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Revision 1.113 by root, Sat May 10 20:30:35 2008 UTC

1	=head1 NAME	1	=head1 NAME
2		2
3	AnyEvent - ???	3	AnyEvent - provide framework for multiple event loops
		4
		5	EV, Event, Glib, Tk, Perl, Event::Lib, Qt, POE - various supported event loops
4		6
5	=head1 SYNOPSIS	7	=head1 SYNOPSIS
6		8
		9	use AnyEvent;
		10
		11	my $w = AnyEvent->io (fh => $fh, poll => "r|w", cb => sub {
		12	...
		13	});
		14
		15	my $w = AnyEvent->timer (after => $seconds, cb => sub {
		16	...
		17	});
		18
		19	my $w = AnyEvent->condvar; # stores whether a condition was flagged
		20	$w->wait; # enters "main loop" till $condvar gets ->send
		21	$w->send; # wake up current and all future wait's
		22
		23	=head1 WHY YOU SHOULD USE THIS MODULE (OR NOT)
		24
		25	Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
		26	nowadays. So what is different about AnyEvent?
		27
		28	Executive Summary: AnyEvent is I<compatible>, AnyEvent is I<free of
		29	policy> and AnyEvent is I<small and efficient>.
		30
		31	First and foremost, I<AnyEvent is not an event model> itself, it only
		32	interfaces to whatever event model the main program happens to use in a
		33	pragmatic way. For event models and certain classes of immortals alike,
		34	the statement "there can only be one" is a bitter reality: In general,
		35	only one event loop can be active at the same time in a process. AnyEvent
		36	helps hiding the differences between those event loops.
		37
		38	The goal of AnyEvent is to offer module authors the ability to do event
		39	programming (waiting for I/O or timer events) without subscribing to a
		40	religion, a way of living, and most importantly: without forcing your
		41	module users into the same thing by forcing them to use the same event
		42	model you use.
		43
		44	For modules like POE or IO::Async (which is a total misnomer as it is
		45	actually doing all I/O I<synchronously>...), using them in your module is
		46	like joining a cult: After you joined, you are dependent on them and you
		47	cannot use anything else, as it is simply incompatible to everything that
		48	isn't itself. What's worse, all the potential users of your module are
		49	I<also> forced to use the same event loop you use.
		50
		51	AnyEvent is different: AnyEvent + POE works fine. AnyEvent + Glib works
		52	fine. AnyEvent + Tk works fine etc. etc. but none of these work together
		53	with the rest: POE + IO::Async? no go. Tk + Event? no go. Again: if
		54	your module uses one of those, every user of your module has to use it,
		55	too. But if your module uses AnyEvent, it works transparently with all
		56	event models it supports (including stuff like POE and IO::Async, as long
		57	as those use one of the supported event loops. It is trivial to add new
		58	event loops to AnyEvent, too, so it is future-proof).
		59
		60	In addition to being free of having to use I<the one and only true event
		61	model>, AnyEvent also is free of bloat and policy: with POE or similar
		62	modules, you get an enourmous amount of code and strict rules you have to
		63	follow. AnyEvent, on the other hand, is lean and up to the point, by only
		64	offering the functionality that is necessary, in as thin as a wrapper as
		65	technically possible.
		66
		67	Of course, if you want lots of policy (this can arguably be somewhat
		68	useful) and you want to force your users to use the one and only event
		69	model, you should I<not> use this module.
		70
7	=head1 DESCRIPTION	71	=head1 DESCRIPTION
8		72
		73	L<AnyEvent> provides an identical interface to multiple event loops. This
		74	allows module authors to utilise an event loop without forcing module
		75	users to use the same event loop (as only a single event loop can coexist
		76	peacefully at any one time).
		77
		78	The interface itself is vaguely similar, but not identical to the L<Event>
		79	module.
