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I am trying to understand hierarchical quorums in Zookeeper. I may not understand the example shown in the documentation (here). Are votes [from at least two servers from each of two different groups] enough to form a legal quorum?

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In my opinion, the example here does not gain the majority of all the weight; it only gains more than 4 ballots. A legal quorum should earn more than 5 ballots (9/2+1).

I also read the source code. The algorithm implementation is shown from line 352 to line 371. Zookeeper only checks if all groups have a majority and if the number of selected groups is larger than half of the group number.

dengzeyuan
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1 Answers1

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Maybe I find the answer.

A different construction that uses weights and is useful in wide-area deployments (co-locations) is a hierarchical one. With this construction, we split the servers into disjoint groups and assign weights to processes. To form a quorum, we have to get a hold of enough servers from a majority of groups G, such that for each group g in G, the sum of votes from g is larger than half of the sum of weights in g. Interestingly, this construction enables smaller quorums. If we have, for example, 9 servers, we split them into 3 groups, and assign a weight of 1 to each server, then we are able to form quorums of size 4.

Note that two subsets of processes composed each of a majority of servers from each of a majority of groups necessarily have a non-empty intersection. It is reasonable to expect that a majority of co-locations will have a majority of servers available with high probability.

dengzeyuan
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