Requests/Second and Concurrent Users?
Are they both the same or is there any slight difference between the two?
Requests can be queued, the idea is that concurrent users represent those requests actually enjoying non-zero time-slices in the web server's multi-threading/multi-process channels. They are usually used interchangably because there's only so much requests you can queue before realizing no furthur benefits.
However if you are talking about Microsoft licensing that's a whole different discussion.
licensing for Windows, IIS or SQL Server?
We use Concurrent users to indicate the number of actual people. A single user can have "n" requests on their behalf, request/second indicate how many requests you are willing to (or can) service.
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