Part nine of a series of posts about automated testing.

Mocking out time required overriding a relatively small number of core functions. For something like filesystem access, there are a wide range of system calls: open, close, read, write, seek, chmod, chdir…

It is a similar situation with databases - in both cases there is a resource which is relatively slow and expensive to access.

It is worth taking a step back and thinking about what needs testing. Often it may be possible to restructure the code itself to separate persistence store access from other business logic:

package Local::Application::ListManager;

use Moo;

has fs => (is => 'ro');

sub is_on_naughty_list {
    my ($self, $name) = @_;

    my $file = $self->fs->open( 'naughty_list.txt' );
    while (my $line = <$file>) {
        if ($line eq $name) {
            return 1;

    return 0;

package Local::Infrastructure::UnixFilesystem;

use Moo;

sub open { ... }


With the classes arranged like this, it becomes relatively easy to test ListManager, by creating a mock filesystem where naughty_list.txt contains test data.

To test the production store implementation will require a small integration test - but this was always true. The implementation of the filesystem class can be kept simple and straightforward, which should make it easier to review. Plus it will be easier to swap out for an alternative implementation later.