Server-Sent Event

  • Is when a web page automatically gets updates from a server.
  • This was also possible before, but the web page would have to ask if any updates were available. With server-sent events, the updates come automatically.
  • Examples: Facebook/Twitter updates, stock price updates, news feeds, sport results, etc.

Receive Server-Sent Event Notifications

var source = new EventSource("demo_sse.php");
source.onmessage = function(event) {
  document.getElementById("result").innerHTML += + "<br>";

//Example explained:

// - Create a new EventSource object, and specify the URL of the page sending
//the updates (in this example "demo_sse.php")
// - Each time an update is received, the onmessage event occurs 
// - When an onmessage event occurs, put the received data into the element
//with id="result"

The EventSource object is used to receive server-sent event notifications.

Check Server-Sent Events Support

if(typeof(EventSource) !== "undefined") {
  // Yes! Server-sent events support!
  // Some code.....
} else {
  // Sorry! No server-sent events support..

Server-Side Code Example

//Code in PHP (demo_sse.php)

header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');

$time = date('r');
echo "data: The server time is: {$time}\n\n";
//Code in ASP (VB) (demo_sse.asp)

Response.ContentType = "text/event-stream"
Response.Expires = -1
Response.Write("data: The server time is: " & now())

For the example to work, you need a server capable of sending data updates (like PHP or ASP).

The server-side event stream syntax is simple. Set the "Content-Type" header to "text/event-stream".

Server-Sent Event — Structure map

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Server-Sent Event — Related pages: