sub flush {
    my $self         = shift;
    my $session_file = yaml_file( $self->id );

    open my $fh, '>', $session_file or die "Can't open '$session_file': $!\n";
    flock $fh, LOCK_EX or die "Can't lock file '$session_file': $!\n";
    set_file_mode($fh);
    print {$fh} YAML::Dump($self);
    close $fh or die "Can't close '$session_file': $!\n";

    return $self;
}