Graceful degradation is the ability of a computer, machine, electronic system or network to maintain limited functionality even when a large portion of it has been destroyed or rendered inoperative. The purpose of graceful degradation is to prevent catastrophic failure. Ideally, even the simultaneous loss of multiple components does not cause downtime in a system with this feature. In graceful degradation, the operating efficiency or speed declines gradually as an increasing number of components fail.
Graceful degradation has been an important consideration in the design and implementation of large communications networks since the Internet was originally conceived by the Advanced Research Projects Agency (ARPA) of the U.S. government. The Internet arose out of a desire on the part of public-service and military officials for a large-scale computer network that could resist massive physical as well as electronic attacks including global nuclear war. Today's Internet continues to maintain this resiliency although not to the extent all engineers would like. Malicious electronic activities in the form of viruses, worms, denial of service attacks and proliferation of spam cause considerable disruption from time to time.
Graceful degradation is sometimes considered equivalent to fault tolerance but there is a significant difference. Fault-tolerant systems are designed so that if a component fails or a network route becomes unusable, a backup component, procedure or route can immediately take its place with no negative impact whatsoever on individual subscribers. Graceful degradation is an outgrowth of effective fault management, which is the component of network management concerned with detecting, isolating and resolving problems.