Collaborative Document Editor

Real-time collaborative editing — where multiple users see each other's changes instantly without conflicts — is one of the most challenging distributed systems problems because it requires resolving concurrent edits while maintaining document consistency and low-latency feedback. In this challenge, you will design a collaborative document editor on AWS that supports Google Docs-style real-time collaboration with cursor presence, change tracking, version history, and offline editing support. The core consistency algorithm uses Conflict-free Replicated Data Types (CRDTs) — specifically, an RGA (Replicated Growable Array) for text content that guarantees eventual consistency without a central coordination server. Each character insertion includes a unique ID (combining client ID and logical timestamp) that provides a total order for conflict resolution. The real-time synchronization layer uses API Gateway WebSocket APIs for persistent connections, with Lambda functions that receive operations from clients, validate them, apply them to the server-side document state in DynamoDB, and broadcast the transformed operations to all other connected clients. The document state is stored as a series of operations (operational log) in DynamoDB with a compacted snapshot stored in S3 at regular intervals — new clients load the latest snapshot and replay subsequent operations to catch up. Cursor presence uses a lightweight heartbeat protocol through the WebSocket connection, with ElastiCache storing cursor positions per user per document with 5-second TTL. Offline editing support works by queuing operations locally on the client and replaying them through the CRDT merge algorithm when connectivity is restored — the CRDT properties guarantee that the result is consistent regardless of the order operations arrive. Version history uses DynamoDB Streams to capture every operation, with a Lambda function that creates named snapshots (versions) on demand and stores them in S3. Access control supports document-level permissions (owner, editor, viewer) stored in DynamoDB with real-time enforcement at the WebSocket authorization layer. This challenge teaches CRDT-based consistency, real-time synchronization protocols, and the architecture patterns behind modern collaborative applications.