What is the biggest concern when designing a fault-tolerant system for a critical application?

When designing a fault-tolerant system for a critical application, the main concern revolves around minimizing downtime. In such environments, the availability of services is crucial. A fault-tolerant system aims to maintain operational functionality even in the face of failures, ensuring that the application continues to run and remain accessible to users or clients at all times.

Downtime can lead to significant operational issues, including lost revenue, diminished user trust, and impacts on service level agreements (SLAs). Therefore, strategies such as redundancy, failover mechanisms, and regular maintenance routines are implemented to ensure that any potential outages or disruptions are addressed swiftly, keeping systems up and running.

While considerations like cost, security, and data integrity are important elements of a comprehensive design, the paramount goal in fault tolerance is always to ensure that critical services maintain availability no matter the circumstances that may threaten it. Hence, the focus on minimizing downtime becomes the essence of fault-tolerant system design.