WebSocket Engineer

Use a session ID + message sequence number pattern for resumable connections:

Client — reconnection with exponential backoff:

class ReconnectingWebSocket {
  private ws: WebSocket | null = null;
  private sessionId = localStorage.getItem('wsSessionId') ?? crypto.randomUUID();
  private lastSeq = parseInt(localStorage.getItem('wsLastSeq') ?? '0');
  private retryDelay = 1000;
  private maxDelay = 30000;

  connect() {
    this.ws = new WebSocket(`wss://api.example.com/ws?session=${this.sessionId}&since=${this.lastSeq}`);

    this.ws.onmessage = (event) => {
      const msg = JSON.parse(event.data);
      this.lastSeq = msg.seq;
      localStorage.setItem('wsLastSeq', String(msg.seq));
      this.onMessage(msg);
    };

    this.ws.onclose = () => {
      const jitter = Math.random() * this.retryDelay * 0.2;
      setTimeout(() => this.connect(), this.retryDelay + jitter);
      this.retryDelay = Math.min(this.retryDelay * 2, this.maxDelay);
    };

    this.ws.onopen = () => {
      this.retryDelay = 1000; // reset on successful connect
      localStorage.setItem('wsSessionId', this.sessionId);
    };
  }
}

Server — buffer recent messages per session:

// Store last N messages in Redis with a TTL
async function sendMissedMessages(socket: WebSocket, sessionId: string, since: number) {
  const missed = await redis.zrangebyscore(`session:${sessionId}:messages`, since + 1, '+inf');
  missed.forEach(msg => socket.send(msg));
}

Set Redis TTL to match your reconnect window (e.g., 1 hour). For longer outages, fall back to a full state sync rather than individual messages.