Which of the following is a key characteristic of observability in systems?

Observability is a key concept in understanding the behavior of complex systems, and it emphasizes the importance of being able to infer the internal state of a system based on external outputs and data. The correct answer highlights that observability focuses on the external data produced by the system, which includes logs, metrics, and traces. This external data provides insights that allow developers and operators to troubleshoot issues, understand performance, and ensure the system behaves as expected.

By analyzing this external data, one can gain a holistic view of the system's operations and potential pain points without needing direct access to its internal workings. This is crucial in modern applications that often run in distributed environments, where direct access may not be practical.

In contrast, the other options do not capture the essence of observability correctly. For instance, relying solely on backend database queries limits the scope of understanding, as this approach does not encompass other critical types of data like logs or traces. Additionally, requiring physical access to servers is impractical given the rise of cloud-based infrastructures where systems can be accessed remotely. Finally, limiting insights to performance metrics only would be too restrictive, as observability is about integrating multiple data types to understand overall system health and behavior comprehensively.