		80
		81	During the first call of any watcher-creation method, the module tries
		82	to detect the currently loaded event loop by probing whether one of the
		83	following modules is already loaded: L<EV>,
		84	L<Event>, L<Glib>, L<AnyEvent::Impl::Perl>, L<Tk>, L<Event::Lib>, L<Qt>,
		85	L<POE>. The first one found is used. If none are found, the module tries
		86	to load these modules (excluding Tk, Event::Lib, Qt and POE as the pure perl
		87	adaptor should always succeed) in the order given. The first one that can
		88	be successfully loaded will be used. If, after this, still none could be
		89	found, AnyEvent will fall back to a pure-perl event loop, which is not
		90	very efficient, but should work everywhere.
		91
		92	Because AnyEvent first checks for modules that are already loaded, loading
		93	an event model explicitly before first using AnyEvent will likely make
		94	that model the default. For example:
		95
		96	use Tk;
		97	use AnyEvent;
		98
		99	# .. AnyEvent will likely default to Tk
		100
		101	The I<likely> means that, if any module loads another event model and
		102	starts using it, all bets are off. Maybe you should tell their authors to
		103	use AnyEvent so their modules work together with others seamlessly...
		104
		105	The pure-perl implementation of AnyEvent is called
		106	C<AnyEvent::Impl::Perl>. Like other event modules you can load it
		107	explicitly.
		108
		109	=head1 WATCHERS
		110
		111	AnyEvent has the central concept of a I<watcher>, which is an object that
		112	stores relevant data for each kind of event you are waiting for, such as
		113	the callback to call, the filehandle to watch, etc.
		114
		115	These watchers are normal Perl objects with normal Perl lifetime. After
		116	creating a watcher it will immediately "watch" for events and invoke the
		117	callback when the event occurs (of course, only when the event model
		118	is in control).
		119
		120	To disable the watcher you have to destroy it (e.g. by setting the
		121	variable you store it in to C<undef> or otherwise deleting all references
		122	to it).
		123
		124	All watchers are created by calling a method on the C<AnyEvent> class.
		125
		126	Many watchers either are used with "recursion" (repeating timers for
		127	example), or need to refer to their watcher object in other ways.
		128
		129	An any way to achieve that is this pattern:
		130
		131	my $w; $w = AnyEvent->type (arg => value ..., cb => sub {
		132	# you can use $w here, for example to undef it
		133	undef $w;
		134	});
		135
		136	Note that C<my $w; $w => combination. This is necessary because in Perl,
		137	my variables are only visible after the statement in which they are
		138	declared.
		139
		140	=head2 I/O WATCHERS
		141
		142	You can create an I/O watcher by calling the C<< AnyEvent->io >> method
		143	with the following mandatory key-value pairs as arguments:
		144
		145	C<fh> the Perl I<file handle> (I<not> file descriptor) to watch
		146	for events. C<poll> must be a string that is either C<r> or C<w>,
		147	which creates a watcher waiting for "r"eadable or "w"ritable events,
		148	respectively. C<cb> is the callback to invoke each time the file handle
		149	becomes ready.
		150
		151	Although the callback might get passed parameters, their value and
		152	presence is undefined and you cannot rely on them. Portable AnyEvent
		153	callbacks cannot use arguments passed to I/O watcher callbacks.
		154
		155	The I/O watcher might use the underlying file descriptor or a copy of it.
		156	You must not close a file handle as long as any watcher is active on the
		157	underlying file descriptor.
		158
		159	Some event loops issue spurious readyness notifications, so you should
		160	always use non-blocking calls when reading/writing from/to your file
		161	handles.
		162
		163	Example:
		164
		165	# wait for readability of STDIN, then read a line and disable the watcher
		166	my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
		167	chomp (my $input = <STDIN>);
		168	warn "read: $input\n";
		169	undef $w;
		170	});
		171
		172	=head2 TIME WATCHERS
		173
		174	You can create a time watcher by calling the C<< AnyEvent->timer >>
		175	method with the following mandatory arguments:
		176
		177	C<after> specifies after how many seconds (fractional values are
		178	supported) the callback should be invoked. C<cb> is the callback to invoke
		179	in that case.
		180
		181	Although the callback might get passed parameters, their value and
		182	presence is undefined and you cannot rely on them. Portable AnyEvent
		183	callbacks cannot use arguments passed to time watcher callbacks.
		184
		185	The timer callback will be invoked at most once: if you want a repeating
		186	timer you have to create a new watcher (this is a limitation by both Tk
		187	and Glib).
