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A cluster with external etcd, instead, is a topology where the distributed data storage cluster provided by etcd is external to the cluster formed by the nodes that run control plane components. So, etcd members run on separate hosts and each etcd host communicates with thethe kube-apiserver of each control plane node. This topology decouples the control plane and etcd member. It therefore provides an HA setup where losing an etcd member has less impact and does not affect the cluster redundancy. On the other hand, higher reliability pays off with more hosts employed: the minimum number of hosts is 3, and then goes up to 5 or 7 (although there is no hard limit, an etcd cluster probably should have no more than seven nodes).
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$ kubectl get componentstatuses
NAME STATUS MESSAGE
etcd-1 Healthy {"health":"true"}
etcd-2 Healthy {"health":"true"}
etcd-0 Healthy {"health":"true"} |
Later we will see how to monitor an external etcd cluster with Prometheus Operator.