When a lower tier quorum comes to its own consensus

Question

Consider this simple scenario:

A <--- B

A.QUORUM_SET = [A]
A.THRESHOLD = 100

B.QUORUM_SET = [A,B]
B.THRESHOLD = 49

So A is a higher tier node that comes to consensus with itself. B is a lower tier node that comes to consensus with 1/2 nodes from the set {A,B}.

The two nodes get disconnected. A continues to reach consensus with itself while B gets disconnected but is still coming to consensus with itself. Someone submits a createAccount operation through B.

At some point the network comes backup. A and B are technically out of sync. A does not have B's new account. How can the two nodes sync up without changing any quorum configurations? Can this be done through archive histories?

Answer 1

A and B cannot sync up with SCP. This is an invalid configuration. B is misconfigured, so should just dump its state and restart with better quorum slices.

SCP only guarantees agreement when there is quorum intersection. (This is one of the reasons stellar-core makes it hard to assign <= 66% threshold. In your example, B would have had to add the configuration line UNSAFE_QUORUM=true to its stellar-core.cfg file.)