Modernwebservicesystemsaredesigned forhigh availability andresiliency. Whichconceptpertains to the ability to detect problems within a system…

Fault tolerance is the design principle that enables a system to continue operating, possibly at a reduced level, even when one or more of its components fail. This concept inherently includes the ability to detect failures and automatically invoke recovery procedures, such as activating redundant components or redirecting traffic, to mitigate the impact of the failure and maintain service availability. The question's description of detecting problems and programmatically invoking redundant systems is the exact definition of a fault-tolerant system in action.

A. Elasticity refers to the ability to dynamically scale computing resources up or down to match workload demands, not to handle failures.

B. Redundancy, the practice of having duplicate components, is a necessary prerequisite for fault tolerance but is not the concept of failure detection and recovery itself.

D. Automation is the mechanism used to implement fault tolerance (e.g., automated failover), but fault tolerance is the overarching design concept being described.

1. National Institute of Standards and Technology (NIST). (2011). NIST Special Publication 500-292: NIST Cloud Computing Reference Architecture. "Resiliency is the ability of a system to provide and maintain an acceptable level of service in the face of faults and challenges to normal operation. ... Fault tolerance is one means of achieving resiliency." (Page 21, Section 5.3.3).

2. Avižienis, A., Laprie, J. C., Randell, B., & Landwehr, C. (2004). Basic concepts and taxonomy of dependable and secure computing. IEEE Transactions on dependable and secure computing, 1(1), 11-33. "Fault tolerance is the attribute that enables a system to avoid service failures in the presence of faults." (Page 16, Section 3.2). DOI: 10.1109/TDSC.2004.2.