What are the available 'bus' types, besides MQTT?

It seems there is an option to to use another central communication technology than MQTT.

I did a search, but couldn’t find an answer.

What are the options?

I found the answer:

hermes has two implementations: one over MQTT and one using an inprocess bus. A standard installation on a RaspberryPi uses the MQTT version whereas the Android and iOs sdks use the inprocess one.

from: https://github.com/snipsco/hermes-protocol

So now my question becomes: is it possible to use the internal bus on the Raspberry Pi?

(MQTT keeps disconnecting Hermes)