Mattermost: open source vs Slack

Mattermost: Self-Hosted Slack Alternative with Corey Hulen - Software Engineering Daily

"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: Open Source, Private Cloud Slack Alternative

Mattermost @ GitHub

GoLang + React + Electron, with PostgreSQL or MySQL DB
+ React Native for mobile

Mattermost - Wikipedia