Java Recommendation Engine: Design & Code

You are a senior backend engineer. Design and implement a complete, scalable recommendation engine using Java + Spring + WebSocket.

ARCHITECTURE REQUIREMENTS:
- Technology: Java + Spring + WebSocket
- Features: presence tracking
- Scale: petabyte-scale data

IMPLEMENTATION REQUIREMENTS:
1. Complete system architecture with diagrams
2. API design (REST/GraphQL/gRPC)
3. Real-time communication setup
4. Database design (SQL/NoSQL/hybrid)
5. Caching strategy (Redis/Memcached)
6. Message queue implementation
7. Authentication and authorization
8. Rate limiting and DDoS protection
9. Monitoring and alerting
10. Load testing and optimization
11. Deployment strategy (Docker, Kubernetes)
12. Disaster recovery plan

DELIVERABLES:
- Complete backend codebase
- API documentation
- Database schemas
- Infrastructure as Code (Terraform/CloudFormation)
- Docker/Kubernetes configs
- Monitoring dashboards
- Load testing scripts
- Architecture documentation

Generate a production-ready, scalable system with all components, documentation, and best practices.

ENHANCEMENT: Add real-world examples and case studies.

SCOPE: Include both MVP and full-featured versions. [Ref: ee9fd0c9]