Cluster Size

For delivery assurance with fault tolerance, use a minimum cluster size of 3 services, so that a quorum can exist even when one service is unavailable.

For replication, use a minimum cluster size of 3 services.

A cluster of 2 services is invalid, because when one service is unavailable, the remaining service cannot form a quorum on its own. It is an error to configure only 2 services.

For apportioning message streams, a cluster of only 1 service is a valid configuration.

Similarly, a message broker cluster of only 1 service is a valid configuration.

Related concepts