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Signed-off-by: Daniele Sciascia <daniele@nats.io>
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This commit fixes a bug where disjoint majorities could form during peer additions. Previously, node would update their cluster size only after the `EntryPeerAdd` log entry was committed by the cluster. This led to a situation where a leader could have a new peer in its peer set, before committing the corresponding EntryPeerAdd, while keeping the quorum size unchanged. This created the opportunity for a partitioned leader to commit EntryPeerAdd using only append entry responses from newly added nodes. For instance, a partitioned leader in a three node cluster, could form a majority with a newly added peer. Resulting in a 4 node membership, now requiring a quorum of 3. A subsequent peer addition would also succeed, using with the help of the two newly added nodes. Resulting in a 5 node membership with 3 functioning nodes on one side of the partition, and the old followers forming a different majority on the other side of the partition. The fix ensures that a node adds a new peer and adjusts its quorum calculation at the same time, right after appending the corresponding EntryAddPeer to its WAL. This is in line with the membership change algorithm given by Raft's original author (section 4.1 of Ongaro's thesis). With the revised logic, a partitioned leader of a 3 node cluster is unable to form a majority using the acknowledgement from the to be added peer. As the quorum now requires at least one acknowledgement coming from the original followers. Signed-off-by: Daniele Sciascia <daniele@nats.io>
Signed-off-by: Daniele Sciascia <daniele@nats.io>
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This PR makes the following changes:
Signed-off-by: Daniele Sciascia daniele@nats.io