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HTML Server-Sent Events API

Server-Sent Events (SSE) 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 79.0 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");
source.onmessage = function(event) {
  document.getElementById("result").innerHTML += event.data + "<br>";
};

Example explained:

  • Create a new EventSource object, and specify the URL of the page sending the updates (in this example "demo_sse")
  • 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..
    }

    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
    header('Content-Type: text/event-stream');
    header('Cache-Control: no-cache');

    $time = date('r');
    echo "data: The server time is: {$time}\n\n";
    flush();
    ?>

    Code in ASP (VB) (demo_sse):

    <%
    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:

    Events Description
    onopen When a connection to the server is opened
    onmessage When a message is received
    onerror When an error occurs

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