During which testing level should non-functional tests be executed?

Non-functional tests are an essential aspect of software testing that evaluate aspects such as performance, usability, reliability, and security. These types of tests seek to assess how well a system performs under various conditions and are not strictly focused on specific functionality.

Executing non-functional tests during every level of testing, including unit, integration, system, and acceptance, ensures that these attributes are evaluated in various contexts and stages of the development lifecycle. For instance, unit testing can include non-functional aspects such as testing the performance of individual components under minimal load. Integration testing might focus on how multiple components behave together under stress. System testing validates the non-functional aspects of the entire application to ensure it meets the defined quality standards. Finally, acceptance testing which is conducted to ensure the software meets business requirements, can also include non-functional criteria to confirm that the system complies with user expectations and regulatory requirements.

By involving non-functional tests at all these levels, teams can identify potential issues early, making it easier to address them before they become more significant problems later in development or post-deployment. This comprehensive approach helps create a robust and reliable software product.