Design a Real-Time Collaborative Editor

Functional Requirements:

• Multiple users can edit the same document at the same time, with changes reflected in real time.
• Users can create, edit, and delete documents.
• Users can see who else is currently viewing or editing a document.
• Basic text formatting (bold, italic, underline, etc.) should be supported.
• Users can add comments to documents.
• Each document maintains a version history, allowing users to view or revert to previous versions.
• User authentication is required to access and edit documents.
• Access control: document owners can share documents and set permissions (view/edit/comment).

Non-Functional Requirements:

• Low latency for real-time collaboration (edits should be visible to others within 200ms).
• High availability: system should target at least 99.9% uptime.
• Scalability: support up to 1 million daily active users.
• Data consistency: ensure all users see the same document state after edits.
• Security: all data in transit and at rest should be encrypted; only authorized users can access documents.
• Auditability: log all document changes for troubleshooting and compliance.
• Reasonable performance for documents up to 1MB in size and up to 50 concurrent editors per document.