Parallel::ForkManager v2: the moonager cometh

August 22nd, 2018

Parallel::ForkManager v2: the moonager cometh

I did a thing. Which might end up be one of the achievements engraved under my name at the CPANtheon, or might cause a swift pitchfork-engineered crowd-sourced demise.

Or both.

Yeah. Both is definitively a possibility.

Anyway. By the time you read this, I will have pushed a new trial version of Parallel-ForkManager to CPAN. This new version flips the major version number from 1 to 2, which has been a sign since medieval times that holy heck, we better hide children and livestock down the cellars ‘cause something dark is coming.

In this instance, the big, huge change is an overhaul of the package to Moo.

So, what does that mean for you?

Good news

If you are a casual user of Parallel::ForkManager, you probably won’t even notice it. I took some effort to ensure that the API didn’t change. To the point that the test cases are still passing unmodified. Of course, disclaimer: while I think the old code should still run without any problem, I’ve been wrong before.

More good news

If you ever wished that Parallel::ForkManager would use a different communication mechanism between parent and children, or that you wanted to tweak some behavior, the change to Moo makes it ridiculously easy to go wild.

For example, you’d like the children to bail out using POSIX::_exit instead of the regular exit()? That’s how you can do it:

use Parallel::ForkManager;

package Parallel::ForkManager::Child::PosixExit {
    use Moo::Role;
    with 'Parallel::ForkManager::Child';

    sub finish  { POSIX::_exit() };

my $fm = Parallel::ForkManager->new(
    max_proc   => 1,
    child_role => 'Parallel::ForkManager::Child::PosixExit'

Want your parent/children communication go over a REST service?

package Parallel::ForkManager::Web {

    use HTTP::Tiny;

    use Moo;
    extends 'Parallel::ForkManager';

    has ua => (
        is => 'ro',
        lazy => 1,
        default => sub {

    sub store {
        my( $self, $data ) = @_;

        $self->ua->post( "http://.../store/$$", { body => $data } );

    sub retrieve {
        my( $self, $kid_id ) = @_;

        $self->ua->get( "http://.../store/$kid_id" )->{content};

my $fm = Parallel::ForkManager::Web->new(2);

   # usual stuff goes here

Under the hood

I’ll keep the technobabble short. The overhaul to Moo mostly collapses a lot of the code into object attributes. The bit of new cleverness I injected is that, instead of having functions trying to figure out if they are in the parent or child process, I now have the forked incarnation of the ForkManager object consume a Parallel::ForkManager::Child role that augments the original object with childish behaviors.

TRIAL and errors

I am usually rather cavalier about major API changes on my distributions. The prerogative of authors who are also the de facto quorum of their userbase, I guess. But Parallel::ForkManager is different. Since it is a rather large fish in the CPAN River, I ought to be slightly more poised and tactful.

Which is why this new version has been released as a TRIAL, and it’ll remain so for at least one month. If by the end of September I didn’t receive any panicked feedback, I’ll take a deep breath and hit the fo’ real release button. If there is any panicked feedback, it’ll be addressed, and the clock will be set back.

That is to say, TL;DR: you do use Parallel::ForkManager, do yourself a favor, download the trial version and give it a whirl. Because it’s incontestably better for anyone if you discover that I’m a genius on that trial release rather than an utter idiot on the next general-use release.