HTML SSE API

1-6-25

Server-Sent Event (SSE) allow a web page to get updates from a server.

Server-Sent Events - One Way Messaging

A 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 event, the updates come automatically.

Examples: Facebook/Twitter updates, stock price updates, news feeds, sport results, etc.

Receive Server-Sent Wvent Notifications

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

Getting server updates

Example explained:

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

Check Server-Sent Events Support

Inthe tyit example above there were some extra lines of code to check browser support for server-sent event:

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

Server-Side Code Examples

For the example above 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". Now you can start ssending streams.

Code in PHP (demo_sse.php):

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

&time = date('r'):
echo "data: The server time is: {&time}/n/n";
flush();
?>

Code in ASP (VB) (demo_sse.asp):

<%

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

Code explained:

• Set the "Content-Type" header to "text/event-stream"
• Specify that the page should not cache
• Output the data to send (Always start with "data: "
• Flush the output data back to the web page

The EventSource Object

In the example above we used the onmessage event to get messages. But other events are also available: