NestJS Enterprise Backend APIs · Lesson

WebSocket Gateways with the Socket.IO Adapter

Implement @WebSocketGateway handlers, rooms, and lifecycle hooks for live communication.

WebSocket Gateways with the Socket.IO Adapter is a free NestJS Enterprise Backend APIs lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the NestJS Enterprise Backend APIs learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why Gateways Exist

REST handles request/response, but real-time features (chat, presence, live dashboards) need a persistent, bidirectional channel. NestJS wraps this with a WebSocket Gateway.

A gateway is a provider decorated with @WebSocketGateway().
By default Nest uses the Socket.IO platform adapter (@nestjs/platform-socket.io).
Gateways are normal Nest classes: they support dependency injection, guards, pipes, and interceptors just like controllers.

Install the deps with npm i @nestjs/websockets @nestjs/platform-socket.io socket.io.

Declaring a Gateway

Decorate a class with @WebSocketGateway() and register it as a provider in a module. You can pass a port and options such as namespace and CORS config.

@SubscribeMessage('event') binds a handler to an inbound Socket.IO event.
The return value (or a WsResponse) is emitted back to the calling client.

Below, clients sending ping receive a pong reply.

import { WebSocketGateway, SubscribeMessage, MessageBody } from '@nestjs/websockets';

@WebSocketGateway({ namespace: '/chat', cors: { origin: '*' } })
export class ChatGateway {
  @SubscribeMessage('ping')
  handlePing(@MessageBody() data: string): { event: string; data: string } {
    return { event: 'pong', data: `received: ${data}` };
  }
}

Accessing the Server Instance

To broadcast to many clients you need the underlying Socket.IO Server. Inject it with the @WebSocketServer() property decorator.

server.emit('event', payload) sends to every connected client in the namespace.
This is the foundation for fan-out features like global announcements.

import { WebSocketGateway, WebSocketServer, SubscribeMessage, MessageBody } from '@nestjs/websockets';
import { Server } from 'socket.io';

@WebSocketGateway({ namespace: '/chat' })
export class ChatGateway {
  @WebSocketServer()
  server: Server;

  @SubscribeMessage('broadcast')
  handleBroadcast(@MessageBody() message: string): void {
    this.server.emit('announcement', message);
  }
}

The Connected Socket

Each client has its own Socket. Inject it per-handler with @ConnectedSocket() to read auth data, the socket id, or to reply only to that client.

client.emit(...) targets just the sender.
client.id uniquely identifies the connection.
client.handshake exposes headers, query, and auth handed in at connect time.

import { WebSocketGateway, SubscribeMessage, MessageBody, ConnectedSocket } from '@nestjs/websockets';
import { Socket } from 'socket.io';

@WebSocketGateway({ namespace: '/chat' })
export class ChatGateway {
  @SubscribeMessage('whoami')
  whoAmI(@ConnectedSocket() client: Socket, @MessageBody() _: unknown): void {
    client.emit('identity', { id: client.id, token: client.handshake.auth?.token });
  }
}

Lifecycle Hooks

Gateways can implement lifecycle interfaces to react to connection events:

OnGatewayInit → afterInit(server) runs once the server is ready.
OnGatewayConnection → handleConnection(client) fires per new client.
OnGatewayDisconnect → handleDisconnect(client) fires when a client leaves.

Use these for presence tracking, auth on connect, and cleanup.

import { WebSocketGateway, OnGatewayInit, OnGatewayConnection, OnGatewayDisconnect } from '@nestjs/websockets';
import { Server, Socket } from 'socket.io';
import { Logger } from '@nestjs/common';

@WebSocketGateway({ namespace: '/chat' })
export class ChatGateway implements OnGatewayInit, OnGatewayConnection, OnGatewayDisconnect {
  private readonly logger = new Logger(ChatGateway.name);

  afterInit(server: Server): void {
    this.logger.log('Gateway initialized');
  }

  handleConnection(client: Socket): void {
    this.logger.log(`Client connected: ${client.id}`);
  }

  handleDisconnect(client: Socket): void {
    this.logger.log(`Client disconnected: ${client.id}`);
  }
}

Rooms: Grouping Sockets

A room is a server-side label you attach to sockets so you can broadcast to a subset. A socket can belong to many rooms.

client.join('room') adds the socket to a room.
client.leave('room') removes it.
Socket.IO automatically puts each socket in a room named after its own client.id.

Rooms are how you build per-channel chat, per-tenant dashboards, or per-document collaboration.

import { WebSocketGateway, SubscribeMessage, MessageBody, ConnectedSocket } from '@nestjs/websockets';
import { Socket } from 'socket.io';

@WebSocketGateway({ namespace: '/chat' })
export class ChatGateway {
  @SubscribeMessage('joinRoom')
  joinRoom(@ConnectedSocket() client: Socket, @MessageBody() room: string): void {
    client.join(room);
    client.emit('joined', room);
  }
}

Broadcasting to a Room

Target a room with server.to('room').emit(...). To exclude the sender, emit from the client socket: client.to('room').emit(...) sends to everyone in the room except that client.

server.to(room).emit(...) → everyone in the room.
client.to(room).emit(...) → everyone in the room but the sender.

