I needed arbitrary precision math in PHP, and wasn’t willing to rebuild PHP to add the bcmath extensions. All hail backticks.

function bceval($expr) {
  return trim(`echo "$expr" | bc`);
}

Used like so

$end = bceval("$start + $batchsize - 1");

Wet cat territory.

This morning I needed to read from a file line by line from the bottom. In PHP. Perl, of course, has a module to do this. A quick view source decided that I didn’t want to get into file seeks before breakfast. Very happy with my solution:

$file = popen("tac $filename",'r');

while ($line = fgets($file)) {
  echo $line;
}

Collapsing the GTalk buddy list, and Reader sharing list was a serious blunder, and one that could use a bit more ink spilled about it. But one click sharing is one of my favorite Reader features.

GData Won’t Save You

Except there is a bit of a problem. I don’t really want to share with other Google Reader users, I’m not even sure I’m destined to be a long time Reader user. I want to share links the way I’m already doing it, through del.icio.us.

No problem, Reader has an Atom feed of shared items. A really good feed, with the source info maintained, well formed, nicely done. Simplest thing in the world to parse the feed, and write the entries back to del.icio.us. And I can tag any post in Reader, which is perfect, easy Ajaxy sharing into del.icio.us with a few minutes work.

Except for reasons I can’t fathom Reader isn’t including my tags in the Shared Items feed. Which all of a sudden makes my data feel a bit more locked up and trapped then I’d really like.

For Our Sins

Casting around a bit for a solution, I noticed the “Email” button, which allows me to send a link via email, along with a short note, and so “Email to del.icio.us” was born.

Super quick and dirty Perl script that:

1. Parse the Google Reader HTML email for the relevant URL (no semantic markup, alas)
2. Pull the del.icio.us link description from the subject
3. Look for a line beginning “tags: ” followed by a space separated list of tags.
4. Look for a line beginning “note: ” for the extended description.

Add the following rule to /etc/aliases file, and away you go.

to_del: | /home/you/email_to_del.pl

Takes 10-15 seconds vs 1 second to share, but much more flexible.

And Perl is still unbeatable when it comes to these kind of scripts.

Mike released HTTP_Request_OAuth today, so I spent a little while this evening coding up Service_Twitter as helper class for making OAuth authorized requests against the Twitter API.

Both are early enough in the dev cycle to be called proof of concepts.

Mostly I wrote it because I had always envisioned there being wrapper libraries around the low level OAuth implementations that wrapped the calls, and constants, and as Mike graciously went out and wrote a low level library I felt compelled to write a wrapper.

Also twittclient, an interactive client for getting an authed access token, essential to bootstrapping development.

And nota bene, HRO currently only supports the MD5 signing algorithm, which is undefined in the core spec, and subject to change. (Just in case you didn’t believe me about the early state of things.)

update 2008/4/18

This code no longer works because Twitter has taken down their (slightly non-compliant) OAuth endpoint. When they add OAuth support back in, I’ll link to it.

j = Jabber::Simple.new(jid, pass)

j.received_messages.each do |mesg|
    if mesg.body.match(/austin|sxsw/i)
        sender = mesg.elements['//screen_name'].text
        j.deliver('twitter@twitter.com', "leave #{sender}")
    end
end

Though truth be told, it isn’t running.

Looking over the PHP5.2 changelog I noticed that somewhere along the way PHP5 seems to have picked up a provocatively named pair of classes, DateTime and DateTimeZone.

There is something fundamentally brash, brazen even, to releasing a class named DateTime. As a calendar geek I imagine upon seeing “new DateTime()” I feel something akin to what an old thespian feels when they see a company putting on a production of the Scottish play — it’s a decidedly mixed emotion. But I’m going to bump my way through learning how to use this new DateTime lib, bringing all my preconceptions about how it should work. The odds of this being interesting to you is probably nil unless you’re in one or two very small cliques, feel free to read on, or browse away.

I’m primarily working in PHP4 right now, so my first step was to grab a copy of MAMP 1.5b getting me a nice PHP5.2 sandbox to play with.

