Performance tests verify the non-functional aspect of how a system performs in terms of responsiveness and correct behavior under certain workloads.

It needs to be ensured that an application can provide business value, not only under laboratory conditions but also in production. In production the load on the system can vary heavily, depending on the nature of the application and its use cases. Applications that are publicly available also run the risk of becoming the target of denial of service attacks.

Performance tests are a helpful tool to detect potential performance issues that are caused by the application. This includes, for example, resource leaks, misconfiguration, deadlock situations, or missing timeouts. Putting the application under simulated workload will bring these issues to light.

However, as we will see in Chapter 9, Monitoring, Performance, and Logging, performance tests aren't necessarily helpful to predict production responsiveness or tune an application's performance. They should be used as a barrier against obvious mistakes, providing fast feedback.

For the rest of this book, I will use the term performance tests to describe performance as well as load or stress tests that put the application under performance load.