polling vs long-polling vs push: пример кода

// SHORT-POLL — fine for "eventually" data (build status, low-stakes counters)
setInterval(async () => {
  const r = await fetch('/api/status');
  render(await r.json());
}, 5000); // ← latency you're accepting: up to 5s

// LONG-POLL — near-real-time with only fetch(). Note the cursor + immediate re-arm.
async function subscribe(cursor = 0) {
  while (true) {
    try {
      const r = await fetch(`/api/updates?since=${cursor}`); // server HOLDS this
      if (r.status === 204) continue;          // server timed out → re-poll, no gap
      const batch = await r.json();
      for (const ev of batch.events) apply(ev);
      cursor = batch.cursor;                    // advance so we never miss/replay
    } catch {
      await new Promise((res) => setTimeout(res, 1000)); // backoff on network error
    }
  }
}

// server (long-poll) — do NOT answer until there's news or a timeout
app.get('/api/updates', async (req, res) => {
  const since = Number(req.query.since);
  const events = await waitForEventsSince(since, { timeoutMs: 30000 });
  if (!events.length) return res.status(204).end();   // let client re-arm
  res.json({ events, cursor: events.at(-1).id });
});