My comment above explains the reason for using the Last-Modified date and Etag headers and making Conditional GET requests.

There’s a fundamental difference in your approach using Max-Age, and my approach using Conditional GET requests. I want to check if there’s a change on the server and know when that change happens ASAP. I have no way to know when the change is going to happen on the server, therefore have no way to accurately set the Max-Age header.

I see what you’re saying about the modern browsers respecting Max-Age and evicting their cached XHR-requested resource representations; this is the exact problem I had, browsers are good at caching the responses of XHR requests.

My question for you is: How do you know what to set the Max-Age response header to? How can you predict when something like a sports score will change?

I want to be in control of how the browsers are caching all resource representations on my applications (static UI files, XHR-requested resources, etc.); this was the motivating factor behind sending over Conditional GET requests on each XHR-poll of a resource. The server should be quick at determining if what the client has cached is stale or not by comparing an Etag; saving the hit of generating the entity body and sending the payload over the wire by simply responding with a 304 Not Modified response.

IE 6 has some gotcha’s so we’re planning on using a random variable to bust through caches on that browser, but everywhere else, just relying on the browser to do the right thing has worked well.

See implementation here (when games are going on):
http://rivals.yahoo.com/ncaa/football/scoreboard

So I question the need for this library.

JS here: http://l.yimg.com/j/static/versioned_asset/v7/js/editorial/js/scorestream.js

include("include/database.php");

$q = "Select * From qryLatestPost";
$result = $database->query($q);
$num_rows = mssql_num_rows($result);

if($num_rows > 0){
	$DatePosted = mssql_result($result,0,"DatePosted");
	$DisplayName = mssql_result($result,0,"DisplayName");
	$PageText = mssql_result($result,0,"PageText");
	$PageText = addslashes($PageText);

	// create ETAG (hash of feed)
	// (HTTP_IF_NONE_MATCH has quotes around it)
	$eTag = md5($DisplayName.$DatePosted);
	header('Etag: '.$eTag);

	$text = "{ \"label\":\"$DisplayName\", \"time\":\"$DatePosted\" }";

	// compare Etag to what we got
	if ($eTag == $_SESSION["HTTP_IF_NONE_MATCH"]) {
		header("HTTP/1.0 304 Not Modified");
		header('Content-Length: 0');
	} else {
	// dump feed
		echo $text;
	}
	$_SESSION["HTTP_IF_NONE_MATCH"] = $eTag;
}

It is technically a dynamic source, but if there hasn’t been a new post, it doesn’t need to update every 5 seconds.

If there is a better way to do this, I’d love to hear about it, otherwise I’ll just keep researching Etags.

I logged into the server and changed it with notepad manually and it did update.

Now I just have to make it read something besides timestamp.

Thanks for the cool API

The data.php file isn’t downloadable (comes down as a blank page), so I’ve been simply changing the timestamp in the data.json file by hand. however, it doesn’t seem to be noticing that the file has been updated.

So either it checks something other than the timestamp, or I’m completely missing something important by not seeing the php file.

Would you mind posting the contents for ajax newbies like me?

@Chris thanks for the heads-up, fixed the typo.