Vielzeug

SSE Events ​

Problem ​

An application needs a long-lived server event stream without losing cancellation ownership.

Solution ​

Use native EventSource for standard cookie-authenticated SSE. It owns frame parsing, reconnects, and Last-Event-ID; Courier remains responsible for ordinary HTTP requests.

ts
const events = new EventSource('/events', { withCredentials: true });

events.addEventListener('message', (event) => {
  const notification = JSON.parse(event.data) as { text: string };
  console.log(notification.text);
});

events.addEventListener('error', () => {
  console.log('Event stream disconnected');
});

function leaveView(): void {
  events.close();
}

When an endpoint requires custom headers or request bodies, request a raw response with timeout: Infinity and use a dedicated SSE parser that supports incremental UTF-8 decoding, multiline data fields, comments, event IDs, and reconnect policy.

Pitfalls ​

  • Do not parse SSE independently per network chunk; frames and UTF-8 code points may span chunks.
  • Native EventSource cannot attach arbitrary request headers.
  • Always close EventSource when its owner ends.
  • Always consume or cancel a raw Courier response body so Courier can release request ownership.