When I set permissions to the rabbitmq user, there is output the vhost:
[root@ha-node1 my.cnf.d]# rabbitmqctl set_permissions openstack ".*" ".*" ".*"
Setting permissions for user "openstack" in vhost "/" ...
What is the meaning of the vhost when I set permission, and what function does it have?
Let me say this by giving you an analogy.
Vhosts are to Rabbit what virtual machines are to physical servers: Vhosts allow you to run data for multiple applications safely and securely by providing logical separation between instances.
This is useful for anything from separating multiple customers on the same Rabbit to avoiding naming collisions on queues and exchanges. Where otherwise you might have to run multiple Rabbits
Every RabbitMQ server has a ability to create virtual message brokers called virtual hosts (vhosts). Each one is essentially a mini-RabbitMQ server with its own queues, exchanges, and bindings ... etc, more important, with its own permissions.
For details information ref: https://livebook.manning.com/book/rabbitmq-in-action/chapter-2/