Having fun with invisible ink

▼ A month ago, I got a message from the OHM2013 speakerdesk:

Since you you'll be giving a presentation, lecture or workshop at OHM2013 (for grownups), we'd like to ask you; do you feel like giving a little talk about your subject for kids?

Thinking that my talk (about federation of online services, where you can have a choice of service providers or even run your own server rather than be beholden to big companies like Google, Facebook and Twitter (PDF of the slides, explanation in Dutch) might be a tad too abstract for kids age 6 - 10, I proposed something else instead: writing secret messages in invisible ink.

So I got to experimenting a bit. A well-known type of invisible ink is lemon juice or sugar water. After writing with it, you heat the paper, the ink turns brown and the message appears. Another option is to write with something that is acidic or basic, and then use red cabbage juice to bring out the message. This has the advantage of not requiring a heat source. So I got a cabbage, cut it into small pieces and cooked it with only a little water to obtain the dark purple juice. You can then easily create your own litmus paper by soaking up some juice in coffee filters.

About 25 kids age 6 - 10 participated. They really do seem to get younger every year... they're so small! I wanted to get them involved, so I brought several kinds of fruit and let them squeeze out juice to be used as ink. The onion wasn't a success, it's almost impossible to get juice out of it and it's not very acidic so it doesn't make for very good ink anyway. Apples are also hard to juice by hand, but are supposed to be quite acidic if you get the right kind. The juice we got was definitely weaker than the lemon juice, though. Grapes are fairly easy to juice, but it helps to have a coffee filter. Use white/green grapes or the ink is too visible... But lemons give you the most bang for your buck by far: just cut them up and squeeze, and you get plenty of juice without much pulp and it's nice and acidic.

The kids were completely unfazed by the fact that cabbage juice turns different colors when exposed to fruit juice and soap, by the way. They also had no problem drawing pictures using the "invisible" ink. (I guess drawing is more fun than writing.) In practice, the ink is not that invisible, because you can see where the paper is wet. I brought a bunch of small brushes to use for this, and that seemed to work well.

After finishing, we let the drawings dry in the sun. Then finally we got all the kids back inside and put all the drawings on a table, and applied the cabbage juice. I put the juice in a spray container, but this didn't work all that well: the juice doesn't get everywhere evenly, and a lot gets into the air, not smelling all that great. Using a big brush to paint it over the drawings worked better.

It takes a minute or two for the reaction to happen, turning the cabbage juice blue on unpainted parts of the paper, and red/purple where there's fruit juice on the paper. Putting cabbage juice on a drawing takes half a minute, so having all the kids come back inside before we started doing this was not the best approach with just one big brush. Having them come in as their drawing had dried would have been better.

If I'm going to do that again, I want to investigate some alkaline materials and see if we can get additional colors. Cabbage juice can be red, purple, blue, green or yellow, so there should be lots more possibilities than just the red/purple/blue that we got with fruit juices as ink. Also, it's easy enough to manufacture the cabbage juice on the spot, it doesn't have to cook long and you can just rip the leaves by hand if you don't want to use a knife. You can also buy prepared canned red cabbage and squeeze the juice out. I don't think the stains are too hard to get out, but you may not want to wear anything too new or too white.

Permalink - posted 2013-08-05

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