HA 最小集群为 5-9
我想了解 3 节点 HA 集群是否是常见做法？ Google 上的大多数参考文献都指向 2 节点集群。但我无法说服自己，需要 5 个 9 的应用程序可以在商用硬件上实现 2 节点 HA 集群。 其背后的原因很简单。如果一台机器上有一个节点离线，那么就只剩下一个节点，没有任何备份。 为了减少对离线节点的依赖，我认为 3 节点集群是最低要求。
I am trying to find out if 3 node HA cluster is common practice? Most of the references on Google point to 2 node cluster. But i not able to convince myself that an application that require 5 Nine's, can implement 2 node HA cluster on commodity hardware.
The reason behind it is simple. If a machine on which one node goes offline, then there will be only one node left without any back up.
To reduce dependency on node that went offline, i think a 3 node cluster is a min requirement.
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为了给出事实答案，需要更多的数据。
但从轶事的角度来看，两个商用硬件节点不足以为您提供任何级别的可靠性（或至少夜间睡眠舒适度）的五个九。
为了便于解释，大多数集群图可能只用两个节点绘制，“如果 A 失败，B 继续工作”。
然而，鉴于您的五个九和“商品硬件”，我会考虑超过三个作为要求；也许多达五个或更多。
如果您确实追求这种可靠性，请记住考虑到网络、电力甚至地理多样性。
In order to give a factual answer, much more data would be required.
But from an anecdotal perspective, two nodes of commodity hardware are not nearly enough to give you five-nines with any level of reliability (or at least sleep-at-night comfort).
Most cluster diagrams are likely drawn with only two nodes for ease of explanation, "If A fails, B keeps working".
Given your five-nines however, and "commodity hardware", I would consider more than three as a requirement; perhaps as many as five or more.
Remember to allow for network, power and perhaps even geographical diversity if you are really after that kind of reliability.