Server-Sent Events

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Server-Sent Events (SSE) is a server push technology enabling a client to receive automatic updates from a server via HTTP connection. Each notification is sent as a block of text terminated by a pair of newlines (learn more here).

Usage#

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


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

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

Warning Server-Sent Events routes must return an Observable stream.

In the example above, we defined a route named sse that will allow us to propagate real-time updates. These events can be listened to using the EventSource API.

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


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

With this in place, we can now create an instance of the EventSource class in our client-side application, passing the /sse route (which matches the endpoint we have passed into the @Sse() decorator above) as a constructor argument.

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

Once the connection is opened, incoming messages from the server are delivered to your code in the form of events. If there is an event field in the incoming message, the triggered event is the same as the event field value. If no event field is present, then a generic message event is fired (source).


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., eventSource.close()), NestJS automatically unsubscribes from the returned Observable, which 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 disconnect). This makes it the right place to release external resources such as database cursors or file handles tied to the stream.

Example#

A working example is available here.