In the case of NuoDB being hung, how do we get the information about a number of connections and others, because in this situation NuoDB doesn’t accept any SQL query?
Use the "monitor" management command.
The monitor command connects to a broker which queries the other brokers, each of which queries the TE(s) on the local machine.
The NuoDB metric "ClientCncts" outputs the number of client connections;
The NuoDB metric "CurrentActiveTransactions" outputs the number of active transactions;
nuodbmgr --broker <$host> --password <Password> --command "monitor database db1" | grep -E "(ClientCncts|CurrentActiveTransactions)"
More information and examples can be found here.