The new objects are documented here, apparently there are functional equivalents for each of the object methods, and they use the PECL timezomedb.

Hey! timezonedb! First fence cleared! A timezone database compiled into a native format based on Olson is the one true solution, and I can update it independently, the most recent release being based on 2007b. Sweet.

Constructor takes an initialization string that it passes to strtotime(), and an optional DateTimeZone obj. Defaults to “now”

$date = new DateTime();
echo $date . "\n";
> Object of class DateTime could not be converted to string 

Oops, no __toString() method defined. You’ll need to use the format() instance method. If you end up using the DateTime objects, you’ll be seeing a lot of format(), more on that in a bit.

format() uses the date() formatting strings (not the strftime format strings). Also takes a number of useful constants, most usefully your pal and mine RFC3339 (aka W3CDTF aka Dublin Core/Atom date format).

echo $date->format(DATE_RFC3339) . "\n";
> 2007-02-22T15:23:47-05:00

Note: thats a constant, if you pass in the string ‘DATE_RFC3339′, and you’ll get odd looking results.

Here we can see the default constructor sets both the time and a timezone — correctly, for the moment, identifying my timezone as America/New_York. That’s somewhat contentious behaviour, some people will tell you that dates with unspecified timezones should either be in UTC or be “floating”, divorced from any timezone. Why? At least in part because across platforms and boxes timezone guessing is going to be non-deterministic — the script that worked when you ran it locally on your Mac laptop in New York, might fail on your ISP’s servers. You get a hint of this reading over the timezone guessing rules on date_default_timezone_get. There is also the fact that I’m currently moving at about 400mph and will be in a different timezone real soon now. However you can set the default to something reasonable in a script, or in the php.ini. (consider this my recommendation)

date_default_timezone_set('UTC');
$date = new DateTime();
echo $date->format(DATE_RFC3339);
> 2007-02-22T20:44:49+00:00

Yay, that worked. Okay, but lets display that datetime in the local timezone. (after all the point of this entire exercise will be the ability to work painlessly in multiple timezones).

$date->setTimezone('America/New_York');
> DateTime::setTimezone() expects parameter 1 to be DateTimeZone

Siiiigh. Not smart enough to cast strings into TimeZone objects (holds true for the constructor as well, so no new DateTime('now', 'UTC')). Now its time to learn how to use DateTimeZone.

Working with DateTimeZone, All Hail Olson

I mentioned briefly earlier that PHP is now shipping with an extension timezonedb, which is a compiled version of the Olson database. The Olson database is a massive, largely volunteer effort to catalog the various timezones both in use, and those that have been in the past. Time is a political issue, particularly day light savings, and as such the rules governing it are arbitrary, whimsical, and subject to frequent change. (p.s. gotten a panicked memo yet about new daylight savings compliance for March 11th? No? Where did you say you worked?)

Note: Olson also uses a longer form of the zone names then we usually see in the U.S., this is to combat ambiguity. See Appendix H for a list of timezone names, including some handy shortcuts.

$tz = new DateTimeZone('America/New_York');
$date->setTimezone($tz);
echo $date->format(DATE_RFC3339) . "\n";
> 2007-02-22T16:02:55-05:00

This is starting to get long winded, but, hey, PHP5 supports object dereferencing on returns. Maybe this will work.

echo $date->setTimezone($tz)->format(DATE_RFC3339) . "\n";
>  Call to a member function format() on a non-object

Nope. Oh well.

Date vs Datetime?

Say I’ve got a nice platonic date, say November 11th. There is no time element associated with this, so timezones are kind of irrelevant. I mean Nov. 11th starts at different times through out the world, but Nov. 11th is universal. (as long as you’re using the same version of Gregorian as most of the rest of us) Ideally this date would float above timezone issues, but that isn’t how PHP does it, 2007-11-11 is treated internally as midnight on the 11th, which is certainly simpler, but disappointing. You can prove this like so:

$date = new DateTime('2007-11-11');
$date->setTimeZone($tz);
echo $date->format(DATE_RFC3339) . "\n";
> 2007-11-10T19:00:00-05:00

The other useful DateTimeZone method is getOffest()

echo $tz->getOffset($date); 
> -18000

Daylight Saving, March 11th, and Why Programmers Are a Grouchy Lot

Note: getOffset, which returns a timezone’s offset in seconds from UTC, takes a DateTime obj because offsets can be date sensitive due to daylight savings. Really without daylight saving this stuff would all be pretty straightforward. Let’s test to make sure the offsets are correct at the boundary.

echo $tz_nyc->getOffset(new DateTime('2007-03-11 1:00')) . "\n";
echo $tz_nyc->getOffset(new DateTime('2007-03-11 2:00')) . "\n";
> -18000
> -14400

(-18000/(60*60) == -5 hours) 
(-14400/(60*60) == -4 hours) 

Yay! They got the memo about U.S. Energy Policy Act of 2005.

The Basics: Accessors and Mutators

So what are some other basic desires?

Get epoch seconds! Except for their kind of limited range epoch seconds are great, and have helped a generation of programmers put off worrying about timezones as long as possible. They’re also the backbone of PHP’s traditional date/time methods.

Alas, there isn’t an accessor method for getting epoch seconds, you’ll have to use format().

In fact DateTime doesn’t expose any of the accessors you’d expect, so you’ll be using format a lot if you want to access pieces of your date. (for you know, display purposes, or manipulation, or building queries, or pretty much doing anything you’d want to do with a date)

examples of the format() as all purpose accessor pattern:

epoch:  $date->format('U'); // 1173596400
year:   $date->format('Y'); // 2007
month:  $date->format('n'); // 3
day:    $date->format('j'); // 11
dow:    $date->format('l'); // Sunday

… etc …

So now you have accessors for the full range of date() formatting strings. You just have to jump through a hope.

Pretty much the only accessor is getTimezone()

echo $date->getTimeZone();   // hope springs eternal!
> Object of class DateTimeZone could not be converted to string
echo $date->getTimeZone()->getName() . "\n";
> America/New_York

Mutators

Speaking of accessors, DateTime is a little sparse on mutators as well: setTime(), setDate(), and the mysteriously named setISODate().

$date->setDate('2007', '1', '1')->format(DATE_RFC3339);  // who am I kidding?
> Call to a member function format() on a non-object 
$date-> setDate('2007', '1', '1');
echo $date->format(DATE_RFC3339) . "\n";
> 2007-01-01T02:00:00-05:00

Now what if I want to set just the day?

Maybe

$date-> setDate(null, null, '11');
echo $date->format(DATE_RFC3339) . "\n";
> -001-12-11T02:00:00-05:00

Nope.

Instead you’ll need to pull out the year and month (using our format() accessors) and pass those back in just to set the day.

$date-> setDate('2007', '1', '1');   // jan 1.  
$date->setDate($date->format('Y'), $date->format('n'), 11);
echo $date->format(DATE_RFC3339) . "\n"; 

Clunky.

setTime() works the same, but for time.

e.g. Setting just the minutes, 33 minutes past, but keep hours, and seconds constant:

$date->setTime($date->format('G'), 33, $date->format('s'));
echo $date->format(DATE_RFC3339) . "\n"; 
> 2007-01-11T02:33:00-05:00

So what is an ISODate? I’m unclear, and so is PHP’s documentation. The docs show the call signature taking a $year, $week, and optional $day, while the description talks about $year, $month, $day. Looking at the code looks like $week is the proper call, $month is cut and paste error from setDate(). So I guess this is a method for setting day by the “week of the year” a concept more popular in Europe then in the US. Not sure what ISO has to do with it. So what is our current week of the year?

echo $date->format('N') . "\n";  // 'N' is new in 5.1.0
> 4 

Jan 11th was in the 4th week of the 2007? Go figure.

$date->setISODate(2007, 4, 5);  // fifth day of the 4th week?
echo $date->format(DATE_RFC3339) . "\n";
> 2007-01-26T02:33:00-05:00

Um. You know what? You’re on your own with setISODate, sorry.

Date Math: Adding and Subtracting Deltas aka $date->modify($str)

PHP5 for better or worse has very limited operator overloading, so no $dt1 + $dt2 * $dt3 / $dt4. Instead the primary method for doing math is modify()

Thankfully PHP’s strtotime() method is a gem, and one of the things it handles is relative dates. strtotime() + relative dates is the secret to doing math with PHP5’s DateTime.

Lets get a basic date to start with:

$date = new DateTime('today');
echo $date->format(DATE_RFC3339) . "\n";
> 2007-02-22T00:00:00+00:00

Note: modify() is destructive. It changes the original datetime object (as the name suggests). You’ll need to jump through some hopes to keep a copy of your original date. More later.

Add/subtract N days:

foreach (range(1,10) as $n) {
   $date->modify("+1 days");
   echo $date->format("Y-m-d") . "\n";
}

> 2007-02-23
> 2007-02-24
> 2007-02-25
> 2007-02-26
> 2007-02-27
> 2007-02-28
> 2007-03-01
> 2007-03-02
> 2007-03-03
> 2007-03-04

$date->modify("-10 days");
echo $date->format("Y-m-d") . "\n";

> 2007-02-22

$date->modify("-1 month");
echo $date->format("Y-m-d") . "\n";
> 2007-01-22
// or alternately
$date->modify("1 month ago");
echo $date->format("Y-m-d") . "\n";
> 2006-12-22

Cloning DateTime objects to work around modify

Of course you usually want to keep the original when doing date math, so modify()’s lack of idempotentce is annoying. Lets say I’m building a SQL query to select events happening in the next 7 days.

In an ideal world the code would like this:

$start = new DateTime('today');
$end = $start + 7;
echo "select between " . $start->format('Y-m-d') . " and " . $end->format('Y-m-d') . "\n";

The above of course is just a pipe dream. But wouldn’t it be nice?

I’d settle for:

$end = $start->calc("+7 days");

Or even:

$end = $start->clone->modify('+7 days');

None of the above examples remotely work.

Instead use:

$start = new DateTime('today');
$end = clone $start;
$end->modify('7 days 3 minutes 42 seconds ago');

Now format our SQL query for our example:

echo "select between " . $start->format(DATE_RFC3339) . " and " . $end->format(DATE_RFC3339) . "\n";
> select between 2007-02-22T00:00:00+00:00 and 2007-02-14T23:56:18+00:00

Awkward, but it gets the job done.

At least the relative date format is super flexible and expressive. As far as I know the closest thing to documentation is from the GNU tar manual on date input formats. (just like CVS) Btw. if you ever want nightmares, take a look at the scan method in PHP’s parse_date.c and be thankful that isn’t your job to maintain

Date Math: Comparison and Differences

Beyond adding deltas (”+7 days”), the other common date math is comparing two datetimes, to find out which is more recent, and getting the difference between them. DateTime supports no methods for comparing two datetimes. The simplest solution for doing comparison is to compare epoch seconds.

Note: This method only works for dates that can be represented by epoch seconds. PHP uses a signed int for epoch seconds, so the range is limited by the size of the max int on your platform. Generally you get approximately 138 years, 1901 to 2038. There are other schemes besides epoch seconds for mapping dates to an easily comparable number; MJDs, and Tai time being two. See also Rheingold & Dershowitz 1997

$d1 = new DateTime("today");
$d2 = new DateTime("tomorrow");
if ($d1->format('U') < $d2->format('U')) {
   echo "true\n";
} 
> true

If you’re going to be comparing a large number of dates you might consider a memoization technique like the Schwartzian transform.

We can get the difference in seconds using the same hack of casting to epochs.

echo $d2->format('U') - $d1->format('U') . "\n";
> 86400

Ideally we’d then divide the difference seconds to get the difference in hours, days, weeks, or months. However the following naive solution won’t work.