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

@WebSocketGateway({ namespace: '/chat' })
export class ChatGateway {
  @WebSocketServer() server: Server;

  @SubscribeMessage('sendToRoom')
  sendToRoom(
    @ConnectedSocket() client: Socket,
    @MessageBody() body: { room: string; text: string },
  ): void {
    client.to(body.room).emit('message', { from: client.id, text: body.text });
  }
}

Acknowledgements with WsResponse

Socket.IO supports request/response style acknowledgements. In Nest you can return a value, a WsResponse<T> object, or even an Observable that streams multiple emissions back.

Return { event, data } to emit a named event back to the caller.
Returning an Observable emits one message per value, useful for progress updates.

import { WebSocketGateway, SubscribeMessage, MessageBody, WsResponse } from '@nestjs/websockets';
import { from, Observable } from 'rxjs';
import { map } from 'rxjs/operators';

@WebSocketGateway({ namespace: '/jobs' })
export class JobsGateway {
  @SubscribeMessage('countdown')
  countdown(@MessageBody() n: number): Observable<WsResponse<number>> {
    const ticks = Array.from({ length: n }, (_, i) => n - i);
    return from(ticks).pipe(map((value) => ({ event: 'tick', data: value })));
  }
}

Validation with Pipes

Gateways reuse Nest's ValidationPipe, but errors are thrown as WsException rather than HTTP exceptions. Apply the pipe at the handler or gateway level and validate a DTO via @MessageBody().

Set transform: true so plain payloads become class instances.
A WsException is serialized to an exception event on the client.

import { WebSocketGateway, SubscribeMessage, MessageBody } from '@nestjs/websockets';
import { UsePipes, ValidationPipe } from '@nestjs/common';
import { IsString, MinLength } from 'class-validator';

class SendMessageDto {
  @IsString() @MinLength(1)
  text: string;
}

@WebSocketGateway({ namespace: '/chat' })
export class ChatGateway {
  @UsePipes(new ValidationPipe({ transform: true }))
  @SubscribeMessage('send')
  send(@MessageBody() dto: SendMessageDto): { event: string; data: string } {
    return { event: 'sent', data: dto.text };
  }
}

Authenticating on Connect

Authenticate clients during handleConnection by reading the token from the handshake and disconnecting unauthorized sockets early. Storing the user on client.data makes it available to every later handler.

Token usually arrives via handshake.auth.token or an Authorization header.
Call client.disconnect() to reject bad clients before they join rooms.

import { WebSocketGateway, OnGatewayConnection } from '@nestjs/websockets';
import { Socket } from 'socket.io';
import { JwtService } from '@nestjs/jwt';

@WebSocketGateway({ namespace: '/chat' })
export class ChatGateway implements OnGatewayConnection {
  constructor(private readonly jwt: JwtService) {}

  async handleConnection(client: Socket): Promise<void> {
    try {
      const token = client.handshake.auth?.token as string;
      client.data.user = await this.jwt.verifyAsync(token);
    } catch {
      client.emit('error', 'Unauthorized');
      client.disconnect();
    }
  }
}

Swapping the Adapter for Scale

Out of the box, Socket.IO state lives in a single process. To run multiple Nest instances behind a load balancer you must share room/emit state. Use a custom adapter with the Redis adapter so broadcasts reach clients on other nodes.

Subclass IoAdapter and attach @socket.io/redis-adapter in createIOServer.
Register it via app.useWebSocketAdapter(new RedisIoAdapter(app)) in main.ts.

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

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

  async connectToRedis(): Promise<void> {
    const pub = createClient({ url: 'redis://localhost:6379' });
    const sub = pub.duplicate();
    await Promise.all([pub.connect(), sub.connect()]);
    this.adapterConstructor = createAdapter(pub, sub);
  }

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

Quick Check

Test your understanding of room broadcasting semantics.

Recap

You built a real-time layer with NestJS WebSocket gateways on the Socket.IO adapter:

Gateway basics: @WebSocketGateway() providers with @SubscribeMessage handlers; inject the server via @WebSocketServer() and the per-client socket via @ConnectedSocket().
Lifecycle hooks: afterInit, handleConnection, and handleDisconnect for init, presence, and cleanup.
Rooms: client.join/leave, then server.to(room) for everyone or client.to(room) to exclude the sender.
Robustness: validate payloads with ValidationPipe (errors become WsException), authenticate in handleConnection, and use acknowledgements/Observables for replies.
Scale: swap in a Redis-backed IoAdapter so broadcasts work across multiple instances.

Frequently asked questions

Is the “WebSocket Gateways with the Socket.IO Adapter” lesson free?

Yes — the full text of “WebSocket Gateways with the Socket.IO Adapter” is free to read here on the web, and the NestJS Enterprise Backend APIs course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the NestJS Enterprise Backend APIs course, upgrade to CoddyKit PRO.

What will I learn in “WebSocket Gateways with the Socket.IO Adapter”?

Implement @WebSocketGateway handlers, rooms, and lifecycle hooks for live communication. You practise NestJS Enterprise Backend APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start NestJS Enterprise Backend APIs?

No prior experience is required. NestJS Enterprise Backend APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “WebSocket Gateways with the Socket.IO Adapter” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this NestJS Enterprise Backend APIs lesson?

Yes. Every NestJS Enterprise Backend APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

WebSocket Gateways with the Socket.IO Adapter
Authenticating and Guarding Socket Connections
Server-Sent Events for One-Way Push
Scaling Realtime with a Redis Pub/Sub Adapter

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