Design WhatsApp or Wechat

Question 1

What questions to ask interviewer?

Answer 1

Ask about features: want service available on phones, 1:1 messaging with just text (baseline)

1) One : One text
2) Sent/received/read
3) Push notifications

Question 2

One to One design/bottleneck

Answer 2

Bob -> Server -> Alice (three components can be asynchronous). Storage optimization = only store in server what is in progress of being delivered

Question 3

Server architecture

Answer 3

Server has number of queues, used to store messages. Once message is received, its removed from queue. Problem is first message goes to server 1 but Bob is offline, second message goes to server 2 (if server 1 queue is full or something) so Bob may get messages in wrong order

Question 4

Sent/received/read feature

Answer 4

Server replies to Alice with “sent” notification. Same with “received” when Bob receives message. Bob could send acknowledgment “message” with ID -> doesn’t matter which server it goes through for “read”

Question 5

Push notifications feature

Answer 5

push notifications are fire / forget. Once server gets ack from Bob that he got push notification, server can delete it. Possibly have separate server just to log push notifications?