Harbor Chat needs durable messaging for 30,000 connected devices. At peak, 12,000 senders each send once every 20 seconds. Deliver a retry-safe send API, ordered history model and socket delivery board. Prove that a lost acknowledgement doesn't duplicate a message and that an offline phone can recover its missing sequence range.
Build the delivery path for Harbor Chat, a fictional team messenger. A sender may lose its connection after the server stores a message but before the acknowledgement reaches the phone. The phone must retry safely. A person reconnecting after an hour needs the missed messages in conversation order, not only whatever arrives on the live socket.
Keep persistence separate from live delivery.
Durable conversation history supplies recovery; a socket gateway gives low-latency updates to connected devices. Define order within a conversation and use a stable client message ID to recognize retries. You'll build the API, model and board in checked steps, then choose how an offline device catches up. Presence can be approximate. Committed message history can't rely on presence being correct.