Caching PHP-generated pages

▼ For years, I always used to include an If-Modified-Since header in my PHP scripts in order to allow their output to be cached by web brow­sers. Turns out, it takes a bit more work.

The idea is that the HTTP server tells browsers when the page was last modified when browsers request a page. Then, when they request the same page again later, the browser can say "only send me the page if it's been modified since <date>". If the page has been changed since <date> the server sends over the page. If the page is still the same, it just sends a 304 response code, "Not Modified". The browser then displays the page from its cache.

(If the browser doesn't supply an If-Modified-Since date, the page is always sent.)

The Apache HTTP server can handle all of this for HTML and other static files. But PHP scripts generate output on the fly, so the concept of a last modified date doesn't really apply, so PHP-generated pages are always sent, even if they are the same as during an earlier visit.

Of course many PHP scripts generate output based on what's in a database, and as long as the contents of that database don't change, there's really no need to run the script again. But the script itself has to be smart enough to handle this. I had a look at this page and then came up with the following.

First, we determine what the last modified date should be: the last time the script itself was changed, or the last time the database was changed, whichever was more recent:

<? $mtime = getlastmod(); $result = mysql("mydb", "select max(unix_timestamp(createts)) as uts from mydb") if ($row = mysql_fetch_array($result) && $row[uts] > $mtime) $mtime = $row[uts];

Then, we check against the If-Modified-Since header supplied by the browser. If the our last modified date is older or the same as the browser's, we send the Not Modified response and call it a day.

$htime = strtotime($HTTP_IF_MODIFIED_SINCE); if ($htime && $mtime <= $htime) { header("HTTP/1.0 304 Not Modified"); exit(); }

(Of course there's some more error checking here and there.)

If the script is still running, the browser had an old version of the page so we're going to send over the latest version. Before we do that, we generate a Last-Modified date so the browser knows what to say in its If-Modified-Since header the next time.

header("Last-Modified: " . date("r", $mtime));

In Safari, it looks like entering a URL in the URL/search bar doesn't use If-Modified-Since, but clicking on a link does. This is now a good deal faster for me. It should help even more for people on slow links or far away from the server.

Permalink - posted 2013-07-18

iOS 7, new iPhone screen sizes and insta-deletion

▼ Decisions, decisions: update your iOS apps now for iOS 7 and be ready on day one, or wait until new iPhones and iPads arrive and incorporate support for a possible new screen size/resolution in the update?

On the Tapadoo blog, Dermot Daly writes that when the iPhone 5 came around, his clients were initially reluctant to spend money updating their iPhone apps to support the bigger screen, but the letterboxing that happened with the unupdated apps made those an ❝insta-delete❞ for many users. He thinks that iOS 7 will play out in much the same way.

(On episode 63 of the Amplified podcast, Dan Benjamin argues that people won't delete apps unless an alternative that does support the bigger / higher resolution screen or new aesthetic is available, and I certainly agree with that.)

However, what if Apple once again releases iPhones (or iPads) with a different screen size/resolution? Unless you really want to have your update ready on day one, it may be prudent to wait for that shoe to drop before finalizing a big update, and certainly start laying the groundwork for another screen size now.

It will be very interesting to see how regular users react to the big changes in iOS 7. It could be like a website redesign, where lots of people complain. But I think even regular users will know there are big changes, and either decide to stick with the old iOS or upgrade understanding that things are going to be different. I also think that old apps won't stick out like a sore thumb immediately like what happened on the iPhone 4's higher resolution display, or to a lesser degree, on the iPhone 5's taller screen. But as users spend more time with updated apps, the older ones will start worse and worse. So there's probably a grace period before that deletion impulse kicks in.

Permalink - posted 2013-07-17

My OHM2013 talk: Federation fosters freedom

▼ Between 31 july and 4 august OHM2013 ("Observe. Hack. Make.") will take place in Holland. I'll have a talk on the 4th at 16.00 about how network protocols that let you run your own server foster freedom, while monolithic services such as Facebook and Twitter do the opposite.

Update: these are the slides (PDF, 4 MB)

This is the abstract of the talk:

Many services are monolithic: you can only Skype using the Skype client, and only Facebook using facebook.com. But it doesn't have to be that way: everyone can run their own mail server. Federation makes for more complex protocols, but is required for freedom.

A decade ago, the only way to use instant messaging was through a service like ICQ, AIM or MSN. Today, everyone can run their own Jabber server, and the servers communicate using the XMPP protocol, the same way as you can run your own mail server, which communicates using the SMTP protocol.

A federation of independent servers means that no single entity can control huge numbers of users. This has huge advantages: no single business can pull the plug on a service, or change it in ways the community doesn't like. Competition gets to drive down the price and/or improve the user experience. Last but not least, users have a choice of jurisdictions and can use servers in a country with at least a modicum of privacy protection on the books.

This talk will outline the challenges an opportunities for federation in areas such search and social networking (Twitter!), drawing inspiration from the XMPP and SIP protocols. It is a high level talk about fundamental choices—there will be no header diagrams or DTDs.

Permalink - posted 2013-07-15