How to capture SSE Eventsource acknowledgement

Question

I'm using this code to send an SSE message to the browser client.

https://nodejs.org/api/http.html#http_response_write_chunk_encoding_callback

Node server

    response.writeHead(200, {  'Content-Type': 'text/event-stream', 'Access-Control-Allow-Origin': '*' });
    response.write(message);
    response.end();

And for the client I'm using this javascript:

var source = new EventSource('myurl');
source.addEventListener('add', addHandler, false);
source.addEventListener('remove', removeHandler, false);

Everything is working fine, but how the server knows for sure that the client actually received it ? I guess SSE is using TCP, is there any way to received the acknowledgement ?

Answer 1

SSEs are a one to many Push protocol. So there is no acknowledgement. You could send an AJAX request back on receipt, but there is nothing in the pattern to provide this functionality.

Answer 2

SSE is a one way communication protocol to push data from server to client.

There no way for a client to ack event reception.

If acknowledge is a must have, you probably need a two way communication like websockets.

Answer 3

I know this is many years old, but none of the answers is correct. 1) TCP does indeed ACK the push stream - its standard http! (though whether your code is at a low-enough level to detect it is a different story)

2) it's not to difficult to develope your own ACK system (I've done it! - To free up resources when last client disappears) and yes, it tends to go against the "spirit" of the protocol and duplicate to a degree the websocket paradigm...but it is wrong to say its impossible. Send a unique per-client "token" in the first message which the browser saves and starts a js "ping" timer which ajaxes a "still alive" message. In your erver code, handle the ajax and restart client-stil-alive timer. If that expires, client has gone, clean up / free resources etc.

Yes its a bit "lumpy" but it works and its not rocket-science difficulty.

just my (very late) 2c worth

The attached image was me diagnosing a case where the ACK was missing, but one every other one apart from the indicated one you can see the ACK