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A High Availability (HA) system is one that is designed to be available 99.999% of the time, or as close to it as possible. Usually this means configuring a failover system that can handle the same workloads as the primary system.
A Fault Tolerant (FT) system is extremely similar to HA, but goes one step further by guaranteeing zero downtime. HA still comes with a small portion of downtime, hence the ideal of a perfect HA strategy reaching “five nines” rather than 100% uptime. The time it takes for the intermediary layer, like the load balancer or hypervisor, to detect a problem and restart the VM can add up to minutes or even hours over the course of yearly runtime.
Disaster Recovery (DR) goes beyond FT or HA and consists of a complete plan to recover critical business systems and normal operations in the event of a catastrophic disaster like a major weather event (hurricane, flood, tornado, etc), a cyberattack, or any other cause of significant downtime. HA is often a major component of DR, which can also consist of an entirely separate physical infrastructure site with a 1:1 replacement for every critical infrastructure component, or at least as many as required to restore the most essential business functions.