HTML SSE API

Server-Sent Events (SSE) allow a website to receive updates from a server .

Server-Sent Events - One-way messaging

Server-Sent Events are when a website automatically receives updates from the server.

This could have happened before, but the website would have had to ask if there were any updates. With Server-Sent Events, updates arrive automatically.

For example, Facebook/Twitter updates, stock price updates, news feed, sports results, etc.

Browser support

The numbers in the table indicate the browser version that first fully supported Server-Sent Events.

Receive Server-Sent Event notifications

The object EventSourceused to receive event notifications sent by the server:

For example:

var source = new EventSource("demo_sse.php"); source.onmessage = function(event) { document.getElementById("result").innerHTML += event.data + "
"; };

The example is explained as follows:

• Create a new object EventSourceand specify the URL of the page that submits updates (in this example, " demo_sse.php ").
• Every time an update is received, the onmessage event will occur.
• When the onmessage event occurs, place the received data into the element with the id="result"

Check Server-Sent Events support

In the example above, there are some additional 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. }

Example of server-side code

For the above example to work, you need a server capable of sending data updates (such as PHP or ASP).

Server-side event stream syntax is very simple. Set the "Content-Type" header to "text/event-stream". Now you can start sending event streams.

Code in PHP (demo_sse.php):

Code in ASP (VB) (demo_sse.asp):

<% Response.ContentType = "text/event-stream" Response.Expires = -1 Response.Write("data: The server time is: " & now()) Response.Flush() %>

The code is explained as follows:

• Set the "Content-Type" header to "text/event-stream".
• Specify that the page should not be cached.
• Export data for sending (Always start with "data:" )
• Return the output data to the website.

EventSource object

In the examples above, the author used the onmessage event to receive the message. But other events are also available: