Leif Pedersen > Proc-SafeExec > Proc::SafeExec::Queue

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NAME ^

Proc::SafeExec::Queue - Uses Proc::SafeExec to manage a group of concurrent processes.

SYNOPSIS ^

        use Proc::SafeExec::Queue;
        my $queue = Proc::SafeExec::Queue->new({
                "soft_limit" => 4,  # Default is 4.
                "hard_limit" => 8,  # Default is soft_limit * 2.
        });

To add a task to the queue:

        my $id = $queue->add({
                # Options describing when to execute.
                "exec_order" => 1,  # Default is 1.
                "callback_prefork" => \&callback_prefork,  # Default is undef.
                "callback_postfork" => \&callback_postfork,  # Default is undef.
                "callback_postwait" => \&callback_postwait,  # Default is undef.
                "callback_error" => \&callback_error,  # Default is undef.
                "exec" => {
                        # Options to new Proc::SafeExec.
                },
                "data" => {
                        # Space for the caller to store any ancillary information, for example, to be
                        # used in the callback functions.
                },
        });

To cancel a task before it starts (okay during prefork):

        $queue->cancel($id);

To wait on exited children and begin executing pending children:

        my $did_something = $queue->do_events();

To get a list of children that haven't finished yet:

        my @list = $queue->remaining_children();

DESCRIPTION ^

Proc::SafeExec::Queue provides a way of managing a group of concurrent processes.

Here's a logical description of what happens. Processes are added to the queue and execute when appropriate. The parent can enqueue them and forget about them. Immediately before a child executes, before forking, the prerun function executes. Immediately after a child is waited on, the postrun function executes.

This decides when to execute a child based on exec_order, soft_limit, and hard_limit. The exec_order option describes the ideal order the children should be executed in, however, it's not strictly enforced, since children may be added with a lower exec_order after one with a higher exec_order began execution. When a new child is added to the queue, it always begins executing immediately if there are fewer than soft_limit already executing. Otherwise, it begins executing only if the number of children with an exec_order lower or equal to the new child is lower than soft_limit and there are fewer than hard_limit children executing. The number of children will never exceed hard_limit. Whenever a child exits, this checks the queue to see if any more should be started. If there is a tie when deciding which child to execute next, the first one added to the queue wins.

Note that setting soft_limit to undef or greater than hard_limit is the same as setting it to the same value as hard_limit, and both must be an integer greater than zero. exec_order may be any numerical value, including negative and floating point values (although it may be most intuitive to limit it to integers greater than zero). Setting soft_limit or hard_limit to 0 means infinity, but this can be dangerous because no computer can handle an infinite number of processes.

USAGE ^

The parent should call $queue->do_events() whenever it receives SIGCHLD, but should never call it directly from the signal handler, because signal handlers may be invoked at times when it is not safe to do anything but set a variable. It is safe to call $queue->do_events() when there is nothing to do. Thus, alternatively to trapping SIGCHLD, the parent may simply call $queue->do_events() whenever it is convenient, such as at the beginning of an event loop. If you choose to do this, consider the loop:

        1 while $queue->do_events();

This is not the default because it starve the parent of time to do other work.

$queue->do_events() first waits on any children that exited and calls the associated callback_postwait functions, then for each child scheduled for execution, it calls the associated callback_prefork function, executes it, and calls the associated callback_postfork function. If there is an error executing a child, it calls callback_error; callback_postfork and callback_postwait are never called. callback_prefork, callback_postfork, and callback_postwait may be undef to indicate a null-op. If callback_error is set to undef and an error occurs, a warning is issued via warn().

$queue->add() adds a child to the queue and, if appropriate, begins executing it. It returns a unique ID representing the child.

The callback functions always receive the original options hash that was passed to $queue->add(), with some additional elements. The callback functions can inspect this hash to find out some things about the child, however this hash is not to be meddled with, except as documented here. If you need to associate your own information with the child, use the "data" subhash, which is entirely reserved for the caller's use and may be modified at any time.

If the child began execution, the element "Proc::SafeExec" is set to the Proc::SafeExec object. (This is always set in callback_postfork and callback_postwait, never in callback_prefork, and sometimes in callback_error.) The query methods of Proc::SafeExec may be called, specifically child_pid, stdin, stdout, stderr, exit_status. The caller may not call the wait method because it affects the execution state. If an error occurred, the element "error" is set to $@ for the duration of the callback_error function. The "id" element is always set to the unique ID assigned to this child. The callback_prefork function may modify the exec hash.

The method $queue->cancel() removes a child from the queue, but only if it hasn't started yet. This may be called during callback_prefork, but no later.

The method $queue->remaining_children() returns the hashes of the children remaining in the queue (including those that are running). (These are the same hashes that are passed to the callback functions.) They're listed in the order they would be started if none were running yet. The ones that are running might not be the first ones in the list if the exec_order decreased in the sequence that children were added.

When used as documented, this module never has an error to report directly to the caller; all errors are reported through callback_error functions. If it does die, there's a good chance the bug or misuse will leave things in an inconsistent state that can't be recovered from automatically. This may be worth improving.

XXX: What to do with running children when the queue is destroyed?

CAVEATS ^

EXAMPLES ^

INSTALLATION ^

VERSION AND HISTORY ^

See Proc::SafeExec.

SEE ALSO ^

See also Proc::SafeExec, the package containing this.

AUTHOR ^

Leif Pedersen, <bilbo@hobbiton.org>

COPYRIGHT AND LICENSE ^

 This may be distributed under the terms below (BSD'ish) or under the GPL.
 
 Copyright (c) 2007
 All Rights Reserved
 Meridian Environmental Technology, Inc.
 4324 University Avenue, Grand Forks, ND 58203
 http://meridian-enviro.com
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
 met:
 
  1. Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
 
  2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the
     distribution.
 
 THIS SOFTWARE IS PROVIDED BY AUTHORS AND CONTRIBUTORS "AS IS" AND ANY
 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL AUTHORS OR CONTRIBUTORS BE
 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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