No database should lack an unsynchronized replica in general. A kubernetes admin could accidentally drain the host, but we run in AWS and AWS can kill nodes a lot faster than we can haha. We have enough servers in AWS that at least 5 fail per day. If some database is not replicated, someone is getting paged for it. We run MySQL with semi-sync replication for example: if a primary has two replicas, one replica must ack a write before the primary commits it, so you can lose the primary at any time and not lose data. That said, kubernetes has prevention mechanisms for this.

PodDisruptionBudgets allow you to specify the minimum number of replicas. The drain command calls evict instead of delete and evict honors these restrictions. This is helpful for node draining, but not necessarily helpful if someone accidentally deletes a StatefulSet. However, nothing in kubernetes deletes PVC objects automatically, so if you delete the pod, the volume remains bound to the PVC until you delete it.

preStop hooks are used in all the database pods to ensure that if the current pod is the primary, leadership is correctly changed to a replica before proceeding with the delete. We combine this with extremely long termination grace periods and alerting if termination is taking too long so that an ops team can look into the issue.

preStop hooks are great, but they also are somewhat of a time bomb if not careful, so some of our database operators make use of finalizers instead. With finalizers, nothing happens when a pod enters the terminating state until an operator marks the pod's finalizer as completed. This flow is really nice because you end up with behavior where the kubernetes cluster admins or built in controllers effectively are really gently requesting that a pod terminate, and it's up to the operator of that pod to decide exactly how and when that occurs.

For non-local volumes, like EBS PVCs, once the PVC is deleted an operator will snapshot the volume during a finalizer before allowing the volume to be fully deleted so that we can recover data if this ever accidentally occurs. This was the first protection we ever implemented when initially adopting kubernetes back when we had no idea what we were doing so that we could always recover data in the event of admin mistakes. We can't do this for local volumes, however, any team making use of local volumes truly needs to stay on top of replication when running in the cloud, whether on kubernetes or not or else you absolutely will lose data. AWS local nvme volumes have limited write cycles just like the ones in on-prem servers. So ideally all databses are configured to not accept writes that cannot be replicated. And even with EBS, at scale we end up with something like 20 unrecoverable volume failures per year