		188
		189	Example:
		190
		191	# fire an event after 7.7 seconds
		192	my $w = AnyEvent->timer (after => 7.7, cb => sub {
		193	warn "timeout\n";
		194	});
		195
		196	# to cancel the timer:
		197	undef $w;
		198
		199	Example 2:
		200
		201	# fire an event after 0.5 seconds, then roughly every second
		202	my $w;
		203
		204	my $cb = sub {
		205	# cancel the old timer while creating a new one
		206	$w = AnyEvent->timer (after => 1, cb => $cb);
		207	};
		208
		209	# start the "loop" by creating the first watcher
		210	$w = AnyEvent->timer (after => 0.5, cb => $cb);
		211
		212	=head3 TIMING ISSUES
		213
		214	There are two ways to handle timers: based on real time (relative, "fire
		215	in 10 seconds") and based on wallclock time (absolute, "fire at 12
		216	o'clock").
		217
		218	While most event loops expect timers to specified in a relative way, they
		219	use absolute time internally. This makes a difference when your clock
		220	"jumps", for example, when ntp decides to set your clock backwards from
		221	the wrong date of 2014-01-01 to 2008-01-01, a watcher that is supposed to
		222	fire "after" a second might actually take six years to finally fire.
		223
		224	AnyEvent cannot compensate for this. The only event loop that is conscious
		225	about these issues is L<EV>, which offers both relative (ev_timer, based
		226	on true relative time) and absolute (ev_periodic, based on wallclock time)
		227	timers.
		228
		229	AnyEvent always prefers relative timers, if available, matching the
		230	AnyEvent API.
		231
		232	=head2 SIGNAL WATCHERS
		233
		234	You can watch for signals using a signal watcher, C<signal> is the signal
		235	I<name> without any C<SIG> prefix, C<cb> is the Perl callback to
		236	be invoked whenever a signal occurs.
		237
		238	Although the callback might get passed parameters, their value and
		239	presence is undefined and you cannot rely on them. Portable AnyEvent
		240	callbacks cannot use arguments passed to signal watcher callbacks.
		241
		242	Multiple signal occurances can be clumped together into one callback
		243	invocation, and callback invocation will be synchronous. synchronous means
		244	that it might take a while until the signal gets handled by the process,
		245	but it is guarenteed not to interrupt any other callbacks.
		246
		247	The main advantage of using these watchers is that you can share a signal
		248	between multiple watchers.
		249
		250	This watcher might use C<%SIG>, so programs overwriting those signals
		251	directly will likely not work correctly.
		252
		253	Example: exit on SIGINT
		254
		255	my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });
		256
		257	=head2 CHILD PROCESS WATCHERS
		258
		259	You can also watch on a child process exit and catch its exit status.
		260
		261	The child process is specified by the C<pid> argument (if set to C<0>, it
		262	watches for any child process exit). The watcher will trigger as often
		263	as status change for the child are received. This works by installing a
		264	signal handler for C<SIGCHLD>. The callback will be called with the pid
		265	and exit status (as returned by waitpid), so unlike other watcher types,
		266	you I<can> rely on child watcher callback arguments.
		267
		268	There is a slight catch to child watchers, however: you usually start them
		269	I<after> the child process was created, and this means the process could
		270	have exited already (and no SIGCHLD will be sent anymore).
		271
		272	Not all event models handle this correctly (POE doesn't), but even for
		273	event models that I<do> handle this correctly, they usually need to be
		274	loaded before the process exits (i.e. before you fork in the first place).
		275
		276	This means you cannot create a child watcher as the very first thing in an
		277	AnyEvent program, you I<have> to create at least one watcher before you
		278	C<fork> the child (alternatively, you can call C<AnyEvent::detect>).
		279
		280	Example: fork a process and wait for it
		281
		282	my $done = AnyEvent->condvar;
		283
		284	AnyEvent::detect; # force event module to be initialised
		285
		286	my $pid = fork or exit 5;
		287
		288	my $w = AnyEvent->child (
		289	pid => $pid,
		290	cb => sub {
		291	my ($pid, $status) = @_;
		292	warn "pid $pid exited with status $status";
		293	$done->send;
		294	},
		295	);
		296
		297	# do something else, then wait for process exit
		298	$done->wait;
		299
		300	=head2 CONDITION VARIABLES
		301
		302	If you are familiar with some event loops you will know that all of them
		303	require you to run some blocking "loop", "run" or similar function that
		304	will actively watch for new events and call your callbacks.
