VerneMQ provides multiple hooks throughout the lifetime of a session. The most important ones are the auth_on_register and auth_on_register_m5 hooks which act as an application level firewall granting or rejecting new clients.
The auth_on_register and auth_on_register_m5 hooks allow your plugin to grant or reject new client connections. Moreover it lets you exert fine grained control over the configuration of the client session. The auth_on_register hook is specified in the Erlang behaviour auth_on_register_hook and the auth_on_register_m5 hook in the auth_on_register_m5_hook behaviour available in the vernemq_dev repo.
Every plugin that implements the auth_on_register or auth_on_register_m5 hooks are part of a conditional plugin chain. For this reason we allow the hook to return different values depending on how the plugin grants or rejects this client. In case the plugin doesn't know the client it is best to return next as this would allow subsequent plugins in the chain to validate this client. If no plugin is able to validate the client it gets automatically rejected.
The on_auth_m5 hook allows your plugin to implement MQTT enhanced authentication, see Enhanced Authentication Flow.
The on_register and on_register_m5 hooks allow your plugin to get informed about a newly authenticated client. The hook is specified in the Erlang behaviour on_register_hook and the on_register_m5_hook behaviour available in the vernemq_dev repo.
Once a new client was successfully authenticated and the above described hooks have been called, the client attaches to its queue. If it is a returning client using clean_session=false or if the client had previous sessions in the cluster, this process could take a while. (As offline messages are migrated to a new node, existing sessions are disconnected). The on_client_wakeup hook is called at the point where a queue has been successfully instantiated, possible offline messages migrated, and potential duplicate sessions have been disconnected. In other words: when the client has reached a completely initialized, normal state for accepting messages. The hook is specified in the Erlang behaviour on_client_wakeup_hook available in the vernemq_dev repo.
This hook is called if a client using clean_session=false closes the connection or gets disconnected by a duplicate client. The hook is specified in the Erlang behaviour on_client_offline_hook available in the vernemq_dev repo.
This hook is called if a client using clean_session=true closes the connection or gets disconnected by a duplicate client. The hook is specified in the Erlang behaviour on_client_gone_hook available in the vernemq_dev repo.