I was asked to design a distributed key-value storage in a DevOps interview, is this normal?

I didn't expect this kind of question and got caught completely off-guard. I answered etcd and Raft, but obviously the interviewer wanted me to design the internals. I couldn't answer anything so I failed. I Googled the Raft implementation right after the interview and understand how it works now.
Is this normal for DevOps interviews? If yes, is there a list of protocol/architectural readings that I need to know before the next one?