shares the Thread Pool.
• A thread is handed a request from the request queue.

Here is a lot of great information about IIS7's architecture

Here is some more information about http.sys.

• HTTP Server I/O Completion Stuff
• Typical Server Tasks

Open questions i still have:

• How the heck does IIS change the Server header if it Uses HTTP.SYS? (See this question)

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Note: I am not sure if/how a "Kernel-mode request queue" corresponds to an IO completion port, I would assume that each request would have its own but I don't know, so I truly hope someone will answer this more thoroughly. I just stumbled on this question and it seems that http.sys does in fact use IO Completion ports, which should provide nearly all of the same benifits that evented IO (node.js, nginx, lighttpd, C10K, etc...) have.

Salomie

Thanks for providing so much more information. I've left the question open for the time being in case anyone else would like to contribute. I've also posted this on the IIS forum to try and encourage those folks to take a bash. – Saporous