To tolerate the loss of two ZooKeeper servers, what is the minimum total number of servers in the ensemble?

• A. 3
• B. 5
• C. 4
• D. 7

Answer: (B)

ZooKeeper needs a majority of the ensemble to be up to make progress, which is called a quorum. To tolerate f server failures, you must have at least 2f + 1 nodes so that, even with f nodes down, the remaining nodes still form a majority. For tolerating two failures, that means at least 5 servers. With five, if two fail, three remain and still constitute a majority to elect a leader and continue operating. A smaller ensemble won’t have a majority with two failures (four nodes left would need three for a majority, but two remaining isn’t enough). A larger ensemble like seven would also tolerate two failures, but it’s not the minimum. Hence the minimum total is five.