How many users can dance on the head of a pin?

We have lost hours of our life, and possibly several friends, to this question. We shudder now to even write it.

What is user concurrency?

You see, one metric that everyone understands is users on a system. We have 10,000 employees and so the system needs to be able to have 10,000 people using it. Right?

Like so much in performance testing, the answer is very definitely maybe. Let’s first define a few different ways of looking at users on the system.

First off, there is the number of named users. That’s the number of people that could use a system. Imagine you have an email system or a support desk system, to use non-SAP examples. Probably all your 10,000 employees have access to those.

Second, there’s logged on users, the number of people actively connected to the system. For the mail system that’s probably most of the 10,000 (unless your company is spread over multiple time zones) and for the support desk, that’s probably a few 100. Probably helpdesk analysts and people logging or chasing issues.

Finally, we need to think about the active logged on users. That’s the people pressing buttons and doing something on the system. For the mails system, it’s likely to be a smaller percentage of the 10,000 as people don’t constantly send emails. (Well most people don’t.) Whereas for the support system, it would be quite high as most people will log on to do something.

For performance testing, it’s the last one that usually matters most. So although an SAP system may have 10,000 users, it may have a lot fewer active users. To get that figure, we need a bit more analysis.