HTML5 Server-Sent Events







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HTML5 Server-Sent Events



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Server-Sent Events 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 events, the updates come automatically.


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



Browser Support


The numbers in the table specify the first browser version that fully support server-sent events.



















API
SSE 6.0 Not supported 6.0 5.0 11.5


Receive Server-Sent Event Notifications


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



Example



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

Try it Yourself »

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..
}








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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}nn";
flush();
?>


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



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



















EventsDescription
onopenWhen a connection to the server is opened
onmessageWhen a message is received
onerrorWhen an error occurs



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