		305
		306	AnyEvent is different, it expects somebody else to run the event loop and
		307	will only block when necessary (usually when told by the user).
		308
		309	The instrument to do that is called a "condition variable", so called
		310	because they represent a condition that must become true.
		311
		312	Condition variables can be created by calling the C<< AnyEvent->condvar
		313	>> method, usually without arguments. The only argument pair allowed is
		314	C<cb>, which specifies a callback to be called when the condition variable
		315	becomes true.
		316
		317	After creation, the conditon variable is "false" until it becomes "true"
		318	by calling the C<send> method.
		319
		320	Condition variables are similar to callbacks, except that you can
		321	optionally wait for them. They can also be called merge points - points
		322	in time where multiple outstandign events have been processed. And yet
		323	another way to call them is transations - each condition variable can be
		324	used to represent a transaction, which finishes at some point and delivers
		325	a result.
		326
		327	Condition variables are very useful to signal that something has finished,
		328	for example, if you write a module that does asynchronous http requests,
		329	then a condition variable would be the ideal candidate to signal the
		330	availability of results. The user can either act when the callback is
		331	called or can synchronously C<< ->wait >> for the results.
		332
		333	You can also use them to simulate traditional event loops - for example,
		334	you can block your main program until an event occurs - for example, you
		335	could C<< ->wait >> in your main program until the user clicks the Quit
		336	button of your app, which would C<< ->send >> the "quit" event.
		337
		338	Note that condition variables recurse into the event loop - if you have
		339	two pieces of code that call C<< ->wait >> in a round-robbin fashion, you
		340	lose. Therefore, condition variables are good to export to your caller, but
		341	you should avoid making a blocking wait yourself, at least in callbacks,
		342	as this asks for trouble.
		343
		344	Condition variables are represented by hash refs in perl, and the keys
		345	used by AnyEvent itself are all named C<_ae_XXX> to make subclassing
		346	easy (it is often useful to build your own transaction class on top of
		347	AnyEvent). To subclass, use C<AnyEvent::CondVar> as base class and call
		348	it's C<new> method in your own C<new> method.
		349
		350	There are two "sides" to a condition variable - the "producer side" which
		351	eventually calls C<< -> send >>, and the "consumer side", which waits
		352	for the send to occur.
		353
		354	Example:
		355
		356	# wait till the result is ready
		357	my $result_ready = AnyEvent->condvar;
		358
		359	# do something such as adding a timer
		360	# or socket watcher the calls $result_ready->send
		361	# when the "result" is ready.
		362	# in this case, we simply use a timer:
		363	my $w = AnyEvent->timer (
		364	after => 1,
		365	cb => sub { $result_ready->send },
		366	);
		367
		368	# this "blocks" (while handling events) till the callback
		369	# calls send
		370	$result_ready->wait;
		371
		372	=head3 METHODS FOR PRODUCERS
		373
		374	These methods should only be used by the producing side, i.e. the
		375	code/module that eventually sends the signal. Note that it is also
		376	the producer side which creates the condvar in most cases, but it isn't
		377	uncommon for the consumer to create it as well.
		378
9	=over 4	379	=over 4
10		380
		381	=item $cv->send (...)
		382
		383	Flag the condition as ready - a running C<< ->wait >> and all further
		384	calls to C<wait> will (eventually) return after this method has been
		385	called. If nobody is waiting the send will be remembered.
		386
		387	If a callback has been set on the condition variable, it is called
		388	immediately from within send.
		389
		390	Any arguments passed to the C<send> call will be returned by all
		391	future C<< ->wait >> calls.
		392
		393	=item $cv->croak ($error)
		394
		395	Similar to send, but causes all call's wait C<< ->wait >> to invoke
		396	C<Carp::croak> with the given error message/object/scalar.
		397
		398	This can be used to signal any errors to the condition variable
		399	user/consumer.
