System design chat
WhatsApp-style chat
1:1, groups, media, online/offline. Connection layer vs message store.
Week 4Thursday 1 OctoberChat
Day 27: Chat and messaging
0 of 4 tasks completed0%
System design chat
Presence, receipts, groups
Last-seen, delivery vs read, fan-out to group members, ordering per conversation.
Estimation
50M concurrent WebSockets
RAM per connection, boxes, sticky LB, and reconnect storms after a blip.
LLD chat
Message store schema + API
send, history, ack. Partition key = conversation_id. Indexes for unread.