NestJS Enterprise Backend APIs · 课时

用于单向推送的服务器发送事件

使用 @Sse 装饰器和 RxJS 可观察对象向客户端推送实时更新。

第 3 / 4 课13 个步骤

用于单向推送的服务器发送事件 是 CoddyKit 上的免费 NestJS Enterprise Backend APIs 课时。这是第 3 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。

Why Server-Sent Events?

Server-Sent Events (SSE) let a server push a continuous stream of updates to a client over a single, long-lived HTTP connection. It is the simplest way to deliver one-way, server-to-client live data such as notifications, progress bars, or dashboard metrics.

One-way only: the server talks, the client listens. There is no client-to-server channel on the same connection.
Plain HTTP: no special protocol upgrade like WebSockets need. It rides on a normal GET request.
Auto-reconnect: the browser's EventSource reconnects automatically if the connection drops.

In NestJS, SSE is a first-class feature exposed through the @Sse() decorator combined with RxJS observables.

SSE vs WebSockets

Both stream live data, but they solve different problems. Choosing the right one is an architectural decision.

SSE: server → client only, text-based, runs over HTTP/1.1 or HTTP/2, built-in reconnection and event IDs. Ideal for feeds, alerts, and progress.
WebSockets: full-duplex (both directions), binary or text, requires a protocol upgrade. Ideal for chat, multiplayer games, and collaborative editing.

If your clients only need to receive updates, SSE is lighter, easier to scale behind standard HTTP infrastructure, and requires no extra client library. Reach for WebSockets only when the client must also push messages in real time.

The wire format

SSE is just a streaming HTTP response with the content type text/event-stream. Each message is a block of text fields separated by newlines, and each block ends with a blank line.

data: the payload (often a JSON string).
event: a named event type the client can listen for.
id: a message identifier used for reconnection via Last-Event-ID.
retry: the reconnection delay in milliseconds.

You rarely format this by hand in NestJS — the framework serializes a typed object into these fields for you — but knowing the shape helps you debug with curl.

// Raw text/event-stream bytes the server emits
// (NestJS builds this for you from a MessageEvent)
const sample =
  'id: 42\n' +
  'event: heartbeat\n' +
  'data: {"status":"ok","ts":1718000000}\n' +
  '\n';

process.stdout.write(sample);

Your first @Sse endpoint

The @Sse() decorator marks a controller method as an SSE stream. Instead of returning a plain value, the method returns an RxJS Observable. Every value the observable emits becomes one SSE message sent to the client.

NestJS expects each emitted value to be a MessageEvent-shaped object with a data property. It automatically sets the text/event-stream headers and keeps the connection open.

import { Controller, Sse, MessageEvent } from '@nestjs/common';
import { interval, map, Observable } from 'rxjs';

@Controller('events')
export class EventsController {
  @Sse('clock')
  clock(): Observable<MessageEvent> {
    return interval(1000).pipe(
      map((n) => ({ data: { tick: n, time: new Date().toISOString() } })),
    );
  }
}

The MessageEvent shape

NestJS exports a MessageEvent interface that maps directly onto the SSE wire fields. Only data is required; the rest are optional.

data — string or object. Objects are JSON-stringified automatically.
type — becomes the event: field (a named event).
id — becomes the id: field, enabling resume-on-reconnect.
retry — becomes the retry: field in milliseconds.

Returning a well-typed object keeps your stream self-documenting and lets clients subscribe to specific named events.

import { MessageEvent } from '@nestjs/common';

function buildEvent(orderId: string, n: number): MessageEvent {
  return {
    id: String(n),
    type: 'order.updated',
    retry: 5000,
    data: { orderId, sequence: n },
  };
}

console.log(buildEvent('ord_123', 7));

Pushing domain events with a Subject

A fixed interval is fine for clocks, but real systems push when something happens. The idiomatic pattern is an RxJS Subject living in a service. Your business logic calls .next() on the subject whenever an event occurs, and the SSE endpoint simply exposes the subject as an observable.

This cleanly decouples the producer (any service) from the transport (the SSE controller).

import { Injectable, MessageEvent } from '@nestjs/common';
import { Subject, Observable } from 'rxjs';

@Injectable()
export class NotificationsService {
  private readonly stream$ = new Subject<MessageEvent>();

  emit(payload: unknown): void {
    this.stream$.next({ type: 'notification', data: payload });
  }

  asObservable(): Observable<MessageEvent> {
    return this.stream$.asObservable();
  }
}

Wiring the service to the controller

The controller injects the service and returns its observable from an @Sse() method. Any other part of the app — a queue consumer, a webhook handler, a cron job — can inject the same service and call emit() to broadcast to every connected client.

