How Slack Supports Billions of Daily Messages

Slack supports billions of daily messages by maintaining over five million simultaneous WebSocket sessions at peak hours, addressing the architectural challenge of real-time interaction. Initially developed from a video game infrastructure, Slack's architecture separates message propagation from business logic and adopted a push-first model using WebSockets. Significant challenges included scaling session initialization for large teams and ensuring message persistence and order through atomic broadcast approximations. Innovations like Flannel and architectural shifts improved the system's resilience and efficiency at scale, embracing complexity where necessary while simplifying other areas.