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).
To enable Server-Sent events on a route (route registered within a controller class), annotate the method handler with the
info Hint The
@Sse()decorator is imported from the
and mapare imported from the
warning Warning Server-Sent Events routes must return an
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.
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:
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
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).
A working example is available here.