There aren't any frameworks or toolkits that you let do so in a scalable, performant way. Most open-source Slack bots out there are meant to be run as single-player mode, but not one to serve hundreds of thousands of teams.
Relax is a Go-based Message Broker for Slack Bots.
What does that mean? If you are running a "bot-as-a-service" for Slack, you have to maintain hundreds (if not thousands) of websocket connections and handle the deluge of events from all these connections.
Relax does all that heavy lifting for you and provides you with a single stream of events that your web app can then take action on. The protocol is JSON based and so any web app can communicate with Relax.
Its benefits are: