TODO

• https://www.youtube.com/watch?v=cSNGGAKJqwk&ab_channel=FOSDEM
• http://highscalability.com/blog/2010/11/16/facebooks-new-real-time-messaging-system-hbase-to-store-135.html
• The Underlying Technology of Messages
• https://vertabelo.com/blog/database-model-for-a-messaging-system/

Functional requirements

1. Messenger should support one-on-one conversations between users.
2. Messenger should keep track of online/offline statuses of its users.
3. Messenger should support persistent storage of chat history.

Non-functional requirements

1. Users should have real-time chat experience with minimum latency.
2. Our system should be highly consistent; users should be able to see the same chat history on all their devices.
3. Messenger’s high availability is desirable; we can tolerate lower availability in the interest of consistency.

Extended requirements

• Group Chats: Messenger should support multiple people talking to each other in a group.
• Push notifications: Messenger should be able to notify users of new messages when they are offline.