Zab [the distributed consensus algorithm that powers ZK] shares some similarities with Paxos, and requires a quorum of nodes to be online.
If you want highly available ZK, your choices are 3, 5, 7... nodes, for which you can have 1, 2, or 3 nodes offline at any one time.
If you have one node fully down on a 3 node cluster, and there is even a tiny network blip or partition (as often happens in cloud environments) then you are down.
If you want highly available ZK, your choices are 3, 5, 7... nodes, for which you can have 1, 2, or 3 nodes offline at any one time.
If you have one node fully down on a 3 node cluster, and there is even a tiny network blip or partition (as often happens in cloud environments) then you are down.