WITHOUT React (that comes ONLY in block 9), a Mercure subscription can ALREADY be tested NOW with basic tools – EITHER in the browser via the JavaScript EventSource API OR via curl.

Via EventSource in the browser

const url = new URL('https://localhost/.well-known/mercure');
url.searchParams.append('topic', 'https://localhost/api/projects/1');

const eventSource = new EventSource(url);

eventSource.onmessage = (event) => {
  const updatedProject = JSON.parse(event.data);
  console.log('Update received:', updatedProject);
};

EXACTLY the standard EventSource mechanism, IDENTICAL to ITS use in ANY other web application – Mercure ONLY adds the topic-based subscription model to this standard.

Testing the subscription in the browser console

1. Run the JavaScript code above in the browser console (F12 → Console).
2. In a SECOND tab/terminal, send a PATCH /api/projects/1 via curl.
3. The console shows the update IMMEDIATELY, WITHOUT reloading the page.

Watching the raw SSE connection via curl

curl -k -N 'https://localhost/.well-known/mercure?topic=https://localhost/api/projects/1'

-N (--no-buffer) is CRITICAL – WITHOUT this flag, curl buffers the response and shows NOTHING until the connection (which DELIBERATELY stays open) eventually gets CLOSED.

event: message
data: {"@id":"/api/projects/1","name":"Updated name", ...}

Subscribing to multiple topics at once

curl -k -N 'https://localhost/.well-known/mercure?topic=https://localhost/api/projects/1&topic=https://localhost/api/projects/2'

Multiple topic parameters in the SAME request subscribe to MULTIPLE topics OVER A single connection – ECONOMICAL compared to a separate connection PER topic.