Definition: Scalability testing checks how a system's performance and behaviour change as the amount of data, users or transactions grows. Rather than testing at one load level, it looks at how the system copes as it scales up, and where it starts to struggle.
So what? Systems that work well at small scale can fail in unexpected ways as they grow. Scalability testing gives teams warning of those limits before customers hit them, and helps them plan capacity.
Examples: Testing whether query times stay acceptable as a data archive grows from millions to billions of records. Testing individual components at the index or metadata layer to understand how they will behave at full scale.