		400
		401	=item $cv->begin ([group callback])
		402
		403	=item $cv->end
		404
		405	These two methods can be used to combine many transactions/events into
		406	one. For example, a function that pings many hosts in parallel might want
		407	to use a condition variable for the whole process.
		408
		409	Every call to C<< ->begin >> will increment a counter, and every call to
		410	C<< ->end >> will decrement it. If the counter reaches C<0> in C<< ->end
		411	>>, the (last) callback passed to C<begin> will be executed. That callback
		412	is I<supposed> to call C<< ->send >>, but that is not required. If no
		413	callback was set, C<send> will be called without any arguments.
		414
		415	Let's clarify this with the ping example:
		416
		417	my $cv = AnyEvent->condvar;
		418
		419	my %result;
		420	$cv->begin (sub { $cv->send (\%result) });
		421
		422	for my $host (@list_of_hosts) {
		423	$cv->begin;
		424	ping_host_then_call_callback $host, sub {
		425	$result{$host} = ...;
		426	$cv->end;
		427	};
		428	}
		429
		430	$cv->end;
		431
		432	This code fragment supposedly pings a number of hosts and calls
		433	C<send> after results for all then have have been gathered - in any
		434	order. To achieve this, the code issues a call to C<begin> when it starts
		435	each ping request and calls C<end> when it has received some result for
		436	it. Since C<begin> and C<end> only maintain a counter, the order in which
		437	results arrive is not relevant.
		438
		439	There is an additional bracketing call to C<begin> and C<end> outside the
		440	loop, which serves two important purposes: first, it sets the callback
		441	to be called once the counter reaches C<0>, and second, it ensures that
		442	C<send> is called even when C<no> hosts are being pinged (the loop
		443	doesn't execute once).
		444
		445	This is the general pattern when you "fan out" into multiple subrequests:
		446	use an outer C<begin>/C<end> pair to set the callback and ensure C<end>
		447	is called at least once, and then, for each subrequest you start, call
		448	C<begin> and for eahc subrequest you finish, call C<end>.
		449
		450	=back
		451
		452	=head3 METHODS FOR CONSUMERS
		453
		454	These methods should only be used by the consuming side, i.e. the
		455	code awaits the condition.
		456
		457	=over 4
		458
		459	=item $cv->wait
		460
		461	Wait (blocking if necessary) until the C<< ->send >> or C<< ->croak
		462	>> methods have been called on c<$cv>, while servicing other watchers
		463	normally.
		464
		465	You can only wait once on a condition - additional calls are valid but
		466	will return immediately.
		467
		468	If an error condition has been set by calling C<< ->croak >>, then this
		469	function will call C<croak>.
		470
		471	In list context, all parameters passed to C<send> will be returned,
		472	in scalar context only the first one will be returned.
		473
		474	Not all event models support a blocking wait - some die in that case
		475	(programs might want to do that to stay interactive), so I<if you are
		476	using this from a module, never require a blocking wait>, but let the
		477	caller decide whether the call will block or not (for example, by coupling
		478	condition variables with some kind of request results and supporting
		479	callbacks so the caller knows that getting the result will not block,
		480	while still suppporting blocking waits if the caller so desires).
		481
		482	Another reason I<never> to C<< ->wait >> in a module is that you cannot
		483	sensibly have two C<< ->wait >>'s in parallel, as that would require
		484	multiple interpreters or coroutines/threads, none of which C<AnyEvent>
		485	can supply.
		486
		487	The L<Coro> module, however, I<can> and I<does> supply coroutines and, in
		488	fact, L<Coro::AnyEvent> replaces AnyEvent's condvars by coroutine-safe
		489	versions and also integrates coroutines into AnyEvent, making blocking
		490	C<< ->wait >> calls perfectly safe as long as they are done from another
		491	coroutine (one that doesn't run the event loop).
		492
		493	You can ensure that C<< -wait >> never blocks by setting a callback and
		494	only calling C<< ->wait >> from within that callback (or at a later
		495	time). This will work even when the event loop does not support blocking
		496	waits otherwise.
