POE::Queue(3) User Contributed Perl Documentation POE::Queue(3)NAMEPOE::Queue - a flexible, generic priority queue API
SYNOPSISPOE::Queue specifies additional methods not illustrated here.
#!perl
use warnings;
use strict;
use POE::Queue::Array;
my $pqa = POE::Queue::Array->new();
# Enqueue a few items.
foreach my $priority (505, 404, 303, 202, 101) {
$pqa->enqueue($priority, "payload $priority");
}
# Dequeue until the queue is drained.
while (1) {
my ($priority, $queue_id, $payload) = $pqa->dequeue_next();
last unless defined $priority;
print(
"dequeued id($queue_id) ",
"priority($priority) ",
"payload($payload)\n",
);
}
Sample output:
dequeued id(5)priority(101) payload(payload 101)
dequeued id(4)priority(202) payload(payload 202)
dequeued id(3)priority(303) payload(payload 303)
dequeued id(2)priority(404) payload(payload 404)
dequeued id(1)priority(505) payload(payload 505)
DESCRIPTION
Priority queues may be implemented a number of ways, but they tend to
behave similar to lists that are kept in order by some kind of
"priority". Enqueued items are stored such that the "next" item to be
retrieved is the one with the highest priority. Subsequent fetches
return the next lowest priority, and so on, until the queue is emptied.
Priority queues (also known as priority heaps) attempt to do this while
consuming the fewest resources. Go read about it! It's fascinating
stuff!
POE::Queue Items
POE::Queue items consist of three fields: A priority, a unique ID
assigned at enqueue time, and a payload. The priority and payload are
specified by the caller, and the unique ID is generated by POE::Queue
when an item is enqueued.
POE::Queue imposes two limitations on priorities: Priorities must be
numeric, and lower numbers indicate higher priorities. Aside from
that, POE::Queue doesn't care what the numbers mean.
Unique IDs are handles into the queue. POE::Queue generates and
returns them as new items are enqueued. Some methods manipulate items,
and they take IDs to identify the items to alter.
Item payloads are arbitrary application data. POE::Queue does not
examine or alter payloads itself. Any methods that need to examine
payloads will accept a filter function. Filter functions examine
payloads so POE::Queue need not.
Public MethodsPOE::Queue is an API specification. Subclasses like POE::Queue::Array
provide actual implementations.
new
Creates a new priority queue. Returns a reference to the queue.
my $queue = POE::Queue::Array->new();
enqueue PRIORITY, PAYLOAD
Enqueues a PAYLOAD, which can be just about anything that will fit into
a Perl scalar, at a particular PRIORITY level. enqueue() returns a
unique ID which can be used to manipulate the payload or its priority
directly.
Following the UNIX tradition, lower priority numbers indicate higher
priorities. The payload with the lowest priority number will be
dequeued first. If two payloads have the same PRIORITY, then they will
be dequeued in the order in which they were enqueued.
In this example, a queue is used to manage a number of alarms. The
"next" alarm will be the one due soonest.
my $payload_id = $queue->enqueue($alarm_time, [ "stuff" ]);
dequeue_next
Removes the next item from the queue, returning it as three fields:
priority, ID and payload.
The "next" item is the one with the lowest priority number. If
multiple items exist with the same priority, dequeue_next() will return
the one that was given the priority first.
ITEM: while (1) {
my ($priority, $id, $payload) = $queue->dequeue_next();
last ITEM unless defined $priority;
...;
}
get_next_priority
Returns the priority of the item at the head of the queue. This is the
lowest numeric priority in the queue.
get_next_priority() can be useful for checking the queue to see if it's
time to dequeue some items. In this case, the queue manages multiple
alarms, and there's nothing to do if the next alarm isn't due yet.
ALARM: while (1) {
my $next_alarm_time = $queue->get_next_priority();
last ALARM unless defined $next_alarm_time;
if ($next_alarm_time - time() > 0) {
sleep($next_alarm_time - time());
}
my ($priority, $id, $payload) = $queue->dequeue_next();
...;
}
get_item_count
Returns the number of items in the queue. It's another way to tell
whether the queue has been fully drained. Here's an alternative
version of the example for get_next_priority().
