балансировка нагрузки (вид клиента): пример кода

// The client-side reality of "any request can hit any server":
// design STATELESS. Never assume server memory between requests.

// ❌ Assumes the upload and the "commit" hit the same server's local temp dir.
await fetch('/upload/chunk', { method: 'POST', body: chunk }); // → server A's disk
await fetch('/upload/commit'); // → maybe server B: chunk isn't there. Fails under LB.

// ✅ Stateless: every request carries what it needs; state lives in shared storage.
const id = await startUpload();                       // returns an id backed by S3/Redis
await fetch(`/upload/${id}/chunk`, { method: 'POST', body: chunk }); // any server
await fetch(`/upload/${id}/commit`);                  // any server — state is shared

// Real-time: a WebSocket lives on ONE server, so the LB must be sticky for it
// AND the client must be able to reconnect (it may land on a different server).
function connect() {
  const ws = new WebSocket('wss://api.app.com/live');
  ws.onclose = () => setTimeout(connect, backoff()); // deploy drains → reconnect elsewhere
  return ws;
}