What is Delay when running a Load Test?

"Delay" in Load Testing

"Delay" when running a load test refers to the intentional pause or wait time introduced between actions performed by virtual users or between the initiation of each virtual user. Delays are used to simulate real-world user behavior more accurately and prevent all virtual users from hitting a system at the exact same moment.

In a real-world scenario, users don't all take actions simultaneously. They might read a piece of content, ponder on a decision, fill out a form, or just casually browse at their own pace. This natural delay between actions can impact the load on the system and the resultant performance metrics. By introducing a delay in a load test, testers aim to emulate this more random, staggered behavior of real users.

Types of Delays in Load Testing

There are various types of delays one might encounter or set during load testing:

  • Think Time: This is the time a virtual user "waits" between actions, simulating a real user reading content, considering options, etc. For instance, after logging in, a real user might wait a few seconds before clicking on a link or button. Introducing "think time" in a script simulates this behavior.

  • Ramp-Up Delay: When starting a load test, it's not always desirable to launch all virtual users simultaneously. A ramp-up delay means that the load test will start with a subset of users and will gradually increase the number of active users until the desired load is reached. For example, if you want to simulate 1000 users over 10 minutes, a linear ramp-up would add 100 users every minute.

  • Startup Delay: This is an initial delay before a virtual user starts its actions. For example, if you have 10 virtual users and a startup delay of 5 seconds, the first user starts immediately, the second user starts after 5 seconds, the third after 10 seconds, and so on.

Importance of Delays

Introducing appropriate delays is crucial for a realistic load test. Without them, the test can produce skewed results as it might place an unnatural, concentrated load on the system, leading to misleading outcomes. It's always important to adjust delays to closely mimic the actual user behavior for the application or system being tested.

In many load testing tools and discussions, when the term "Delay" is used in isolation, it often refers specifically to the Ramp-up Delay. This is an especially important metric as it dictates the pacing at which virtual users are introduced during the test.

The Ramp-up Delay ensures that not all virtual users are bombarding the system simultaneously right from the start. Instead, they're gradually added, mirroring a more realistic scenario where users might flock to a website or application over a span of time rather than all at once.

Properly configuring this Ramp-up Delay is pivotal to preventing sudden, unrealistic spikes in load, which can skew test results or unduly stress the system. When fine-tuning the parameters of a load test, it's essential to give special attention to this specific delay to ensure the authenticity of the test conditions.