Load Testing: Concurrent Users verses Simultaneous Users

It seems like on every new project, there is a need to educate members of the project team on the difference between concurrent users and simultaneous users as it applies to load testing applications.

Concurrent = users on the system using resources.
Simultaneous = users executing the same code at the exact same time.

Concurrency has various levels. I generally it explain things this way to my clients at the initial project kickoff meeting to make sure everyone agrees with the definitions:

• Application Concurrency – how many vusers are on the system doing something?
• Business Process Concurrency – how many are doing a specific business process right now?
• Transaction concurrency – how many are executing the same code at the exact same time at a given moment?

The relevance of any of these depends on what your testing objective is. Most projects that are trying to create a load test (not a stress test), and will prefer application concurrency.

Do you define concurrency differently? Let me know your thoughts on this by putting a comment in the section below.