HTML5 Server-Sent Events | SSE One Way Messaging | SSE Notifications | SSE Support | SSE Object

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 events, the updates come automatically.

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

Browser Support

Server-Sent Events are supported in all major browsers, except Internet Explorer.

Receive Server-Sent Event Notifications

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

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

Check Server-Sent Events Support

In the tryit example above there were some extra lines of code to check browser support for server-sent events:

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

Server-Side Code Example

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 sending event 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):

<% Response.ContentType="text/event-stream" Response.Expires=-1 Response.Write("data: " & now()) Response.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 examples above we used the onmessage event to get messages. But other events are also available:

Events	Description
onopen	When a connection to the server is opened
onmessage	When a message is received
onerror	When an error occurs