Provided by: libanyevent-perl_7.170-2build6_amd64 
NAME
AnyEvent::Impl::POE - AnyEvent adaptor for POE
SYNOPSIS
use AnyEvent;
use POE;
# this module gets loaded automatically as required
DESCRIPTION
This module provides transparent support for AnyEvent. You don't have to do anything to make POE work
with AnyEvent except by loading POE before creating the first AnyEvent watcher. There are some cases
where POE will issue spurious (and non-suppressible) warnings. These can be avoided by loading
AnyEvent::Impl::POE before loading any other modules using POE and AnyEvent, i.e. in your main program.
AnyEvent::Impl::POE will output some spurious message how to work around POE's spurious messages when it
detects these cases.
Unfortunately, POE isn't generic enough to implement a fully working AnyEvent backend: POE is too badly
designed, too badly documented and too badly implemented.
Here are the details, and what it means to you if you want to be interoperable with POE:
Weird messages
If you only use "run_one_timeslice" (as AnyEvent has to for its condition variables), POE will print
an ugly, unsuppressible, message at program exit:
Sessions were started, but POE::Kernel's run() method was never...
The message is correct, the question is why POE prints it in the first place in a correct program
(this is not a singular case though).
AnyEvent consequently patches the POE kernel so it thinks it already ran. Other workarounds, even the
one cited in the POE documentation itself, have serious side effects, such as throwing away events.
The author of POE verified that this is indeed true, and has no plans to change this.
POE has other weird messages, and sometimes weird behaviour, for example, it doesn't support
overloaded code references as callbacks for no apparent reason.
One POE session per Event
AnyEvent has to create one POE::Session per event watcher, which is immensely slow and makes watchers
very large. The reason for this is lacking lifetime management (mostly undocumented, too). Without
one session/watcher it is not possible to easily keep the kernel from running endlessly.
This is not just a problem with the way AnyEvent has to interact with POE, but is a principal issue
with POEs lifetime management (namely that stopping the kernel stops sessions, but AnyEvent has no
control over who and when the kernel starts or stops w.r.t. AnyEvent watcher creation/destruction).
From benchmark data it is not clear that session creation is that costly, though - the real
inefficiencies with POE seem to come from other sources, such as event handling.
One watcher per fd/event combo
POE, of course, suffers from the same bug as Tk and some other badly designed event models in that it
doesn't support multiple watchers per fd/poll combo. The workaround is the same as with Tk:
AnyEvent::Impl::POE creates a separate file descriptor to hand to POE, which isn't fast and certainly
not nice to your resources.
Of course, without the workaround, POE also prints ugly messages again that say the program *might*
be buggy.
While this is not good to performance, at least regarding speed, with a modern Linux kernel, the
overhead is actually quite small.
Timing deficiencies
POE manages to not have a function that returns the current time. This is extremely problematic, as
POE can use different time functions, which can differ by more than a second - and user code is left
guessing which one is used.
In addition, most timer functions in POE want an absolute timestamp, which is hard to create if all
you have is a relative time and no function to return the "current time".
And of course POE doesn't handle time jumps at all (not even when using an event loop that happens to
do that, such as EV, as it does its own unoptimised timer management).
AnyEvent works around the unavailability of the current time using relative timers exclusively, in
the hope that POE gets it right at least internally.
Lack of defined event ordering
POE cannot guarantee the order of callback invocation for timers, and usually gets it wrong. That is,
if you have two timers, one timing out after another (all else being equal), the callbacks might be
called in reverse order.
How one manages to even implement stuff that way escapes me.
Child watchers
POE offers child watchers - which is a laudable thing, as few event loops do. Unfortunately, they
cannot even implement AnyEvent's simple child watchers: they are not generic enough (the POE
implementation isn't even generic enough to let properly designed back-end use their native child
watcher instead - it insist on doing it itself the broken way).
Unfortunately, POE's child handling is inherently racy: if the child exits before the handler is
created (because e.g. it crashes or simply is quick about it), then current versions of POE (1.352)
will never invoke the child watcher, and there is nothing that can be done about it. Older versions
of POE only delayed in this case. The reason is that POE first checks if the child has already
exited, and then installs the signal handler - aa classical race.
Your only hope is for the fork'ed process to not exit too quickly, in which case everything happens
to work.
Of course, whenever POE reaps an unrelated child it will also output a message for it that you cannot
suppress (which shouldn't be too surprising at this point). Very professional.
As a workaround, AnyEvent::Impl::POE will take advantage of undocumented behaviour in POE::Kernel to
catch the status of all child processes, but it cannot guarantee delivery.
How one manages to have such a glaring bug in an event loop after ten years of development escapes
me.
(There are more annoying bugs, for example, POE runs "waitpid" unconditionally at finaliser time, so
your program will hang until all child processes have exited.)
