010. HTML5 Server Sent Events

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Conventional web applications generate events which are dispatched to the web server. For example a simple click on a link requests a new page from the server.

The type of events which are flowing from web browser to the web server may be called client-sent events.

Along with HTML5, WHATWG Web Applications 1.0 introduces events which flow from web server to the web browsers and they are called Server-Sent Events (SSE). Using SSE you can push DOM events continously from your web server to the visitor's browser.

The event streaming approach opens a persistent connection to the server, sending data to the client when new information is available, eliminating the need for continuous polling.

Server-sent events standardizes how we stream data from the server to the client.

Web Application for SSE:

To use Server-Sent Events in a web application, you would need to add an <eventsource> element to the document.

The src attribute of <eventsource> element should point to an URL which should provide a persistent HTTP connection that sends a data stream containing the events.

The URL would point to a PHP, PERL or any Python script which would take care of sending event data consistently. Following is a simple example of web application which would expect server time.

<!DOCTYPE HTML>
<html>
<head>
<script type="text/javascript">
/* Define event handling logic here */
</script>
</head>
<body>
<div id="sse">
   <eventsource src="/cgi-bin/ticker.cgi" />
</div>
<div id="ticker">
   <TIME>
</div>
</body>
</html>

Server Side Script for SSE:

A server side script should send Content-type header specifying the type text/event-stream as follows.

print "Content-Type: text/event-stream\n\n";

After setting Content-Type, server side script would send an Event: tag followed by event name. Following example would send Server-Time as event name terminated by a new line character.

print "Event: server-time\n";

Final step is to send event data using Data: tag which would be followed by integer of string value terminated by a new line character as follows:

$time = localtime();
print "Data: $time\n";

Finally, following is complete ticker.cgi written in perl:

#!/usr/bin/perl

print "Content-Type: text/event-stream\n\n";
while(true){
   print "Event: server-time\n";
   $time = localtime();
   print "Data: $time\n";
   sleep(5);
}

Handle Server-Sent Events:

Let us modify our web application to handle server-sent events. Following is the final example.

<!DOCTYPE HTML>
<html>
<head>
<script type="text/javascript">
   document.getElementsByTagName("eventsource")[0].
            addEventListener("server-time", eventHandler, false);
   function eventHandler(event)
   {
       // Alert time sent by the server
       document.querySelector('#ticker').innerHTML = event.data;

   }
</script>
</head>
<body>
<div id="sse">
   <eventsource src="/cgi-bin/ticker.cgi" />
</div>
<div id="ticker" name="ticker">
   [TIME]
</div>
</body>
</html>

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