HTML - Server Sent Events

Server Sent Events are a way of sending data from a server to a web page, without requiring the page to refresh or make requests. These events are useful for creating real-time applications, such as chat, news feeds, or notifications. Using SSE, we can push DOM events continuously from our 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 standardize how we stream data from the server to the client.

How to use SSE in Web Application?

To use Server-Sent Events in a web application, we need to add an <eventsource> element to the document. The src attribute of <eventsource> element should point to an URL which provides a persistent HTTP connection that sends a data stream containing the events. Furthermore, the URL points to a PHP, PERL or any Python script which would take care of sending event data consistently.

Instance

Following is a sample HTML code 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 code snippet 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>

Before testing Server-Sent events, I would suggest that you make sure your web browser supports this concept.

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