System design interview question
Design a Real-time Collaborative Document Editor with Conflict Resolution
Design a low-level architecture for a Google Docs-like collaborative editor supporting concurrent multi-user edits, operational transformation (OT), and conflict resolution. Key actors include Users, Documents, EditSessions, and a ConflictResolver. The system must handle: (1) concurrent insert/delete operations from multiple clients with deterministic conflict resolution, (2) undo/redo stacks per user that respect causality, (3) cursor position tracking and synchronization across clients, (4) composable formatting operations (bold, italics, links) that must commute correctly during transformation, and (5) efficient broadcasting of operations to subscribers. Edge cases include: rapid successive edits from same user, network reordering of operations, users joining/leaving mid-session, and transformation of operations against multiple concurrent changes. Consider the Observer pattern for change propagation, Command pattern for operation encapsulation, and Strategy pattern for conflict resolution algorithms.
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