NestJS Enterprise Backend APIs · 课时

使用 Redis 发布/订阅适配器扩展实时能力

使用 Redis IoAdapter 在多个实例之间同步 Socket 状态，实现水平扩展。

第 4 / 4 课13 个步骤

使用 Redis 发布/订阅适配器扩展实时能力 是 CoddyKit 上的免费 NestJS Enterprise Backend APIs 课时。这是第 4 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 NestJS Enterprise Backend APIs 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 NestJS Enterprise Backend APIs 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

The Multi-Instance Problem

A single NestJS WebSocket gateway keeps every connected socket in the memory of one Node process. The moment you scale horizontally behind a load balancer, that assumption breaks.

Client A connects to instance 1.
Client B connects to instance 2.
When instance 1 emits to a room, instance 2 never hears about it — so Client B misses the message.

To fix this we need a shared message bus that lets every instance broadcast events to every other instance. Redis Pub/Sub is the canonical choice, and Socket.IO ships an adapter for exactly this.

How the Redis Adapter Works

The @socket.io/redis-adapter replaces Socket.IO's in-memory adapter. Whenever your code calls server.to(room).emit(...), the adapter publishes that event to a Redis channel instead of only delivering it locally.

Every instance subscribes to the same Redis channels.
An emit on instance 1 is published to Redis, then every subscribed instance (including instance 2) receives it and delivers it to its own local sockets.
Redis is only a relay — socket connections still live on each node; no socket state is stored in Redis.

This means room membership, broadcasts, and even server.emit() fan out correctly across the whole cluster.

Installing the Pieces

You need three packages: a Redis client (ioredis or the official redis client), the Socket.IO Redis adapter, and Socket.IO itself (already pulled in by @nestjs/platform-socket.io).

The adapter needs two Redis connections: one pubClient for publishing and one subClient for subscribing. A connection in subscribe mode cannot issue normal commands, which is why they must be separate.

// package.json dependencies (excerpt)
{
  "dependencies": {
    "@nestjs/platform-socket.io": "^11.0.0",
    "@nestjs/websockets": "^11.0.0",
    "@socket.io/redis-adapter": "^8.3.0",
    "socket.io": "^4.7.0",
    "ioredis": "^5.4.0"
  }
}

A Custom IoAdapter

NestJS wraps Socket.IO behind an IoAdapter class. To inject the Redis adapter, you subclass IoAdapter and override createIOServer so that every namespace server is given the Redis adapter via server.adapter(...).

connectToRedis() creates the duplicated pub/sub clients once at bootstrap.
createIOServer() attaches the adapter created by createAdapter(pubClient, subClient).

This is the central piece of the whole lesson.

// redis-io.adapter.ts
import { IoAdapter } from '@nestjs/platform-socket.io';
import { ServerOptions } from 'socket.io';
import { createAdapter } from '@socket.io/redis-adapter';
import { Redis } from 'ioredis';

export class RedisIoAdapter extends IoAdapter {
  private adapterConstructor: ReturnType<typeof createAdapter>;

  async connectToRedis(url: string): Promise<void> {
    const pubClient = new Redis(url);
    const subClient = pubClient.duplicate();
    this.adapterConstructor = createAdapter(pubClient, subClient);
  }

  createIOServer(port: number, options?: ServerOptions): any {
    const server = super.createIOServer(port, options);
    server.adapter(this.adapterConstructor);
    return server;
  }
}

Wiring It at Bootstrap

The adapter must be connected to Redis before the app starts listening, and registered with app.useWebSocketAdapter(). Do this in main.ts.

Instantiate RedisIoAdapter with the app instance.
await the Redis connection so a failure aborts startup cleanly.
Register it, then call app.listen().

// main.ts
import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { RedisIoAdapter } from './redis-io.adapter';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);

  const redisIoAdapter = new RedisIoAdapter(app);
  await redisIoAdapter.connectToRedis(
    process.env.REDIS_URL ?? 'redis://localhost:6379',
  );
  app.useWebSocketAdapter(redisIoAdapter);

  await app.listen(3000);
}
bootstrap();

The Gateway Stays Unchanged

The beauty of this approach: your @WebSocketGateway code does not change at all. You keep using server.to(room).emit() and the adapter transparently fans it out across instances.

Join a room with client.join(room) as usual.
Broadcast with this.server.to(room).emit(event, payload).

The same gateway now works whether you run 1 or 50 replicas.

// chat.gateway.ts
import {
  WebSocketGateway,
  WebSocketServer,
  SubscribeMessage,
  MessageBody,
  ConnectedSocket,
} from '@nestjs/websockets';
import { Server, Socket } from 'socket.io';

@WebSocketGateway({ cors: { origin: '*' } })
export class ChatGateway {
  @WebSocketServer() server: Server;

  @SubscribeMessage('joinRoom')
  onJoin(@ConnectedSocket() client: Socket, @MessageBody() room: string) {
    client.join(room);
    return { joined: room };
  }

  @SubscribeMessage('sendMessage')
  onMessage(@MessageBody() data: { room: string; text: string }) {
    // Fans out to every instance thanks to the Redis adapter
    this.server.to(data.room).emit('message', data.text);
  }
}

Sticky Sessions vs. Polling

The Redis adapter fixes broadcasting, but it does not fix the HTTP long-polling handshake. With multiple instances, Socket.IO's polling transport sends several HTTP requests during the upgrade; if those requests hit different instances, the handshake fails.

