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>"; }; |
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(); ?> |
<% Response.ContentType="text/event-stream" Response.Expires=-1 Response.Write("data: " & now()) Response.Flush() %> |
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 |