<aside> 💁🏻‍♂️ Solution A → http://blog.gainlo.co/index.php/2016/04/19/design-facebook-chat-function/ (kinda sucks) Solution B → https://eileen-code4fun.medium.com/design-facebook-messenger-438d76639985

</aside>

1. Outline problem, use cases and constraints

Gather requirements and scope the problem.

Design a system very much like Facebook messenger.

Use cases

• User can see who in contacts list is online/offline (if other users make that visible)
• User can send messages to any contact and recipient can get immediately notified
  • messages displayed in chronological order
• User can receive messages from any contact in near real-time
• User can participate in group chats

Out of scope

• Searching through a chat history
• Read reciepts
• User sees ‘now typing’ notifications for other chat participants
• User deletes messages
• User sends image or video or GIFs
• User authentication/authorization

Constraints and assumptions

• Traffic is not evenly distributed. Some users are heavy users, and some are basically churned.
  • Heavy users may send 5,000-10,000 messages a day. Light users 10 or fewer.
• 2,000,000,000 - 2 billion users
  • Say 1,000,000,000 active users. 50%.
• Average messages sent per day 100