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For more complex upgrades, that involve database changes, all nodes may need to be taken down prior to the upgrade.
Historic Availability (last 12m)
This is the history from NewRelic, we find it very useful, but even these availability results aren't strictly accurate, the portray the ability of NewRelic to connect to JEMHC, but, in some cases, we find that due to load balancing over nodes running in different data centers (Multi-AZ), there is 'no downtime' at all.