$diff / (60*60*24);  // calculate difference in days, **BADLY**

Why not? Because days don’t always have 24 hours. Sometimes they have 23 hours, sometimes they have 25. Daylight saving strikes again. (If you want to be even more pedantic, minutes are also not 60 seconds long, sometimes they’re 61 seconds long if we have a leap second)

Basically you need to break yourself of thinking of datetime units as being fungible. You can’t simply calculate minutes from seconds, or days from hours. Just like you can’t divide days by 30 to get an accurate number of months. There are solutions, but they’re a bit beyond this blog post.

new DateTime from Epoch Seconds

So, non-fungible, remember that.

But sometimes you’ve cast DateTimes down to epochs to do math. And then you’ll want to cast back to a DateTime.

Alas DateTime doesn’t have a constructor that takes an epoch, and passing a epoch to the default constructor will throw an exception, rather you want:

$from_epoch = new DateTime(date('c', '-568080000'));
echo $from_epoch->format('Y-m-d') . "\n";
> 1952-01-01   // expected result

Conclusions

DateTime/DateTimeZone get timezones right, and for solving that hard problem they deserve all possibles accolades.

The rest of the API however is kind of simplistic, awkward to work with, and verbose.

Single most useful change: have DateTime methods actually return the object making it possible to use a slightly more abbreviated calls.

I had thought about writing up a few more recipes, like nth dow of the month, and such. But we were coming in for descent, and it was time to be done. Might happen in the future.

Also if anyone has any power to enhance the DateTime object, I hope some of the above can act as a road map for a more expressive and powerful core library. Or ping me, happy to chat.

And while we’re talking about recent hacks, Blaine and I whipped up a Jabber bot using his Jabber::Simple and the Yahoo weather feeds, to provide twice daily weather updates via Twitter.

Jabber is an intriguing platform to build on top of, and the more I play with it the more potential I find. I keep checking in on it every few years (since MetaEvents days), but recently its gotten much more interesting. In part thats Google’s adoption of the standard (and the subsequent enhancement in tools, libraries, and clients), and partially standards bake slowly, but at the core of it I think we’re reaching a point in the evolution of the Web where Internet-scale deployed messaging standards have a lot to offer of us. A protocol for when HTTP fails you.

If you follow these bots, you’ll receive those updates wherever you normally get your Twitters; IM, Phone, RSS, or just on the web. So far, we have bots for the following cities: Boston, Brighton, Chicago, Helsinki, London, Los Angeles, New York, Paris, Portland, San Francisco, Seattle, Singapore, and Vancouver. If you’d like to see another city, just ask and we’ll provide.

Slightly out of date source available at twitter-weather - Google Code

And taking requests for new cities. Probably do a big batch of new ones sometime next week. (not really an automated process)

Photo by bonsaikiptb

One of the side effect of overloading (perverting?) the Wordpress category system to do tagging is you end up with over 1000 categories. The posting interface gets unhappy, and the wp_list_cats template tag becomes pretty much useless.

(This post is another Wordpress meta post, one of several I’ve got queued up in my head, and probably interesting to 3 people in the known universe, but here you go.)

better_cat_lists is Wordpress plugin that adds a couple of more flexible methods.

wp_list_popular_categories

works like wp_list_cats, but only categories with $cat_threshold posts in them.

wp_list_popular_categories('sort_column=name&cat_threshold=20');

wp_list_recent_categories

works like wp_list_cats, but only from posts with the last $n days. You can also limit the total number with $cat_limit

wp_list_recent_categories('days_ago=180&cat_limit=80');

Both methods use the same hack, they overload the list_cats_exclusions callback to do positive match instead of the intended negative match - appending and cat_ID in ($cat_ids) to the exclusion string. PHP at its finest, quick and dirty, like monkey patching but without language level support for it.