Because a plain Subject is multicast, all subscribers receive each emission. This is exactly what you want for a shared notification feed.

import { Controller, Post, Body, Sse, MessageEvent } from '@nestjs/common';
import { Observable } from 'rxjs';
import { NotificationsService } from './notifications.service';

@Controller('notifications')
export class NotificationsController {
  constructor(private readonly notifications: NotificationsService) {}

  @Sse('stream')
  stream(): Observable<MessageEvent> {
    return this.notifications.asObservable();
  }

  @Post()
  publish(@Body() body: { message: string }) {
    this.notifications.emit(body);
    return { accepted: true };
  }
}

Per-user filtered streams

A global subject broadcasts to everyone. In an enterprise API you usually want each client to receive only their events. Use RxJS operators like filter and map to tailor the stream per request, reading the user from a route param or the authenticated request.

The @Sse() method can accept normal route decorators such as @Param() and @Req(), so you can scope the observable to the current user.

import { Controller, Param, Sse, MessageEvent } from '@nestjs/common';
import { Observable, filter, map } from 'rxjs';
import { EventsBus } from './events.bus';

@Controller('users')
export class UserFeedController {
  constructor(private readonly bus: EventsBus) {}

  @Sse(':userId/feed')
  feed(@Param('userId') userId: string): Observable<MessageEvent> {
    return this.bus.events$.pipe(
      filter((e) => e.userId === userId),
      map((e) => ({ type: e.kind, data: e.payload })),
    );
  }
}

Heartbeats keep the connection alive

Proxies, load balancers, and browsers may close an idle connection. A heartbeat — a periodic comment or no-op event — keeps the pipe warm. With RxJS you merge your real event stream with a slow timer.

Send heartbeats as a distinct event type (or as SSE comment lines) so clients can ignore them. A common interval is every 15–30 seconds, comfortably under typical proxy idle timeouts.

import { merge, interval, map, Observable } from 'rxjs';
import { MessageEvent } from '@nestjs/common';

export function withHeartbeat(
  source$: Observable<MessageEvent>,
): Observable<MessageEvent> {
  const heartbeat$ = interval(15000).pipe(
    map((): MessageEvent => ({ type: 'heartbeat', data: 'ping' })),
  );
  return merge(source$, heartbeat$);
}

Cleanup, errors, and backpressure

When a client disconnects, NestJS unsubscribes from your observable. Make sure your stream releases resources on unsubscribe — use finalize() for cleanup and never leak timers or listeners.

Use catchError to convert errors into a final event instead of crashing the stream.
Use finalize to log or decrement a connection counter when the client leaves.
Beware backpressure: a fast producer with a slow client buffers in memory. Throttle or sample high-frequency sources.

import { Observable, catchError, finalize, of } from 'rxjs';
import { MessageEvent } from '@nestjs/common';

export function safeStream(
  source$: Observable<MessageEvent>,
  onClose: () => void,
): Observable<MessageEvent> {
  return source$.pipe(
    catchError((err) =>
      of<MessageEvent>({ type: 'error', data: { message: err.message } }),
    ),
    finalize(onClose),
  );
}

Consuming the stream from a client

Browsers consume SSE with the native EventSource API. It connects, dispatches messages, and reconnects automatically. Listen to the default message event for unnamed data, or add listeners for your named event types.

Note that EventSource only supports GET and cannot set custom headers, so auth is usually done via cookies or a token in the query string. Tools like curl -N are great for quick debugging from the terminal.

// Browser-side consumer
const es = new EventSource('/notifications/stream');

es.addEventListener('notification', (e) => {
  const payload = JSON.parse(e.data);
  console.log('new notification', payload);
});

es.addEventListener('heartbeat', () => {
  // keep-alive, ignore
});

es.onerror = () => console.warn('reconnecting...');

Quick Check

Test your understanding of when and how to use SSE in NestJS.

在交互式课程中回答

Recap

You learned how to stream one-way live updates from NestJS using Server-Sent Events:

SSE is server-to-client only over plain HTTP, with built-in browser auto-reconnect — choose it over WebSockets when clients only need to receive.
The @Sse() decorator turns a controller method into a stream that returns an Observable<MessageEvent>; each emission becomes one message.
A MessageEvent maps to the wire fields data, type, id, and retry; objects are JSON-serialized for you.
Push real domain events with a shared Subject in a service, and scope per-user streams with filter and map.
Keep connections healthy with heartbeats via merge, and clean up safely using catchError and finalize.
Consume on the client with the native EventSource API.

免费开始

用 AI 导师学习 TypeScript — 免费

在浏览器中编写并运行真实代码，获得全天候 AI 导师的即时帮助，并在网页或应用中继续学习。

课程: 20
课程: 76

免费开始学习查看完整课程

常见问题解答

「用于单向推送的服务器发送事件」课时是免费的吗？

是的 — 「用于单向推送的服务器发送事件」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。

「用于单向推送的服务器发送事件」这节课中我会学到什么？

使用 @Sse 装饰器和 RxJS 可观察对象向客户端推送实时更新。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 3 节课，共 4 节。

「用于单向推送的服务器发送事件」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。

我能在这节 NestJS Enterprise Backend APIs 课中编写并运行代码吗？

能。每节 NestJS Enterprise Backend APIs 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

← 返回 NestJS Enterprise Backend APIs