MySQL Cluster Manager uses the XCom (a Paxos variant) as its consensus protocol. Before XCom allows a new message to be delivered, it requires a majority vote by all agents to decide who can send the next message. This majority is referred to as a "quorum." In general, a simple majority of the agents (that is, half of the total number of agents plus one) constitutes a quorum. So, the quorum for a cluster with 4 agents is 3, and for 6 agents is 4, and so on. Here are some special cases:
An agent for a single-host cluster forms a quorum. All messages are automatically delivered.
A 2-host group does not require a simple majority of agents—if full consensus by both agents is not obtained, XCom allows a message to be sent only by the agent on the first host in the host list of the
On top of the quorum requirement described above, for any execution plans, any agent that has work to do in the plan needs to accept the plan before it can be executed.