In a messenger app, what does the client have to guarantee so messages never get lost? Cover acks and resends in both directions, ordering, and deduplication.
The sender stores the message locally first (sending) under a client-generated id and resends from the outbox until a server ack (which assigns a server seq) arrives, while the server dedups by client id. The receiver checks that per-conversation server seqs are contiguous, pulls to fill any gap, and sends a receive ack.
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