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x.vweb.sse #

Server Sent Events

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


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 vweb isn't able to process any other requests.

We can let vweb 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 vweb.no_result(). Vweb will not close the connection and we can handle the connection in a separate thread.


import x.vweb.sse

// endpoint handler for SSE connections
fn (app &App) sse(mut ctx Context) vweb.Result {
    // let vweb know that the connection should not be closed
    // 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 vweb.no_result()

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

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

Javascript code:

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

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

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

[mozilla_sse]: [whatwg html spec]:

fn start_connection #

fn start_connection(mut ctx vweb.Context) &SSEConnection

start an SSE connection

struct SSEConnection #

struct SSEConnection {
pub mut:
	conn &net.TcpConn @[required]

fn (SSEConnection) send_message #

fn (mut sse SSEConnection) send_message(message SSEMessage) !

send_message sends a single message to the http client that listens for SSE. It does not close the connection, so you can use it many times in a loop.

fn (SSEConnection) close #

fn (mut sse SSEConnection) close()

send a 'close' event and close the tcp connection.

struct SSEMessage #

struct SSEMessage {
pub mut:
	id    string
	event string
	data  string
	retry int

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

Event stream format > The event stream is a simple stream of text data which must be encoded using UTF-8. > Messages in the event stream are separated by a pair of newline characters. > A colon as the first character of a line is in essence a comment, and is ignored. > Note: The comment line can be used to prevent connections from timing out; > a server can send a comment periodically to keep the connection alive. > > Each message consists of one or more lines of text listing the fields for that message. > Each field is represented by the field name, followed by a colon, followed by the text > data for that field's value.