WhatsApp

How this question breaks down

1. 1PresenceCore25 minKeep one long-lived encrypted connection open from every online device, remember which gateway holds it, and show "online", "typing" and "last seen" only to the people looking.
2. 2DeliveryCore27 minTake a message from one phone to every device of the recipient exactly once and in order, and turn each step into a tick: one grey for stored, two grey for delivered, blue for read.
3. 3GroupsCore24 minSend one encrypted copy to the server and let it fan out to up to 1,024 members' devices, in one order, with membership changes that lock leavers out and receipts that don't flood the sender.
4. 4OfflineCore30 minHold every envelope for an absent device for up to 30 days, wake it with a push that is only a doorbell, and on reconnect drain the backlog in order, in acked batches, without losing or doubling anything.

Work through the parts in order and you’ve covered the full design. Each part is one topic.