"Slack is a web application that is hosted in the cloud, and regulated industries such as banking often need to run their applications on their own on-prem infrastructure.
Mattermost is an open source alternative to Slack that can be self-hosted."
Mattermost @ GitHub
GoLang + React + Electron, with PostgreSQL or MySQL DB
+ React Native for mobile
GoLang + React + Electron, with PostgreSQL or MySQL DB
+ React Native for mobile