Why is there a potential for a quorum split in this example quorum configuration?

Question

The QuorumIntersectionChecker has a getPotentialSplit function. It doesn't seem to be documented.

I've tried to use it while experimenting with various quorum configuration. Here is one such configuration:

n0: QuorumConfig(thresh: 3, nodes: ["n0", "n6", "n7"])
n1: QuorumConfig(thresh: 3, nodes: ["n1", "n6", "n7"])
n2: QuorumConfig(thresh: 3, nodes: ["n2", "n6", "n7"])
n3: QuorumConfig(thresh: 3, nodes: ["n3", "n5", "n6", "n7"])
n4: QuorumConfig(thresh: 3, nodes: ["n3", "n4", "n6", "n7"])
n5: QuorumConfig(thresh: 3, nodes: ["n4", "n5", "n7"])
n6: QuorumConfig(thresh: 2, nodes: ["n6", "n7"])
n7: QuorumConfig(thresh: 2, nodes: ["n6", "n7"])

In this case getPotentialSplit returns: Splits: first: [] second: [n6, n7].

I'm trying to understand the return value of getPotentialSplit and why in this configuration n6 and n6 can cause a "potential split". What exactly does it mean that there is a potential split?

Answer 1

I can only assume that thresholds are not taken into account and "first" refers to a potential split if one node fails and "second" refers to a potential split if two nodes fail.

• If any single node fails, all other nodes will still be somehow connected with each other.
• If any two nodes (except [n6, n7]) fail, all other nodes will still be somehow connected.
• If [n6, n7] fail, the quorum will be fragmented into [n0], [n1], [n2], [n3, n4, n5] and it would be advisable that those should be better connected. For example n0, n1 and n2 should include one of [n3, n4, n5] in their qset.

With a different qset like this, there should be a potential "first [C]" split, which would result in [A, B], [D, E] fragments:

A: [A, B]
B: [A, B]
C: [A, C, D]
D: [D, E]
E: [D, E]