Outline is a collaborative knowledge base platform built with React and Node.js. It provides real-time document editing via ProseMirror/Yjs, team workspaces with granular permissions, and extensibility through a plugin system. The application features rich text editing, document organization into collections, search functionality, notifications, and integrations with services like Slack, GitHub, and Notion. Architecture follows a monorepo structure with frontend (React/MobX/Styled Components), backend (Node.js/Koa/Sequelize), and shared utilities. Key entry points include the React root (app/index.tsx), Koa server (server/index.ts), and API routes (server/routes/api/). The system relies on PostgreSQL for primary data storage, Redis for caching and job queues, and S3/local storage for file attachments.