Lazy Loading

Lazy Loading is a technique for performance optimisation. The idea is not to load code modules into memory until you are sure they are going to be required.

For example, consider a plugin module called FrobnozzPlugin that requires the Perl modules TWiki::Contrib::Frob and TWiki::Contrib::Nozz to process %FROBNOZZ% tags. These two modules are not required by the TWiki core, so they are only loaded by the plugin.

Normally you would put the lines:

use TWiki::Contrib::Frobb;
use TWiki::Contrib::Nozz;

To avoid this inefficiency it is better to use these modules only if they are required, by lazy loading them when the tag is seen.

use vars qw ( $initialised );

BEGIN {
    $initialised = 0;
};

sub commonTagsHandler {
    $_[0] =~ s/%FROBNOZZ%/&_handleFrobnozz()/ge;
}

sub _handleFrobnozz {
    unless ( $initialised ) {
        eval 'use TWiki::Contrib::Frobb';
        if ( $@ ) {
            TWiki::Func::writeWarning("Failed to use Frobb: $@");
            return;
        }
        eval 'use TWiki::Contrib::Nozz';
        if ( $@ ) {
            TWiki::Func::writeWarning("Failed to use Nozz: $@");
            return;
        }
        $initialised = 1;
    }
    ...
}

Lazy loading can give significant performance improvements, especially where the referenced modules are large.

-- CrawfordCurrie - 13 Aug 2004

• CPAN:autouse.pm (http://search.cpan.org/dist/perl/lib/autouse.pm)
• CPAN:AutoLoader (http://www.perldoc.com/perl5.8.0/lib/AutoLoader.html)

-- WillNorris - 13 Aug 2004

on the flip side, without a good code coverage set of tests (or people that don't run those tests before commiting) lazyloading produces hidden bugs

-- SvenDowideit - 23 Jul 2006