Backpressure Handling Prevents WebSocket Server Memory Exhaustion

A WebSocket server broadcasting high-frequency data to a slow client buffers unsent messages in memory. With hundreds of slow clients, the server runs out of memory and crashes.

Check ws.bufferedAmount before sending. If the send buffer is too large, drop the message or close the slow client rather than queueing more data.

import { WebSocketServer, WebSocket } from 'ws';

const MAX_BUFFER = 1024 * 1024; // 1MB

function broadcast(wss: WebSocketServer, data: string) {
  for (const client of wss.clients) {
    if (client.readyState !== WebSocket.OPEN) continue;

    // Drop message for slow clients rather than buffering
    if ((client as any).bufferedAmount > MAX_BUFFER) {
      console.warn('Dropping message for slow client');
      continue;
    }

    client.send(data);
  }
}

// Alternative: close clients that fall too far behind
function strictBroadcast(wss: WebSocketServer, data: string) {
  for (const client of wss.clients) {
    if ((client as any).bufferedAmount > MAX_BUFFER) {
      client.close(1008, 'Too slow');
      continue;
    }
    if (client.readyState === WebSocket.OPEN) client.send(data);
  }
}

TCP's send buffer provides flow control within a connection, but the application layer can still queue data faster than TCP drains it. The ws library buffers unsent frames in JavaScript heap memory with no built-in limit.