Server-Sent Events

Server-Sent Events (SSE) is a server push technology that enables a client to receive automatic updates from a server over an HTTP connection. Each notification is sent as a block of text terminated by a pair of newlines (see Server-sent events on MDN to learn more).

Usage#

To enable Server-Sent Events on a route (a route registered within a controller class), annotate the route handler with the @Sse() decorator.

@Sse('sse')
sse(): Observable<MessageEvent> {
  return interval(1000).pipe(map((_) => ({ data: { hello: 'world' } })));
}

Hint The @Sse() decorator and the MessageEvent interface are imported from the @nestjs/common package, while Observable, interval, and map are imported from the rxjs package.

Warning Server-Sent Events routes must return an Observable stream (or a Promise that resolves to one, as shown below).

In the example above, we defined a route named sse that propagates real-time updates. Clients can listen to these events using the EventSource API.

The sse() method returns an Observable that emits multiple MessageEvent objects (in this example, a new MessageEvent every second). Each MessageEvent object should respect the following interface to match the specification:

export interface MessageEvent {
  data?: string | object;
  id?: string;
  type?: string;
  retry?: number;
  comment?: string;
}

Object data is serialized to JSON. The type property is sent as the SSE event field, and comment is sent as a comment line, which EventSource clients ignore (useful, for example, as a keep-alive). If you don't set id, Nest assigns an incrementing ID to each event.

With this in place, you can create an instance of the EventSource class in your client-side application, passing the /sse route (which matches the path passed to the @Sse() decorator above) as a constructor argument.

An EventSource instance opens a persistent connection to an HTTP server, which sends events in text/event-stream format. The connection remains open until it's closed by calling EventSource.close().

Once the connection is open, incoming messages from the server are delivered to your code in the form of events. If the incoming message has an event field, the triggered event has the same name as the field value. If no event field is present, a generic message event is fired (see the EventSource documentation on MDN).

const eventSource = new EventSource('/sse');
eventSource.onmessage = ({ data }) => {
  console.log('New message', JSON.parse(data));
};

Client disconnection#

When a client closes the SSE connection (e.g., with eventSource.close()), Nest automatically unsubscribes from the returned Observable. This stops the event stream and cleans up any associated resources, including the interval timer in the example above.

To run custom teardown logic when a client disconnects, use the finalize operator:

@Sse('sse')
sse(): Observable<MessageEvent> {
  return interval(1000).pipe(
    map((_) => ({ data: { hello: 'world' } })),
    finalize(() => console.log('Client disconnected')),
  );
}

Hint The finalize operator (imported from rxjs) executes its callback whenever the Observable terminates: by completion, error, or unsubscription (which includes client disconnection). This makes it the right place to release external resources, such as database cursors or file handles, tied to the stream.

An @Sse() handler can be asynchronous, returning a Promise<Observable> rather than an Observable directly. This is common when the stream needs expensive setup before the first event can be produced, such as opening a database cursor, acquiring a model session, or authorizing against a downstream service.

@Sse('stream')
async stream(): Promise<Observable<MessageEvent>> {
  const session = await createSession();

  return new Observable(subscriber => {
    // ...produce events from `session`
  });
}

This leaves a gap. If the client disconnects while the promise is still resolving, Nest never subscribes to the returned Observable, because it doesn't start a producer whose consumer has already gone away. That's the right behavior for the stream, but it means the Observable's teardown logic never runs, and anything allocated during setup (the session above) leaks.

To close that gap, inject the request's AbortSignal with the @SseSignal() decorator:

import { MessageEvent, Sse, SseSignal } from '@nestjs/common';
import { EMPTY, Observable } from 'rxjs';

@Sse('stream')
async stream(@SseSignal() signal: AbortSignal): Promise<Observable<MessageEvent>> {
  const session = await createSession();

  if (signal.aborted) {
    // The client disconnected during setup. The Observable below will never be
    // subscribed, so release the resource here.
    await session.close();
    return EMPTY;
  }

  return new Observable(subscriber => {
    const stream = session.start();
    stream.on('data', data => subscriber.next({ data }));

    return () => {
      stream.stop();
      session.close();
    };
  });
}

Signal lifetime#

The signal represents the lifetime of the SSE response, not only the connection. It's aborted once the stream terminates for any reason:

• the client disconnected;
• the Observable completed;
• the Observable errored.

This makes the signal a single cleanup hook for the whole request. Instead of duplicating teardown across an abort listener and the Observable's own teardown function, you can wire resources to the signal once and have them released on every exit path:

@Sse('stream')
async stream(@SseSignal() signal: AbortSignal): Promise<Observable<MessageEvent>> {
  // The signal aborts when the response ends, so the fetch is canceled whether
  // the client disconnected or the stream simply finished.
  const upstream = await fetch(UPSTREAM_URL, { signal });

  return new Observable(subscriber => {
    // ...
  });
}

Because the signal also aborts on normal completion, signal.aborted is only meaningful as a "did the client go away?" check during setup, before the Observable is returned. At that point, the stream can't have completed yet, so an aborted signal unambiguously means the client disconnected.

Notice Cleanup wired to the abort event may run alongside the Observable's teardown function, so make it idempotent.

Inside a producer, you can also use the signal to end the stream when the client leaves:

return new Observable<MessageEvent>(subscriber => {
  const timer = setInterval(() => subscriber.next({ data: 'tick' }), 1000);
  const onAbort = () => subscriber.complete();

  signal.addEventListener('abort', onAbort, { once: true });

  return () => {
    clearInterval(timer);
    signal.removeEventListener('abort', onAbort);
  };
});

Hint@SseSignal() is only populated on @Sse() routes; on any other route handler, it resolves to undefined. It works the same way on the Express and Fastify platforms.

Example#

A working example is available in the 28-sse sample.