Re^5: Making an automatic counter adder for all webpages

[polettix]

Nothing prevents you from using the full file name in the "pagename" column :)

Regarding how to get the page name, you provided little insight about how a page name is "generated"; in any case, whatever your approach (the one suggested by Fletch, or the ALTER TABLE one), you have to find some way to decide which counter you want to increase. Which means that this is not a problem tied to this specific solution. If the page name is tied to the current script, anyway, you might benefit from the contents of either $0 (perlvar) or of __FILE__ (perldata), whatever applies best to your case.

I'd make the "pagename" column a primary key for the table, and just do something like this:

# Unconditionally try to create a new record.
eval {
   $db->do('INSERT INTO counters (pagename, pagecounter) VALUES ($, 0)
+',
      undef, $pagename);
};

# Unconditionally increase the counter
$db->do('UPDATE counters SET pagecounter = pagecounter + 1 WHERE pagen
+ame = ?',
   undef, $pagename);

If the record for the given page does not exist, it's created and the counter is initialised to 0. If it already exists, the INSERT fails due to the fact that the pagename column is a primary key, and you can't have two records with the same primary key. In either case, after the eval you're sufficiently confident^* that a record for the given page actually exists in the table.

After this, you increase the counter. If the record was just created, it is correctly bumped to 1. In the other case, it is simply increased, which is what you want.

No checks (the RDBMS does these for you), no race conditions... maybe a little dirt? Acceptable, IMHO.

^* "sufficiently confident" means that the INSERT could fail for other reasons apart the field being there. If all the rest is working properly, than you can be confident that after the eval you'll have something to increase in that table.

[Nik]

I can't beleive it can be done in a so few lines!

I though i had to connect to the database table and check if the specific field is present at the table 'counters' but with the eval you give i dont have too....

Unfortunately i didn't understand how the eval() is working here and what the 'undef, $pagename)' is doing after the eval(). Inever used eval before!

Eval is trying to evaluate the statement closed within its parenthesis? Can you explain a bit more if its possible?

The $ stands for the name of the current webpage thst is running? is it with or witout the '.html'? Also because you ask we dont need to know how the current webpage loaded just its name.

in your second statemtn alsoin, the increament $db->do(), you say where pagename = ?, whats the value of ?

$db->do( 'CREATE TABLE counters   (pagename text primary key, pagecoun
+ter int default 0)' );

eval {
   $db->do('INSERT INTO counters (pagename, pagecounter) VALUES ($, 0)
+',        undef, $pagename);
};

$db->do('UPDATE counters SET pagecounter = pagecounter + 1 WHERE pagen
+ame = ?', undef, $pagename);

Also i forgot to tell you that i also need to print the current webpage's counter to the output after the increment so i need to store it in a variable called $pagecounter.

print table( {class=>'info'},
        Tr( td( {class=>'lime'},    a( {href=>'/cgi-bin/show.pl?name=s
+howlog'},  h1( {class=>'lime'}, $host )))),
        Tr( td( {class=>'yellow'},  $display_date)),
        Tr( td( {class=>'cyan'},    $pagecounter ))
);

[polettix]

There is a lot of context that I left out, let's proceed in order.
• I'm assuming that the counters TABLE is already present in the database, so you don't need to create it every time you want to increase a counter. Creating tables is something that I usually avoid doing inside the code: I create them "offline", then assume that they're there when coding.
• I usually put a RaiseError => 1 when connecting to the database. This means that any error (e.g. trying to insert a duplicate record which violates the primary key constraint) just dies. To prevent this in this specific case (in which the error is something that I consider "physiological"), I put an eval block around the $db->do(...) statement, which executes the code block but doesn't exit from the program. This is the Perl way to exception handling, you can read all this in eval's documentation.
• I made a mistake and used a $ instead of ? in the UPDATE query, sorry!
• The $db->do(...) can often be used to merge the prepare and execute phases into a single call, which can be easier to both write and read. The "extended" interface call is $rv  = $dbh->do($statement, \%attr, @bind_values); (see DBI docs), which means that I'm passing an undef attribute hash reference \%attr. So,

  $db->do('UPDATE counters SET pagecounter = pagecounter + 1 WHERE pagen
  +ame = ?',
     undef, $pagename);
  

  is equivalent to

  my $sth = $db->prepare(
     'UPDATE counters SET pagecounter = pagecounter + 1 WHERE pagename =
  + ?');
  $sth->execute($pagename);
  

• Regarding the pagename value, what I'm saying is that you can use the full name instead of chopping ".html" out. As long as different pages have different names, and the same page has the same name, you can use whatever you like to name them inside the table.
• After the two lines of code to increase the counter, you can grab the counter directly from the database, with a simple query:

  my ($counter) = $db->selectrow_array('SELECT pagecounter FROM counters
  + WHERE pagename = ?', undef, $pagename);
  

  As you can see, there's a lot to read in the DBI documentation ;)
Summing it up:

sub increase_pagecount_for {
   my ($db, $pagename) = @_;

   eval { # Just ignore errors if the record already exists
      $db->do('INSERT INTO counters (pagename, pagecounter) VALUES (?,
+ 0)',
         undef, $pagename);
   };
   $db->do(
       'UPDATE counters SET pagecounter = pagecounter + 1 '
        . ' WHERE pagename = ?',
      undef, $pagename
   );
   my ($counter) = $db->selectrow_array(
      'SELECT pagecounter FROM counters WHERE pagename = ?',
      undef, $pagename
   );

   return $counter;
} ## end sub increase_pagecount_for

[Nik]

Thank you very much for the detailed explaining, a more simple, less code needed and straightforward approach that the initail if(...) structure i had in mind

I didn't knew i could pass variables as arguments to $db->do(...) like that, i was under the impression that i only was used when one had a fixed mysql statement with all arguments known (no vars)! If not i was using prepare and execute, i mean in case of unknown variables like '?' for '$pagename'. Can the 'undef' be ommited too as 'statemnt,,@bind_vars' ? The 'undef' is still a mystery to me on what it does although i read the help section.

Another question is that i didnt understand why you uses this version of select:

   my ($counter) = $db->selectrow_array(
      'SELECT pagecounter FROM counters WHERE pagename = ?',
      undef, $pagename
   );

If i wanted to dot he same i would do something like:

   my $counter = 'SELECT pagecounter FROM counters WHERE pagename = ?'
+,
      undef, $pagename
   );

or

my $sth = $db->prepare('SELECT pagecounter FROM counters WHERE pagenam
+e = ?');
   $sth->execute($pagename);

Why the need for ($counter) and the method $db->selectrow_array ?