Send notification from server to client on server event

Question

How can I send a message from server to client with PHP, avoiding extraneous Ajax calls.

Here is the idea:

1. User: Alice makes a change which gets sent to the server.

2. The Server then checks to see which users are not up to date, and if not calls some code to send the information pertaining to the change to Bob (who is not up to date in this case).

How do I send Bob the message?

Answer 1

You can use Server Sent Events.

These are the often over looked cousin of websockets that are lighter weight and one way. They essentially allow the client to wait for a message from the server (which you can then respond to via a different channel such as AJAX)

Example Client Code:

var source = new EventSource('/newFile');

source.addEventListener('message', function(e) {
  // Use AJAX and pull new file here
}, false);

Unfortuantely it seems (of course) that there is no IE support. One could use a library such as EventSource HQ to support cross browser server sent events. It will abstract away needing to handle the devil's browser.