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High availability is a property of network resilience, the ability to "provide and maintain an acceptable level of service in the face of faults and challenges to normal operation." [ 3 ] Threats and challenges for services can range from simple misconfiguration over large scale natural disasters to targeted attacks. [ 4 ]
Resilience is a multi-facet property, covering four dimensions: technical, organization, social and economic. [6] Therefore, using one metric may not be representative to describe and quantify resilience. In engineering, resilience is characterized by four Rs: robustness, redundancy, resourcefulness, and rapidity.
Redundancy is a form of resilience that ensures system availability in the event of component failure. Components (N) have at least one independent backup component (+1).The level of resilience is referred to as active/passive or standby as backup components do not actively participate within the system during normal operation.
Geographic redundancy corrects the vulnerabilities of redundant devices deployed by geographically separating backup devices. Geographic redundancy reduces the likelihood of events such as power outages, floods, HVAC failures, lightning strikes, tornadoes, building fires, wildfires, and mass shootings disabling most of the system if not the entirety of it.
The most common size for an HA cluster is a two-node cluster, since that is the minimum required to provide redundancy, but many clusters consist of many more, sometimes dozens of nodes. The attached diagram is a good overview of a classic HA cluster, with the caveat that it does not make any mention of quorum/witness functionality (see above).
The first type of resilience engineering work is determining how to best take advantage of the resilience that is already present in the system. Cook uses the example of setting a broken bone as this type of work: the resilience is already present in the physiology of bone, and setting the bone uses this resilience to achieving better healing ...
Search and rescue workers dig through the rubble left behind by the Eaton Fire, in Altadena, Calif., Tuesday, Jan. 14, 2025. / Credit: Jae C. Hong / AP
Computer clustering capability with failover capability, for complete redundancy of hardware and software. Dynamic software updating to avoid the need to reboot the system for a kernel software update, for example Ksplice under Linux. Independent management processor for serviceability: remote monitoring, alerting and control.