What should be the majority in an ensemble for Zookeeper

Question

I am trying to understand Zookeeper using this book - Zookeeper By Flavio Junqueira, Benjamin Reed , it is mentioned that we need to select a majority of servers for quorum as stated here:

Say that we use four servers for an ensemble. A majority of servers is comprised of three servers. However, this system will only tolerate a single crash, because a double crash makes the system lose majority. Consequently, with four servers, we can only tolerate a single crash, but quorums now are larger, which implies that we need more acknowledgments for each request. The bottom line is that we should always shoot for an odd number of servers.

Please help me in understanding this.

How do we select the majority of servers for a given ensemble?

Why does this statement say quorums now are larger and why do we need more acknowledgments for each request?

Answer 1

It just means that more servers should be up than down where each server in the ensemble should be accounted for, or that more servers have acknowledged message receipt than those that have not. With 4 servers you need 3 servers to be up to satisfy that condition, with 3, only 2. In each instance you can only tolerate the failure of one server for the cluster to still be up. The 4 node cluster is worse because you now have an extra server that is essentially not making your cluster any more fault tolerant than just a 3 node one.

Additionally, if you had 3 nodes, you would require just 2 acknowledgements to meet the quorum requirement. With 4, you need 3 acks. That would lead to a slower cluster. That's what the ' Consequently, with four servers...' statement means.