This might be a bit of a silly question but;
If I have two people logging on to my site at exactly the same time, will the server side code be executed one after the other or will they be executed simultaneously in separate threads?
I’m curious in regards to a denial of service attack on a website login. Does the server slow down because it has a massive queue of logins or is it slow because it has a billion simultaneous logins!
Answers:
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Method 1
This is not related to ASP.NET per se (I have very little knowledge in that area), but generally web servers. Most web servers use threads (or processes) to handle requests, so basically, whatever snippet of code you have will be executed for both connections in parallel. Of course, if you access a database or some other backend system where a lock is placed, allowing just one session to perform queries, you might have implicitly serialized all requests.
Web servers typically have a minimum and maximum number of workers, which are tuned to the current hardware (CPUs, memory, etc). If these are exhausted, new requests will be queued waiting for a worker to become available, or until a maximum queue length of pending requests has been reached at which point it disregards new connections, effectively denying service (if this is on purpose, it’s called a denial of service or DoS attack).
So, in your terms it’s a combination, it’s a huge number of simultaneous requests filling up the queue.
Method 2
It should use a thread pool. Note that they are still in the same application, so application level items like static variables are still shared between them.
Method 3
from this article
“Remember ISAPI is multi-threaded so requests will come in on multiple threads through the reference that was returned by ApplicationDomainFactory.Create(). Listing 1 shows the disassembled code from the IsapiRuntime.ProcessRequest method that receives an ISAPI ecb object and server type as parameters. The method is thread safe, so multiple ISAPI threads can safely call this single returned object instance simultaneously.”
So yes, in the case of a DOS attack, it would be slow because of the large number of connections
Method 4
As others said, most webservers use multiple processes or threads (better) to serve multiple requests at a time. In particular, you can set each ASP.NET application pool with a max number of queued requests and max worker processes. Each process has multiple threads up to a maximum (not configurable AFAIK, I may be wrong), and incoming requests are processed on a first-in-first-out basis.
Moreover, ASP.NET processes one single request for each session – but a malicious user can open as many sessions as she wants.
Multiple logins will probably hit the database and bring it to its knees probably before the webserver itself.
As far as I know, there is not a built-in way to throttle ASP.NET requests other than setting the max number of queued requests (waiting to be processed). This number should be ideally very small. You can monitor the number of queued ASP.NET requests using performance counters. Say you find that, on peak traffic, this number is 100. You can then update application so that it refuses login attempts when this number is above 100 so that the database is not hit (never did that, just a thought).
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