Understanding IIS7 Request Restrictions on Windows Vista

Note: This blog entry used to be on my old blog. I'm about to shut it down so I thought I replicate some of the content here.

You might have run into the IIS connection limit on Windows XP. IIS 5.1 on Windows XP allowed 10 concurrent connections before it displayed a 403.9 error message (Access Forbidden: Too many users are connected). Not too user friendly.

The only restrictions we have on Windows Vista is the number of concurrent requests IIS will execute. This is fundamentally different from the Windows XP connection limit. By limiting concurrent requests you will probably never see an error message. Request that cannot be handled because the concurrent request limit is reached will still be queued.

There is only a problem if the request queue overflows - a "500 Server Busy" error. The request queue is pretty big though. Thousands of requests by default. The net effect is that the execution of too many concurrent requests will not generate an error but it might slow your server down for a little bit. Windows Server 2008 doesn't have any request restrictions.

Here is how many concurrent requests IIS7 allows on Windows Vista SKUs:

Windows Vista Home Basic* 3
Windows Vista Home Premium 3
Windows Vista Ultimate 10
Windows Vista Professional 10
All Windows Server 2008 SKUs unlimited

*Home Basic contains the IIS process activation and HTTP processing infrastructure, but it cannot be used as a traditional web server. It also doesn’t include the IIS administration tool.

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