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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"; |
print "Event: server-time\n"; |
$time = localtime(); print "Data: $time\n"; |
#!/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);
}
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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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