One thing to remember is that a HA cluster is for handling node failure (power loss, faulty hardware, faulty software, etc). It is not for handling capacity related failure. If the servers are overloaded with too many requests, they will fail regardless of the HA setup. Capacity monitoring and capacity planning are still needed to maintain uptime.
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They actually have a pretty good HA setup.
One thing to remember is that a HA cluster is for handling node failure (power loss, faulty hardware, faulty software, etc). It is not for handling capacity related failure. If the servers are overloaded with too many requests, they will fail regardless of the HA setup. Capacity monitoring and capacity planning are still needed to maintain uptime.