It must be possible to define a cluster connection without static connectors or discovery-group

I constrained the messaging subsystem to not accept a cluster-connection without either a static-connectors or a discovery-group-name.

However there is one use case where we must accept a cluster-connection without either: when a node must be part of a cluster, accepts connection but not attempts to connect to other nodes. This is shown by HornetQ clustered-static-oneway example where server3 accepts connections from server2 but does not connect to any other cluster nodes.

The constraints must be relaxed to accept a cluster connection without a static connectors or a discovery-group-ref element