What are concurrent users?
I found that there is no clear definition of the number of concurrent users a system must support.
When a system is used by a large group of users, not all users are active all the time. For instance, if your organization has 10,000 employees, not everyone is in the office working every day. People have holidays, or can be sick. And if they are in the office, they are not behind their desks all the time, as they can be in meetings, standing at the coffee machine, etc. And when they are at their desk, using the system, they are not always active using the system’s back-end systems. For instance, when they are reading an article fetched from the internet, only the fetching of the document puts a load on the system. The time the user is reading the text, does not put a load on the system.
Consider the following example.
|Total number of employees||10000|
|Only 80% is at the office||8000|
|70% of their time is spent at their desk||5600|
|At their desk, they use the system 70% of the time||3920|
In that time, they only perform actions that put a
This means that during the day, on average, of all employees, only 196 people are actively using the infrastructure at any given moment.
As an alternative, we can use the ratio between usage of the system and “thinking time”. In our example, the percentage thinking time is 100% - 10000/196 = 98%.
A further breakdown could show how the system is used:
|Load file||Open file in an office application (like Excel or Word)|
|Save file||Save document from an office application|
|Browse files||Open file explorer|
|Send HTTP request||Push a button in a browser-based application, leading to sending data, or use AJAX calls|
|Receive HTTP data||Receive data from a webserver when using a browser-based application, or use AJAX calls|
|Send data to the Internet||Push a button on an internet page, use AJAX calls or send data using protocols like FTP|
|Receive data from the Internet||Receive a web page from the internet, use AJAX calls or receive data using protocols like FTP|
|Send email/calendar||Send a typed email to the email server, or update calendar items|
|Receive email/calendar updates||Receive new emails from the email server|
|Send VDI/SBC data||In a SBC or VDI environment, send keyboard and mouse input to the server|
|Receive VDI/SBC data||In a SBC or VDI environment, receive screen output from the server|
|Send and receive data from DNS||Use DNS to resolve IP addresses|
|Send and receive data from AD||Use AD to handle login/logout or to check credentials|
|Other||Other uses of the infrastructure|
Using such a categorization, the actual load on the infrastructure can be calculated, if we know how the system is setup, how the actions relate to a certain load and what a typical user’s behavior is. Not all users are alike. By observing groups of people, their typical behavior can be mapped to the defined categories over time. For instance, a group called secretaries will typically:
- Open 25 existing Word documents
- Save 40 Word documents (including new documents)
- Send 25 emails
- Receive 25 emails
Based on these numbers, and with the insight in the setup of the system, the actual load on the various parts of the infrastructure can be calculated. This calculation can then be used to shape performance tests.
This entry was posted on Friday 23 January 2015