Design a Realtime Chat App like WhatsApp

Design a chat app like WhatsApp.

• A user can send and receive text messages in real-time
• Chat logs are persistent in the cloud
• When the user gets online, she receives all the messages addressed to her during her offline time.
• Once a message is delivered to a user, they will be store on user’s devices and we no longer need to store them on the server (like WhatsApp and WeChat).
• The user shouldn’t see duplicate messages
• [optional] Support group chat

• 100M DAU
• 20 average daily messages per user
• Data retention for 10 years.
• Assuming each message, with meta-data, is roughly 200 bytes.
• Assuming that the average storage duration for messages is 4 months.