In a Docker Swarm cluster, if you have three manager nodes, what is the minimum number of managers that must be online for the cluster to maintain its quorum and continue operating?

In a Docker Swarm cluster, manager nodes are responsible for maintaining the desired state of the swarm, scheduling tasks, and providing the control plane for the cluster. To ensure consistency and fault tolerance across these distributed manager nodes, Docker Swarm utilizes the Raft consensus algorithm. A critical aspect of Raft is maintaining "quorum." Quorum refers to the minimum number of voting manager nodes that must be online and in communication with each other for the cluster to function correctly and make consistent decisions. It prevents a 'split-brain' scenario where different parts of the cluster might have conflicting views of the truth. The rule for maintaining quorum dictates that a strict majority of the total manager nodes must be online. This is often expressed as (N / 2) + 1, where N is the total number of manager nodes. For example, if there are five manager nodes, at least three must be online for quorum to be maintained. In the specific case of a Docker Swarm cluster with three manager nodes (N=3), the calculation for the minimum number required for quorum is (3 / 2) + 1. Since we need a strict majority, this means at least two manager nodes must be online. If only one manager node remains online, it no longer constitutes a majority of the original three, and thus the cluster loses quorum. When a Docker Swarm cluster loses quorum, it can no longer elect a leader, process new management commands, or make changes to the cluster state. Existing services might continue to run, but no new services can be deployed, scaled, or updated until quorum is restored by bringing enough manager nodes back online.