Hi
VerneMQ comes with file based authentication and authorization out of the box. Here are links to the docs: http://vernemq.com/docs/configuration/authentication.html & http://vernemq.com/docs/configuration/authorization.html. If this is not enough (and it rarely is for large scale systems), VerneMQ can be extended through it's plugin mechanisms: plain erlang, lua scripts and webhooks.
To get the currently connected clients you can use the vmq-admin session list
command, which will display all current cilent sessions (use with care as it doesn't support paging at the moment).
We're working on introducing an expressive query language (SQL-like) to inspect the state of the broker/cluster.
We plan to make SQL integration a first class citizen of VerneMQ, so it 'just works' out of the box with minimal configuration. For now we indirectly support a bunch of databases (Mongo, Redis, Postgres, Mysql a.o.) via the vmq_diversity
plugin: https://github.com/erlio/vmq_diversity. This plugin enables you to write authentication and authorization logic in Lua and provides a driver for the mentioned database.
So unfortunately for now it's not an 'out-of-the-box' experience.
If you're not afraid of trying out and writing Erlang code, you can also write extension using that. Plugins written in Erlang have the added benefit of being more efficient as VerneMQ is implemented in Erlang. The docs for that and how the plugin system works can be found here: http://vernemq.com/docs/plugindevelopment/