Reconnect & resync

{ "id": 999, "cmd": "ping" }

{ "id": 999, "type": "pong", "ts": 1776949200000 }

let reconnectDelay = 1000;

function connect() {
  // Node / server: pass X-Api-Key as a header.
  // Browser:       use `${WSS}/ws/user?key=<encodeURIComponent(apiKey)>`
  //                because the WebSocket(…) ctor can't set custom headers.
  const ws = new WebSocket(`${WSS}/ws/user`, {
    headers: { 'X-Api-Key': apiKey },
  });

  ws.onopen = () => {
    reconnectDelay = 1000;
    // Always re-subscribe from scratch on a new connection.
    ws.send(JSON.stringify({
      id: nextId(),
      cmd: 'subscribe',
      params: { subscriptions: mySubscriptions },
    }));
  };

  ws.onclose = () => {
    // Drop sid → handler map; on next open you'll re-subscribe and
    // get fresh sids.
    sidHandlers.clear();
    setTimeout(connect, reconnectDelay);
    reconnectDelay = Math.min(reconnectDelay * 2, 30_000);
  };

  ws.onmessage = handleMessage;
}

const resyncing = new Set<string>(); // tokenIds with a snapshot request in flight

if (frame.type === 'book_snapshot') {
  replaceBook(frame.id, frame.data.bids, frame.data.asks);
  lastSeq[frame.id] = frame.seq;
  resyncing.delete(frame.id);
} else if (frame.type === 'book_snapshot_failed') {
  resyncing.delete(frame.id); // back off, then request again
} else if (frame.type === 'book_delta') {
  if (resyncing.has(frame.id)) {
    // Already asked: skip deltas until the snapshot lands.
  } else if (frame.data.prevSeq !== lastSeq[frame.id]) {
    // Gap: ask the gateway to re-push a snapshot for this token on this sid.
    resyncing.add(frame.id);
    ws.send(JSON.stringify({
      id: nextId(),
      cmd: 'get_book_snapshot',
      params: { sid: frame.sid, ids: [frame.id] },
    }));
  } else {
    applyChanges(frame.id, frame.data.changes);
    lastSeq[frame.id] = frame.seq;
  }
}