Multiplayer State Sync Needs a Single Source of Truth for Initial Load

New users joining a multiplayer session see an empty or stale state because they connect to the WebSocket server after the initial state was broadcast. They miss all previous events.

Maintain authoritative state on the server (or in a CRDT document). When a new client connects, send the full current state as a snapshot before streaming live events. Use a sequence number to prevent gaps.

// Server
wss.on('connection', (ws) => {
  // Send current snapshot immediately
  ws.send(JSON.stringify({
    type: 'snapshot',
    state: room.currentState,
    seq: room.lastSeq
  }));

  // Then stream live events from that seq forward
  ws.on('message', handleMessage);
  room.addSubscriber(ws);
});

// Client
ws.onmessage = (event) => {
  const msg = JSON.parse(event.data);
  if (msg.type === 'snapshot') {
    store.setState(msg.state);
    store.setSeq(msg.seq);
  } else if (msg.type === 'event' && msg.seq === store.seq + 1) {
    store.applyEvent(msg.event);
    store.setSeq(msg.seq);
  }
};

WebSocket is a live stream — it does not replay history. Without a snapshot handshake, late joiners receive only events that occur after their connection, leaving them in an inconsistent state.