		497
		498	=item $bool = $cv->ready
		499
		500	Returns true when the condition is "true", i.e. whether C<send> or
		501	C<croak> have been called.
		502
		503	=item $cb = $cv->cb ([new callback])
		504
		505	This is a mutator function that returns the callback set and optionally
		506	replaces it before doing so.
		507
		508	The callback will be called when the condition becomes "true", i.e. when
		509	C<send> or C<croak> are called. Calling C<wait> inside the callback
		510	or at any later time is guaranteed not to block.
		511
		512	=back
		513
		514	=head1 GLOBAL VARIABLES AND FUNCTIONS
		515
		516	=over 4
		517
		518	=item $AnyEvent::MODEL
		519
		520	Contains C<undef> until the first watcher is being created. Then it
		521	contains the event model that is being used, which is the name of the
		522	Perl class implementing the model. This class is usually one of the
		523	C<AnyEvent::Impl:xxx> modules, but can be any other class in the case
		524	AnyEvent has been extended at runtime (e.g. in I<rxvt-unicode>).
		525
		526	The known classes so far are:
		527
		528	AnyEvent::Impl::EV based on EV (an interface to libev, best choice).
		529	AnyEvent::Impl::Event based on Event, second best choice.
		530	AnyEvent::Impl::Perl pure-perl implementation, fast and portable.
		531	AnyEvent::Impl::Glib based on Glib, third-best choice.
		532	AnyEvent::Impl::Tk based on Tk, very bad choice.
		533	AnyEvent::Impl::Qt based on Qt, cannot be autoprobed (see its docs).
		534	AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse.
		535	AnyEvent::Impl::POE based on POE, not generic enough for full support.
		536
		537	There is no support for WxWidgets, as WxWidgets has no support for
		538	watching file handles. However, you can use WxWidgets through the
		539	POE Adaptor, as POE has a Wx backend that simply polls 20 times per
		540	second, which was considered to be too horrible to even consider for
		541	AnyEvent. Likewise, other POE backends can be used by AnyEvent by using
		542	it's adaptor.
		543
		544	AnyEvent knows about L<Prima> and L<Wx> and will try to use L<POE> when
		545	autodetecting them.
		546
		547	=item AnyEvent::detect
		548
		549	Returns C<$AnyEvent::MODEL>, forcing autodetection of the event model
		550	if necessary. You should only call this function right before you would
		551	have created an AnyEvent watcher anyway, that is, as late as possible at
		552	runtime.
		553
		554	=item $guard = AnyEvent::post_detect { BLOCK }
		555
		556	Arranges for the code block to be executed as soon as the event model is
		557	autodetected (or immediately if this has already happened).
		558
		559	If called in scalar or list context, then it creates and returns an object
		560	that automatically removes the callback again when it is destroyed. See
		561	L<Coro::BDB> for a case where this is useful.
		562
		563	=item @AnyEvent::post_detect
		564
		565	If there are any code references in this array (you can C<push> to it
		566	before or after loading AnyEvent), then they will called directly after
		567	the event loop has been chosen.
		568
		569	You should check C<$AnyEvent::MODEL> before adding to this array, though:
		570	if it contains a true value then the event loop has already been detected,
		571	and the array will be ignored.
		572
		573	Best use C<AnyEvent::post_detect { BLOCK }> instead.
		574
		575	=back
		576
		577	=head1 WHAT TO DO IN A MODULE
		578
		579	As a module author, you should C<use AnyEvent> and call AnyEvent methods
		580	freely, but you should not load a specific event module or rely on it.
		581
		582	Be careful when you create watchers in the module body - AnyEvent will
		583	decide which event module to use as soon as the first method is called, so
		584	by calling AnyEvent in your module body you force the user of your module
		585	to load the event module first.
		586
		587	Never call C<< ->wait >> on a condition variable unless you I<know> that
		588	the C<< ->send >> method has been called on it already. This is
		589	because it will stall the whole program, and the whole point of using
		590	events is to stay interactive.
		591
		592	It is fine, however, to call C<< ->wait >> when the user of your module
		593	requests it (i.e. if you create a http request object ad have a method
		594	called C<results> that returns the results, it should call C<< ->wait >>
		595	freely, as the user of your module knows what she is doing. always).
