mattermost/mattermost

Mattermost is an open-source, self-hosted team collaboration platform, essentially an alternative to Slack or Microsoft Teams that organizations can run entirely on their own infrastructure. The core problem it solves is the tension between modern team communication tools (real-time chat, file sharing, workflow automation) and enterprise requirements for data control, security, and compliance. Commercial SaaS messaging platforms store all your conversations on their servers, Mattermost lets you keep everything inside your own network. The platform provides persistent team channels, direct messages, threaded conversations, voice calling, screen sharing, file sharing, and search. It has an app marketplace with over 700 integrations, a webhook and bot API for automation, and built-in support for AI integrations. The web interface is built in React, and native clients exist for iOS, Android, Windows, macOS, and Linux. The server is written in Go and ships as a single Linux binary, which makes deployment straightforward: you run the binary, point it at a PostgreSQL database, and it is running. Docker, Kubernetes, Helm, and various Linux package install paths are all supported. The codebase is a monorepo that includes both the server and the web client. New releases follow a monthly schedule, releasing on the 16th under an MIT license for the core platform. You would deploy Mattermost if your organization handles sensitive information (government, healthcare, finance, defense) and cannot use cloud SaaS messaging, or if you need deep customization, custom workflows, compliance integrations, or specialized automations, that commercial tools don't support. The tech stack is Go for the backend server, TypeScript and React for the web client, and PostgreSQL for the database.