Chat and real-time messaging

WebSocket send    { convId, clientMsgId, body }   ack { seq }
WebSocket receive { convId, seq, from, body }
GET /conversations/{id}/messages?afterSeq=...    (sync after reconnect)

messages(conv_id, seq, sender_id, body, created_at)  PRIMARY KEY (conv_id, seq)
presence: user to gateway server, in Redis with a heartbeat TTL

Per conversation, not globally. One sequence counter per conversation (Redis INCR, or a single owner per conversation). Clients sort by seq.

The client sends a clientMsgId and the server dedupes on it, so resending after a lost ack doesn't post twice.

Look up the recipient's gateway in the presence map and forward via pub/sub. If they are offline, store it and send a push notification.

The client sends the last seq it saw and gets everything after it. A gap in seq numbers tells the client something is missing.

Copy// The server stamps a per-conversation sequence number. Clients may get messages late, twice, or out of order
// (reconnects, retries), so they buffer and show them in seq order.
class InOrderInbox {
  constructor() {
    this.next = 1;
    this.buf = new Map();
  }
  receive(msg) {
    if (msg.seq < this.next || this.buf.has(msg.seq)) return []; // duplicate
    this.buf.set(msg.seq, msg);
    const ready = [];
    while (this.buf.has(this.next)) {
      ready.push(this.buf.get(this.next));
      this.buf.delete(this.next++);
    }
    return ready; // show these now, in order
  }
  gap() {
    // the range to fetch from the server: GET /messages?afterSeq=next-1
    return this.buf.size ? [this.next, Math.min(...this.buf.keys()) - 1] : null;
  }
}

• WebSockets for real time, long polling as a fallback.
• Order per conversation is cheap. Global order is expensive and nobody needs it.