WhatsApp guarantees message delivery with acknowledgments:
Message states:
- Sent (single check): Server received
- Delivered (double check): Recipient device received
- Read (blue checks): Recipient opened chat
Delivery flow: Client sends message to server Server stores and acknowledges Server pushes to recipient (or queues if offline) Recipient acknowledges receipt Server notifies sender of delivery
Offline handling: Messages queue on server. Delivered when recipient reconnects. Queue has size and time limits.
Ordering: Sequence numbers ensure messages display in order.