Load Testing

Counting concurrent users

With a new, untried system, the client knew that load testing would be critical to a successful launch. The client had budgeted a full month for load testing, longer than I’d ever seen. Before the site could launch, marketing had declared that it must support 25,000 concurrent users.

Counting concurrent users is a misleading way of judging the capacity of the system. If 100 percent of the users are viewing the front page and then leaving, your capacity will be much, much higher than if 100 percent of the users are actually buying something.

We can’t measure the concurrent users. There’s no long-standing connection, just a series of discrete impulses as requests arrive. The servers receive this sequence of requests that they tie together by some identifier. As shown in the following figure, this series of requests are identified with a session, an abstraction to make programming applications easier.