Option A: Enable sticky sessions on the load balancer so a client always reaches the same instance.
Option B: Force the WebSocket transport only, skipping polling entirely.

Most production setups enable sticky sessions at the ingress/load balancer layer.

// Force WebSocket-only to sidestep multi-request polling handshakes
@WebSocketGateway({
  transports: ['websocket'],
  cors: { origin: '*' },
})
export class ChatGateway {}

Graceful Reconnection of the Bus

If Redis goes down, the adapter cannot relay events between instances. With ioredis you get automatic reconnection out of the box, but you should log and observe failures so you know broadcasts are degraded.

Attach listeners to both pub and sub clients.
A retryStrategy lets you cap backoff and avoid hammering Redis.

// redis-io.adapter.ts (hardened connect)
async connectToRedis(url: string): Promise<void> {
  const pubClient = new Redis(url, {
    retryStrategy: (times) => Math.min(times * 100, 3000),
  });
  const subClient = pubClient.duplicate();

  for (const client of [pubClient, subClient]) {
    client.on('error', (err) => console.error('Redis adapter error', err));
    client.on('reconnecting', () => console.warn('Redis adapter reconnecting'));
  }

  this.adapterConstructor = createAdapter(pubClient, subClient);
}

Reasoning About Fan-Out Cost

Every cross-instance emit becomes Redis traffic. Understanding the fan-out helps you size Redis. A pure standalone calculation makes the scaling intuition concrete: if each broadcast must reach N instances, Redis relays roughly one publish that N−1 other instances consume.

The snippet below is a plain TypeScript program estimating published messages per second across a cluster — no framework or Redis needed to run it.

// Estimate Redis pub/sub relay load for a cluster
function relayLoad(
  instances: number,
  broadcastsPerSecPerInstance: number,
): { publishes: number; deliveries: number } {
  const publishes = instances * broadcastsPerSecPerInstance;
  // each publish is consumed by the other (instances - 1) nodes
  const deliveries = publishes * (instances - 1);
  return { publishes, deliveries };
}

const scenarios = [
  { instances: 2, rate: 100 },
  { instances: 10, rate: 100 },
  { instances: 50, rate: 100 },
];

for (const s of scenarios) {
  const { publishes, deliveries } = relayLoad(s.instances, s.rate);
  console.log(
    `${s.instances} instances -> ${publishes} publishes/s, ${deliveries} deliveries/s`,
  );
}

Targeting Specific Sockets Across Nodes

Beyond rooms, the Redis adapter also supports cluster-wide operations Socket.IO exposes on the server object:

server.fetchSockets() returns socket info from all instances.
server.in(socketId).disconnectSockets() can disconnect a socket living on another node.
server.socketsJoin(room) / socketsLeave(room) move sockets cluster-wide.

These are async because they round-trip through Redis to reach remote instances.

// admin.gateway.ts
@SubscribeMessage('kick')
async onKick(@MessageBody() socketId: string) {
  // Works even if that socket is connected to another instance
  const remoteSockets = await this.server.in(socketId).fetchSockets();
  for (const s of remoteSockets) {
    s.emit('kicked', { reason: 'admin action' });
    s.disconnect(true);
  }
}

Redis Pub/Sub vs. Streams Adapter

Socket.IO offers two Redis-based adapters with different delivery guarantees:

Pub/Sub adapter (@socket.io/redis-adapter): fire-and-forget. If an instance is briefly disconnected from Redis, it misses messages during that window. Lowest latency, simplest.
Streams adapter (@socket.io/redis-streams-adapter): uses Redis Streams so a reconnecting instance can replay missed messages. Slightly more overhead but better for at-least-once needs.

For typical chat/notification fan-out, the Pub/Sub adapter is the default recommendation. Choose Streams when missed broadcasts during a blip are unacceptable.

Quick Check: Why Two Clients?

The Redis adapter requires a separate pubClient and subClient. Why can't you reuse a single Redis connection for both?

在交互式课程中回答

Recap

You learned how to scale a NestJS realtime app horizontally with a Redis Pub/Sub adapter:

In-memory Socket.IO state does not span instances — Redis Pub/Sub relays broadcasts across the cluster.
Subclass IoAdapter, connect a pubClient and subClient, and attach createAdapter(...) in createIOServer.
Register it in main.ts with app.useWebSocketAdapter() before listen(); your gateway code stays unchanged.
Enable sticky sessions (or force the WebSocket transport) so the polling handshake survives load balancing.
Cluster-wide ops like fetchSockets() and disconnectSockets() round-trip through Redis.
Choose the Streams adapter instead when you need replay of broadcasts missed during a Redis blip.

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常见问题解答

「使用 Redis 发布/订阅适配器扩展实时能力」课时是免费的吗？

是的 — 「使用 Redis 发布/订阅适配器扩展实时能力」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 NestJS Enterprise Backend APIs 课程的其余内容，请升级到 CoddyKit PRO。 NestJS Enterprise Backend APIs 课程共包含 4 节课。

「使用 Redis 发布/订阅适配器扩展实时能力」这节课中我会学到什么？

使用 Redis IoAdapter 在多个实例之间同步 Socket 状态，实现水平扩展。你通过在浏览器中直接运行的动手代码来练习 NestJS Enterprise Backend APIs，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 NestJS Enterprise Backend APIs 需要有经验吗？

无需任何先前经验。CoddyKit 上的 NestJS Enterprise Backend APIs 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 4 节课，共 4 节。

「使用 Redis 发布/订阅适配器扩展实时能力」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

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