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.

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Scott Moore

About Scott Moore (153 articles)

With over 20 years of IT experience with various platforms and technologies, Scott has tested some of the largest applications and infrastructures in the world. He is a Certified Instructor and Certified Product Consultant in HP’s LoadRunner and Performance Center products. He currently holds HP certifications for ASE, ASC, and CI. A thought leader in the APM space, he speaks regularly at IT conferences and events

  • http://pretenseofknowledge.com/ speedmaster

    Good post, I run into this issue often when describing to others what a test is doing.

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