Doing some PHP hacking tonight, I was missing Rails’ “flash”. So I coded up a quick and dirty implementation of the read-once session status notification pattern. The code

I use Rails’ database session backend for LM. (for login, as well as “flash”) Without any sort of built in garbage collection the sessions table gets very large, very quickly. Beyond aesthetic issues, this can also cause MySQL’s key buffer to fill up. (which on Debian is by default set quite low)

So I wrote up a quick flush method, and saved it in a file models/session.rb.

class CGI::Session::ActiveRecordStore::Session
  def self.flush_old_empty_sessions
     self.delete_all "DATE_SUB(NOW(),INTERVAL 6 HOUR) > 
     updated_at and BIT_LENGTH(data) <= 688"
  end
end

This says nuke all sessions which are over 6 hours old, and which are empty. (688 is the length of the serialized session with an empty flash)

MySQL specific, and susceptible to changes in either session structure or its serialization. But it was quick and easy and worked for me.

Then you simply need a cron job like: ruby script/runner 'CGI::Session::ActiveRecordStore::Session.flush_old_empty_sessions'

Trying to use:

pear install Some_Package 

but getting

No release with state equal to: 'stable' found for Some_Package?

Some people will tell you to try:

pear install -f Some_Package 

(i.e. force install) A better solution is:

pear remote-info Some_Package

You’ll see near the top the latest version, e.g. Lastest: 0.8RC3.

Then you can do:

pear install Some_Package-0.8RC3

Sometimes you’ll already have a package installed (e.g. "Package 'Foo_Package' is already installed"), but need to upgrade to a unstable version.

pear remote-info Foo_Package

Package details:
================
Latest      1.0.2
Installed   1.0.1
...

pear install -f Foo_Package-1.0.2

At work we’re using Class Table Inheritance to model the core data structures of our as yet nameless open source CRM. (actually it has a code name, but I don’t like it, so we’ll pretend it’s nameless)

This week as I learned both the name of this pattern, and the SQL to implement it efficiently in MySQL I thought I’d share some notes on what we’ve come up with.

Problem: You want to display the first N characters of a string.

Solution 1: Use substr. Unfortunately substr will more often then not leave a word fragment dangling at the end of your new string, which looks broken.

Solution 2: Use substr, but add an ellipsis (…) at the end of the string. Now the user has a clue as to why there is a dangling word fragment, but the ellipsis is misleading if the your original string was actually less then N characters long initially.

Solution 3: Check if the string is longer then N characters, and if it is use Solution 2, otherwise leave it alone. This is an improvement, but in practice those dangling fragments, even with their ellipsises are kind of unsightly.

Solution 4: Try to find a natural breaking point within the desired crop length, and break on that. If not found revert to Solution 2. Below is some quick code to do this in PHP. It uses strpos instead of regex for speed. It doesn’t handle “quoted text” intelligently, and it doesn’t look for paragrah breaks (I’m using it for shorter crop lengths, 100-150 characters), but it is a starting point. Also when cropping strings, it is often important to strip the HTML from them first, or you’re essentially guarenteed to end up with broken HTML.

function crop($str, $len) {
    if ( strlen($str) <= $len ) {
        return $str;
    }

    // find the longest possible match
    $pos = 0;
    foreach ( array('. ', '? ', '! ') as $punct ) {
        $npos = strpos($str, $punct);
        if ( $npos > $pos && $npos < $len ) {
            $pos = $npos;
        }
    }

    if ( !$pos ) {
        // substr $len-3, because the ellipsis adds 3 chars
        return substr($str, 0, $len-3) . '...'; 
    }
    else {
        // $pos+1 to grab punctuation mark
        return substr($str, 0, $pos+1);
    }
}

Several months ago, inspired as I recall by Kate’s How to Avoid Writing Code article, I started playing with Class::DBI and Template Toolkit as a rapid development environment.

