=pod

=encoding UTF-8

=head1 NAME

Parallel::WorkUnit - Provide multi-paradigm forking with ability to pass back data

=head1 VERSION

version 2.181840

=head1 SYNOPSIS

  #
  # Standard Interface
  #
  my $wu = Parallel::WorkUnit->new();
  $wu->async( sub { ... }, \&callback );

  $wu->waitall();


  #
  # Limiting Maximum Parallelization
  #
  $wu->max_children(5);
  $wu->queue( sub { ... }, \&callback );
  $wu->waitall();


  #
  # Ordered Responses
  #
  my $wu = Parallel::WorkUnit->new();
  $wu->async( sub { ... } );

  @results = $wu->waitall();

  #
  # Spawning off X number of workers
  # (Ordered Response paradigm shown with 10 children)
  #
  my $wu = Parallel::WorkUnit->new();
  $wu->asyncs( 10, sub { ... } );

  @results = $wu->waitall();


  #
  # AnyEvent Interface
  #
  use AnyEvent;
  my $wu = Parallel::WorkUnit->new();

  $wu->use_anyevent(1);
  $wu->async( sub { ... }, \&callback );
  $wu->waitall();  # Not strictly necessary

=head1 DESCRIPTION

This is a very simple forking implementation of parallelism, with the
ability to pass data back from the asyncronous child process in a
relatively efficient way (with the limitation of using a pipe to pass
the information, serialized, back).  It was designed to be very simple
for a developer to use, with the ability to pass reasonably large amounts
of data back to the parent process.

This module is also designed to work with AnyEvent when desired.

There are many other Parallel::* applications in CPAN - it would be worth
any developer's time to look through those and choose the best one.

=head1 ATTRIBUTES

=head2 use_anyevent

  $wu->use_anyevent(1);

If set to a value that is true, creates AnyEvent watchers for each
asyncronous or queued job.  The equivilent of an C<AnyEvent> condition
variable C<recv()>, used when all processes finish executing, is the
C<waitall()> method.  However, the processes are integrated into a
standard C<AnyEvent> loop, so it isn't strictly necessary to call C<waitall()>.
In addition, a call to C<waitall()> will execute other processes in
the C<AnyEvent> event loop.

The default value is false.

=head2 max_children

  $wu->max_children(5);
  $wu->max_children(undef);

  say "Max number of children: " . $wu->max_children();

If set to a value other than zero or undef, limits the number of outstanding
queue children (created by the C<queue()> method) that can be executing
at any given time.

This defaults to 5.

This attribute does not impact the C<async()> method's ability to
create children, but these children will count against the limit used
by C<queue()>.

Calling without any parameters will return the number of children.

=head1 METHODS

=head2 new

Create a new workunit class.  Optionally, takes a list that corresponds
to a hashref, in the form of key and value.  This accepts the key
C<max_children>, which, if present (and not undef) will limit the
number of spawned subprocesses that can be active when using the
C<queue()> method.  Defaults to 5.  See the C<max_children> method
for additional information.

=head2 async( sub { ... }, \&callback )

  $wu->async( sub { return 1 }, \&callback );

  # To get back results in "ordered" return mode
  $wu->async( sub { return 1 } );
  @results = $wu->waitall();

Spawns work on a new forked process.  The forked process inherits
all Perl state from the parent process, as would be expected with
a standard C<fork()> call.  The child shares nothing with the
parent, other than the return value of the work done.

The work is specified either as a subroutine reference or an
anonymous sub (C<sub { ... }>) and should return a scalar.  Any
scalar that L<Storable>'s C<freeze()> method can deal with
is acceptable (for instance, a hash reference or C<undef>).

The result is serialized and streamed back to the parent process
via a pipe.  The parent, in a C<waitall()> call, will call the
callback function (if provided) with the unserialized return value.

If a callback is not provided, the parent, in the C<waitall()> call,
will gather these results and return them as an ordered list.

In all modes, should the child process C<die>, the parent process
will also die (inside the C<waitall()> method).

The PID of the child is returned to the parent process when
this method is executed.

The C<max_children> attribute is not examined in this method - you
can spawn a new child regardless of the number of children already
spawned. However, you children started with this method still count
against the limit used by C<queue()>.

Note: on Windows with threaded Perl, if C<AnyEvent> is not installed,
threads instead of forks are used.  See C<thread> for the caveats
that apply.  The PID returned is instead a meaningless (outside of
this module) counter, not associated with any Windows thread identifier.

=head2 asyncs( $children, sub { ... }, \&callback )

  $wu->asyncs( 10, sub { return 1 }, \&callback );

  # To get back results in "ordered" return mode
  $wu->asyncs( 10, sub { return 1 } );
  @results = $wu->waitall();

Added in 1.117.

This functions similarly to the C<async()> method, with a couple
key differences.

First, it takes an additional parameter, C<$children>, that specifies
the number of child threads to spawn.  Like the C<async()> method,
the children are spawned immediately, regardless of the value of
the C<max_children> attribute.

In addition, when the sub/coderef is executed, it is called with a
single parameter representing the child number in that particular
instance (between zero to C<$children-1>).

Returns the number of children spawned.

See C<async()> for more details on how this function works.

=head2 waitall()

Called from the parent method while waiting for the children
to exit.  This method handles children that C<die()> or return
a serializable data structure.  When all children return, this
method will return.

If a child dies unexpectedly, this method will C<die()> and propagate a
modified exception.

In the standard (not ordered) mode, I.E. where a callback was passed
to C<async()>, this will return nothing.

In the ordered mode, I.E. where no callbacks were provided to C<async()>,
this will return the results of the async calls in an ordered list.  The
list will be ordered by the order in which the async calls were executed.

=head2 waitone()

This method similarly to C<waitall()>, but only waits for
a single PID.  It will return after any PID exits.

If this method is called when there is no processes executing,
it will simply return undef. Otherwise, it will wait and then
return 1.

=head2 wait($pid)

This functions simiarly to C<waitone()>, but waits only for
a specific PID.  See the C<waitone()> documentation above
for details.

If C<wait()> is called on a process that is already done
executing, it simply returns.  Otherwise, it waits until the
child process's work unit is complete and executes the callback
routine, then returns.

=head2 count()

This method returns the number of currently outstanding
threads (in either a running state or a waiting to send their
output).

=head2 queue( sub { ... }, \&callback )

Spawns work on a new forked process, doing so immediately if less
than C<max_children> are running.  If there are already
C<max_children> are running, this will run the process once a slot
becomes available.

This method should be treated as nearly identical to C<async()>,
with the only difference being the above behavior (limiting to
C<max_children>) and not returning a PID.  Instead, a value of 1
is returned if the process is immediately started, C<undef>
otherwise.

The result is serialized and streamed back to the parent process
via a pipe.  The parent, in a C<waitall()> call, will call the
callback function (if provided) with the unserialized return value.

If a callback is not provided, the parent, in the C<waitall()> call,
will gather these results and return them as an ordered list.

=head1 AUTHOR

Joelle Maslak <jmaslak@antelope.net>

=head1 COPYRIGHT AND LICENSE

This software is copyright (c) 2018 by Joelle Maslak.

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut
