Server-Sent Events (SSE API)
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 the <eventsource> element should point to an URL that 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 that would take care of sending event data consistently.
Instance
Following is a sample HTML code of a web application that would expect server time:
<!DOCTYPE html><html><head><script type="text/javascript"></script></head><body><div id="sse"><eventsource src="/cgi-bin/ticker.cgi" /></div><div id="ticker"><TIME></div></body></html>/* Define event handling logic here */
Server-side Script for SSE
The following are the steps for sending server-sent events (SSE) from a server-side script:
1. Set the Content-Type Header
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";
2. Send an Event Name
After setting Content-Type, the server-side script would send an Event: tag followed by the event name. Following code snippet would send Server-Time as an event name terminated by a new line character.
print "Event: server-time\n";
3. Send Event Data
The final step is to send event data using Data: tag which would be followed by an integer of a string value terminated by a new line character as follows −
$time = localtime(); print "Data: $time\n";
4. Combine Steps into a Complete Script
Finally, following is a complete “ticker.cgi” written in Perl −
#!/usr/bin/perlprint"Content-Type: text/event-stream\n\n";while(true){print"Event: server-time\n";$time= localtime();print"Data: $time\n";
sleep(5);}
Handle Server-Sent Events
You can also modify the web application to listen for and process server-sent events using an eventsource object. Let us modify our web application to handle server-sent events.
Example
The following example demonstrates handling server-sent events:
<!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></pre>Note: Before testing Server-Sent events, I would suggest that you make sure your web browser supports this concept.
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