Not So Fast

  our $FIELDPROFILE = {
          fields  => 
                  [qw(name widgetstyle note expirationdate) ],
          types   => { 
                  widgetstyle     => 'select', 
                  note             => 'textarea', 
                  expirationdate  => 'date'
          },
          options  => {
                  widgetstyle     => [qw(blue green chrome)]
          },
          required  => [qw(name widgetstyle)]
  };

  sub new {
          my $class = shift;
          my $self = $class->SUPER::new(@);
          $self->addfields($FIELDPROFILE);
          $self->submitfield('addwidget');
          return $self;
  }

  (else where in your application)

  my $form = MyApp::AddWidgetForm->new({query => $query, ...});
  if ($form->issubmitted and $form->validates ) {
          MyAdd::Widget->newwidget({
                  name => $form->name->value,
                  widgetstyle => $form->widgetstyle->value,
                  note => $form->note->value,
                  expiration-date => $form->expirationdate->value

          });
          displaywidgetaddedconfirmation();
  }
  else {
          # form hasn't been submitted, or we encountered an error
          displaypage($template, { form => $form } );
  }

  (mean while, in a nearby template)

  [% IF form.errors %]
  <ul class="errors">
    [% FOREACH e = form.errors %]<li>[% e %]</li>[% END %]
  </ul>
  [% END %]

  Name:       [% form.name.html(size => '40') %]<br />
  Style:      [% form.widgetstyle.html(default => 'blue') %] <br />
  Expiration: [% form.expirationdate.html %]
  Note: <br />
  [% form.note.html(rows =>4, cols=>50) %]<br />
  <input type="submit" name="[%form.submitfield%]" value="Add Widget" />               


...,

Calendaring can be a fraught and tricky business. Probably doesn’t compare to building software to do precise robotic control of surgical tools (though I have a friend who worked for a summer putting Windows into operating rooms), or high volume real time transaction management, but its does have its pitfalls and tricks.

The Case of the Missing Timezone

A classic mistake when one sets out to write calendar software, particularly web calendar software, is to ignore timezones. “Now” is treated as the current time on the web server, new events are added to the database without reference to the timezone of their creator. And amazingly, by and large this will work, for a while. Often your first set of users will be in your timezone, or thereabouts, perhaps you aren’t displaying hours, so it’s just a few hours early in the morning, and late at night when people notice your calendar is displaying the wrong date.

Problems start to crop up as your audience becomes more international, or you start trying to add more time sensitive services. So you try to retro-fit timezones, add them on. And now you’re living in a world of pain.

I was arguing recently that implementing a conditional GET with LWP is trivial and there was no reason why someone wouldn’t support it. I assumed there must be a dozen examples of how to do this. Afterall O’reilly has “open sourced” their original LWP book, there is an LWP cookbook, and reams of POD.

No Such Luck

Conditional GET

This is something of a toy example, but I try to be as correct as possible with it. Noteable in its absence is doing anything with the file you’ve fetched. (for example parsing and storing an RSS feed) Also I use a simple file to store ETag and Last-Modified, you might want to use a different backend.
See the Code

Example Code

use LWP::UserAgent;
use HTTP::Request;

my $url = "http://localhost/rss/laughingmeme.rdf";
my $cache_file = 'cache';
my %headers;

if ( -e $cache_file ) {
    open (CACHE, "< $cache_file") or die "Couldn't open: $!";
    %headers = (
        If_None_Match => <CACHE>,
        If_Last_Modified => <CACHE>
    );
    close CACHE;
}

my $ua = new LWP::UserAgent();
$ua->agent("Conditionally Enabled v0.1");

my $req = HTTP::Request->new( GET => $url );
$req->header(%headers);

my $res = $ua->request($req);
if ($res->is_success) {
    print "new!\n";
    # save ETag & Last-Modified
    open (CACHE, "> $cache_file") or die "Couldn't open: $!";
    print CACHE $res->header('ETag'), "\n";
    print CACHE $res->header('Last-Modified'), "\n";
    close CACHE;
}
elsif ( $res->code() eq '304' ) {
    print "not modified, go to cache\n";
    # do logic for RSS not modified
}
else {
    print "fooey! somthing went wrong\n";
}

So I wrote up my own little SOAP client to Allconsuming, which, while not nearly as cool as booktalk, works nicely to maintain my little READING sidebar. (though as you can see in the case of Applying Patterns there are still some aesthetic tweaks to make). Get the script and the template.

By the way, it looks a little different then DJ’s because SOAP::Lite’s autodispatch+ feature breaks Template Toolkit (and is kind of icky anyway)