Design an Online Messaging App like WhatsApp or Slack

Functional Requirements:

• Users can register and log in to the chat application.
• Users can have one-on-one chats with other users (send and receive messages).
• Users can create group chats, add/remove members, and send messages to groups.
• Users can view chat history for both individual and group chats.
• Messages are stored and retrievable for up to 1 year.

Non-Functional Requirements:

• The system should support both web and mobile clients.
• Should be scalable to handle up to 100 million daily active users.
• Message delivery latency should be low (ideally under 300ms for most users).
• High availability: the system should be resilient to server or network failures.
• Reasonable consistency: messages should appear in order for users, but strict ACID guarantees are not required (eventual consistency is acceptable within 1 second).
• Basic security: user authentication and authorization for chat access.
• The system should handle common failure scenarios, such as network interruptions, gracefully.