Improve event stream resilience

packages/web/server/lib/event-stream/protocol.js

@@ -1,6 +1,9 @@
 export const MESSAGE_STREAM_GLOBAL_WS_PATH = '/api/global/event/ws';
 export const MESSAGE_STREAM_DIRECTORY_WS_PATH = '/api/event/ws';
 export const MESSAGE_STREAM_WS_HEARTBEAT_INTERVAL_MS = 15 * 1000;
+// Per-client pending outbound WS buffer, not a payload or stream-size limit.
+// Healthy clients stay near 0; this only trips when a client is far behind.
+export const MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES = 4 * 1024 * 1024;
 
 export function parseSseEventEnvelope(block) {
   if (!block || typeof block !== 'string') {
@@ -64,8 +67,23 @@ export function sendMessageStreamWsFrame(socket, payload) {
     return false;
   }
 
+  if (typeof socket.bufferedAmount === 'number' && socket.bufferedAmount > MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES) {
+    try {
+      socket.close(1013, 'Message stream client is too slow');
+    } catch {
+    }
+    return false;
+  }
+
   try {
     socket.send(JSON.stringify(payload));
+    if (typeof socket.bufferedAmount === 'number' && socket.bufferedAmount > MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES) {
+      try {
+        socket.close(1013, 'Message stream client is too slow');
+      } catch {
+      }
+      return false;
+    }
     return true;
   } catch {
     return false;

packages/web/server/lib/event-stream/protocol.test.js

@@ -3,6 +3,7 @@ import { describe, expect, it } from 'vitest';
 import {
   MESSAGE_STREAM_DIRECTORY_WS_PATH,
   MESSAGE_STREAM_GLOBAL_WS_PATH,
+  MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES,
   parseSseEventEnvelope,
   sendMessageStreamWsEvent,
   sendMessageStreamWsFrame,
@@ -63,6 +64,30 @@ describe('event stream protocol helpers', () => {
     expect(rawPayload).toBe('{"type":"ready"}');
   });
 
+  it('closes slow websocket clients instead of adding more buffered data', () => {
+    let closeCall = null;
+    let sendCalls = 0;
+    const socket = {
+      readyState: 1,
+      bufferedAmount: MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES + 1,
+      send() {
+        sendCalls += 1;
+      },
+      close(code, reason) {
+        closeCall = { code, reason };
+      },
+    };
+
+    const sent = sendMessageStreamWsFrame(socket, { type: 'ready' });
+
+    expect(sent).toBe(false);
+    expect(sendCalls).toBe(0);
+    expect(closeCall).toEqual({
+      code: 1013,
+      reason: 'Message stream client is too slow',
+    });
+  });
+
   it('serializes event frames with routing metadata', () => {
     let rawPayload = null;
     const socket = {