ALARM: while ($queue->get_item_count()) {
my $next_alarm_time = $queue->get_next_priority();
if ($next_alarm_time - time() > 0) {
sleep($next_alarm_time - time());
}
my ($priority, $id, $payload) = $queue->dequeue_next();
...;
}
remove_item ITEM_ID, FILTER_FUNCTION
Removes a single item by its ID, but only if a FILTER_FUNCTION approves
of the item's payload.
If a payload is found with the given ITEM_ID, it is passed to
FILTER_FUNCTION for examination. If FILTER_FUNCTION returns true, the
item is removed from the queue and is returned as three fields.
my ($priority, $id, $payload) = $queue->remove_item(
$target_id, \&monkeys
);
sub monkeys {
my $payload = shift;
$payload->{type} eq "monkey";
}
The returned $priority will be undef on failure, and $! will be set to
the reason why the item couldn't be removed. That will be ESRCH if the
ITEM_ID was not found in the queue, or EPERM if the filter function
returned false.
remove_items FILTER_FUNCTION [, MAX_ITEM_COUNT ]
Removes and returns items from the queue that match a FILTER_FUNCTION.
remove_items() will return immediately if MAX_ITEM_COUNT items is
specified and that many items have been removed from the queue.
MAX_ITEM_COUNT is a bit of optimization if the application knows in
advance how many items will match the FILTER_FUNCTION.
Returns a list of items that were removed. Each item is an array
reference containing a priority, item ID, and payload. Returns nothing
if FILTER_FUNCTION matched nothing.
# Remove up to 12 monkeys.
my @monkeys = $queue->remove_items(\&monkeys, 12);
foreach my $monkey (@monkeys) {
my ($priority, $id, $payload) = @$monkey;
print(
"Removed monkey:\n",
" priority = $priority\n",
" queue id = $id\n",
" payload = $payload\n",
);
}
There is no guarantee which items will be removed if MAX_ITEM_COUNT is
specified too low.
peek_items FILTER_FUNCTION [, MAX_ITEM_COUNT ]
peek_items() returns up to MAX_ITEM_COUNT items that match a given
FILTER_FUNCTION without removing them from the queue.
my @entire_queue = $queue->peek_items(sub { 1 });
foreach my $item (@entire_queue) {
my ($priority, $id, $payload) = @$item;
print(
"Item:\n",
" priority = $priority\n",
" queue id = $id\n",
" payload = $payload\n",
);
}
adjust_priority ITEM_ID, FILTER_FUNCTION, DELTA
Changes the priority of an item by DELTA. The item is identified by
its ITEM_ID, and the change will only happen if the item's payload
satisfies a FILTER_FUNCTION. Returns the new priority, which is the
previous priority + DELTA. DELTA may be negative.
my $new_priority = $queue->adjust_priority(
$item_id, \&one_of_mine, 100
);
sub one_of_mine {
my $payload = shift;
return $payload->{owner} == $me;
}
Returns undef if the item's priority could not be adjusted, and sets $!
to explain why: ESRCH means that the ITEM_ID could not be found, and
EPERM means that the FILTER_FUNCTION was not satisfied.
set_priority ITEM_ID, FILTER_FUNCTION, ABSOLUTE_PRIORITY
Sets an item's priority to a new ABSOLUTE_PRIORITY. The item is
identified by its ITEM_ID, and the change will only be allowed to
happen if the item's payload satisfies a FILTER_FUNCTION. Returns the
new priority, which should match ABSOLUTE_PRIORITY.
Returns undef if the item's priority could not be set, and sets $! to
explain why: ESRCH means that the ITEM_ID could not be found, and EPERM
means that the FILTER_FUNCTION was not satisfied.
my $new_priority = $queue->set_priority(
$item_id, \&one_of_mine, time() + 60
);
unless (defined $new_priority) {
die "one of our submarines is missing: $item_id" if $! == ESRCH;
die "set_priority disallowed for item $item_id" if $! == EPERM;
die $!;
}
sub one_of_mine {
$_[0]{owner} == $me;
}
SEE ALSO
POE, POE::Queue::Array
BUGS
None known.
AUTHORS & COPYRIGHTS
Please see POE for more information about authors, contributors, and
POE's licensing.
POD ERRORS
Hey! The above document had some coding errors, which are explained
below:
Around line 293:
A non-empty Z<>
Around line 296:
A non-empty Z<>
perl v5.14.2 2011-12-15 POE::Queue(3)
