Recover the world, too

A resumed connection keeps the player. A fresh usable snapshot makes the world ready to play again.

Two different boundaries

GameClient.onSession reports a new or resumed logical session. Same-session event sequence numbers and the lobby seat survive a temporary disconnect. The browser retries RTC after a failed attempt and can keep using WebSocket while it does so.

GameClient.receiveState owns snapshot recovery. Disconnect invalidates readiness. A welcome resets game history by default; only an accepted, sufficiently fresh snapshot from this connection makes the receiver ready. Reliable events and outgoing input continue independently of that playback gate.

const receiver = net.receiveState({
  stream: 0,
  reset: () => { history.clear(); deltaBases.clear(); },
  apply: state => {
    const snapshot = decodeWithBase(state.data, deltaBases);
    if (!snapshot) return false;  // malformed data or a missing base cannot release recovery
    history.push(snapshot);
    return true;
  },
});

function frame() {
  if (receiver.ready) draw(history.sample(net.clock.serverTime(performance.now())));
  else drawReconnecting();
}

The decoder, world history and draw functions above are your game. The runnable starter contains a complete implementation using JSON snapshots.

Grouped snapshots

For a world split across streams, omit stream, reject unrelated or incomplete groups in apply, and return true only after the snapshot group your game requires is usable. One nearby entity packet cannot prove an entire world is complete. Keep each group and its pending parts bounded.

Retaining history

Use resume: 'retain' only when your same-session interpolation and delta histories remain valid. Readiness still waits for fresh state. A new session always resets histories. A lost session requires an explicit newSession(); pending actions from the old player must not become actions by a new one.

Checkpointing reloads

Call checkpoint(), store it in sessionStorage or another private per-player store, and restore it into a new GameClient with the exact same URL and namespace. Bind page lifecycle with unload: 'pause' so reloading keeps the seat during the grace period. Close or leave deliberately when the player chooses to exit.

Verify your game

Test a short network interruption, page reload, background/foreground transition, blocked datagrams, repeated RTC failure and reconnect after the grace period. Assert player ID, room, event counts and fresh gameplay snapshots—not just an open socket. Sessions demo · Client API