Documentation quality
At the time of this writing, POE was in its tenth year. Still, its documentation is extremely
lacking, making it impossible to implement stuff as trivial as AnyEvent watchers without having to
resort to undocumented behaviour or features.
For example, the POE::Kernel manpage has nine occurrences of the word TODO with an explanation of
whats missing. In general, the POE man pages are littered with comments like "section not yet
written".
Some other gems:
This allows many object methods to also be package methods.
This is nice, but since it doesn't document which methods these are, this is utterly useless
information.
Terminal signals will kill sessions if they are not handled by a
"sig_handled"() call. The OS signals that usually kill or dump a
process are considered terminal in POE, but they never trigger a
coredump. These are: HUP, INT, QUIT and TERM.
Although AnyEvent calls "sig_handled", removing it has no apparent effects on POE handling SIGINT.
refcount_increment SESSION_ID, COUNTER_NAME
Nowhere is explained which COUNTER_NAMEs are valid and which aren't - not all scalars (or even
strings) are valid counter names. Take your guess, failure is of course completely silent. I found
this out the hard way, as the first name I came up with was silently ignored.
get_next_event_time() returns the time the next event is due, in a form
compatible with the UNIX time() function.
And surely, one would hope that POE supports sub-second accuracy as documented elsewhere, unlike the
explanation above implies. Yet:
POE::Kernel timers support subsecond accuracy, but don’t expect too
much here. Perl is not the right language for realtime programming.
... of course, Perl is not the right language to expect sub-second accuracy - the manpage author must
hate Perl to spread so much FUD in so little space. The Deliantra game server logs with
100µs-accuracy because Perl is fast enough to require this, and is still able to deliver map updates
with little jitter at exactly the right time. It does not, however, use POE.
Furthermore, since the Kernel keeps track of everything sessions do, it
knows when a session has run out of tasks to perform.
This is impossible - how does the kernel know that a session is no longer watching for some
(external) event (e.g. by some other session)? It cannot, and therefore this is wrong - but you would
be hard pressed to find out how to work around this and tell the kernel manually about such events.
It gets worse, though - the notion of "task" or "resource", although used throughout the
documentation, is not defined in a usable way. For example, waiting for a timeout is considered to be
a task, waiting for a signal is not (a session that only waits for a signal is considered finished
and gets removed). The user is left guessing when waiting for an event counts as task and when not
(in fact, the issue with signals is mentioned in passing in a section about child watchers and
directly contradicts earlier parts in that document).
One could go on endlessly - ten years, no usable documentation.
It is likely that differences between documentation, or the one or two things I had to guess, cause
unanticipated problems with this adaptor.
Fragile and inconsistent API
The POE API is extremely inconsistent - sometimes you have to pass a session argument, sometimes it
gets ignored, sometimes a session-specific method must not use a session argument.
Error handling is sub-standard as well: even for programming mistakes, POE does not "croak" but, in
most cases, just sets $! or simply does nothing at all, leading to fragile programs.
Sometimes registering a handler uses the "eventname, parameter" ordering (timeouts), sometimes it is
"parameter, eventname" (signals). There is little consistency overall.
Lack of knowledge
The IO::Poll event loop provides an alternative that theoretically
scales better than select().
The IO::Poll "event loop" (who in his right mind would call that an event loop) of course scales
about identically (sometimes it is a bit faster, sometimes a bit slower) to select in theory, and
also in practise, of course, as both are O(n) in the number of file descriptors, which is rather bad.
This is just one place where it gets obvious how little the author of the POE manpage understands.
No idle events
The POE-recommended workaround to this is apparently to use "fork". Consequently, idle watchers will
have to be emulated by AnyEvent.
Questionable maintainer behaviour
The author of POE is known to fabricate statements and post these to public mailinglists -
apparently, spreading FUD about competing (in his eyes) projects or their maintainers is acceptable
to him.
This has (I believe) zero effects on the quality or usefulness of his code, but it does completely
undermine his trustworthyness - so don't blindly believe anything he says, he might have just made it
up to suit his needs (benchmark results, the names of my ten wifes, the length of my penis, etc.
etc.). When in doubt, double-check - not just him, anybody actually.
Example: <http://www.nntp.perl.org/group/perl.perl5.porters/2012/01/msg182141.html>. I challenged
him in that thread to provide evidence for his statement by giving at least two examples, but of
course since he just made it up, he couldn't provide any evidence.
On the good side, AnyEvent allows you to write your modules in a 100% POE-compatible way (bug-for-bug
compatible even), without forcing your module to use POE - it is still open to better event models, of
which there are plenty.
Oh, and one other positive thing:
RUNNING_IN_HELL
POE knows about the nature of the beast!
SEE ALSO
AnyEvent, POE.
AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://anyevent.schmorp.de
perl v5.38.2 2024-03-31 AnyEvent::Impl::POE(3pm)