Design the system architecture for WhatsApp.

Focus on outlining a scalable and reliable architecture that can handle billions of messages and users simultaneously. Start by identifying the key components of WhatsApp, such as message delivery, user authentication, and multimedia handling. Discuss how you would implement a microservices architecture to manage different functionalities, ensuring each service can scale independently. Address performance by considering data storage solutions (e.g., SQL vs. NoSQL), caching strategies, and load balancing. Finally, emphasize reliability through redundancy, failover mechanisms, and data consistency strategies to ensure messages are delivered even in case of server failures.