v/vlib/veb/sse/README.md

Server Sent Events

This module implements the server side of Server Sent Events, SSE. See mozilla SSE as well as whatwg for detailed description of the protocol, and a simple web browser client example.

Usage

With SSE we want to keep the connection open, so we are able to keep sending events to the client. But if we hold the connection open indefinitely veb isn't able to process any other requests.

We can let veb know that it can continue processing other requests and that we will handle the connection ourself by calling ctx.takeover_conn() and returning an empty result with veb.no_result(). veb will not close the connection and we can handle the connection in a separate thread.

Example:

import veb.sse

// endpoint handler for SSE connections
fn (app &App) sse(mut ctx Context) veb.Result {
	// let veb know that the connection should not be closed
	ctx.takeover_conn()
	// handle the connection in a new thread
	spawn handle_sse_conn(mut ctx)
	// we will send a custom response ourself, so we can safely return an empty result
	return veb.no_result()
}

fn handle_sse_conn(mut ctx Context) {
	// pass veb.Context
	mut sse_conn := sse.start_connection(mut ctx.Context)

	// send a message every second 3 times
	for _ in 0.. 3 {
		time.sleep(time.second)
		sse_conn.send_message(data: 'ping') or { break }
	}
	// close the SSE connection
	sse_conn.close()
}

Javascript code:

const eventSource = new EventSource('/sse');

eventSource.addEventListener('message', (event) => {
	console.log('received message:', event.data);
});

eventSource.addEventListener('close', () => {
	console.log('closing the connection');
	// prevent browser from reconnecting
	eventSource.close();
});