		596
		597	=head1 WHAT TO DO IN THE MAIN PROGRAM
		598
		599	There will always be a single main program - the only place that should
		600	dictate which event model to use.
		601
		602	If it doesn't care, it can just "use AnyEvent" and use it itself, or not
		603	do anything special (it does not need to be event-based) and let AnyEvent
		604	decide which implementation to chose if some module relies on it.
		605
		606	If the main program relies on a specific event model. For example, in
		607	Gtk2 programs you have to rely on the Glib module. You should load the
		608	event module before loading AnyEvent or any module that uses it: generally
		609	speaking, you should load it as early as possible. The reason is that
		610	modules might create watchers when they are loaded, and AnyEvent will
		611	decide on the event model to use as soon as it creates watchers, and it
		612	might chose the wrong one unless you load the correct one yourself.
		613
		614	You can chose to use a rather inefficient pure-perl implementation by
		615	loading the C<AnyEvent::Impl::Perl> module, which gives you similar
		616	behaviour everywhere, but letting AnyEvent chose is generally better.
		617
		618	=head1 OTHER MODULES
		619
		620	The following is a non-exhaustive list of additional modules that use
		621	AnyEvent and can therefore be mixed easily with other AnyEvent modules
		622	in the same program. Some of the modules come with AnyEvent, some are
		623	available via CPAN.
		624
		625	=over 4
		626
		627	=item L<AnyEvent::Util>
		628
		629	Contains various utility functions that replace often-used but blocking
		630	functions such as C<inet_aton> by event-/callback-based versions.
		631
		632	=item L<AnyEvent::Handle>
		633
		634	Provide read and write buffers and manages watchers for reads and writes.
		635
		636	=item L<AnyEvent::HTTPD>
		637
		638	Provides a simple web application server framework.
		639
		640	=item L<AnyEvent::DNS>
		641
		642	Provides asynchronous DNS resolver capabilities, beyond what
		643	L<AnyEvent::Util> offers.
		644
		645	=item L<AnyEvent::FastPing>
		646
		647	The fastest ping in the west.
		648
		649	=item L<Net::IRC3>
		650
		651	AnyEvent based IRC client module family.
		652
		653	=item L<Net::XMPP2>
		654
		655	AnyEvent based XMPP (Jabber protocol) module family.
		656
		657	=item L<Net::FCP>
		658
		659	AnyEvent-based implementation of the Freenet Client Protocol, birthplace
		660	of AnyEvent.
		661
		662	=item L<Event::ExecFlow>
		663
		664	High level API for event-based execution flow control.
		665
		666	=item L<Coro>
		667
		668	Has special support for AnyEvent via L<Coro::AnyEvent>.
		669
		670	=item L<AnyEvent::AIO>, L<IO::AIO>
		671
		672	Truly asynchronous I/O, should be in the toolbox of every event
		673	programmer. AnyEvent::AIO transparently fuses IO::AIO and AnyEvent
		674	together.
		675
		676	=item L<AnyEvent::BDB>, L<BDB>
		677
		678	Truly asynchronous Berkeley DB access. AnyEvent::AIO transparently fuses
		679	IO::AIO and AnyEvent together.
		680
		681	=item L<IO::Lambda>
		682
		683	The lambda approach to I/O - don't ask, look there. Can use AnyEvent.
		684
		685	=back
		686
11	=cut	687	=cut
12		688
13	package AnyEvent;	689	package AnyEvent;
14		690
		691	no warnings;
		692	use strict;
		693
15	use Carp;	694	use Carp;
16		695
17	$VERSION = 0.1;	696	our $VERSION = '3.4';
		697	our $MODEL;
18		698
19	no warnings;	699	our $AUTOLOAD;
		700	our @ISA;
		701
		702	our $verbose = $ENV{PERL_ANYEVENT_VERBOSE}*1;
		703
		704	our @REGISTRY;
20		705
21	my @models = (	706	my @models = (
22	[Coro => Coro::Event::],	707	[EV:: => AnyEvent::Impl::EV::],
23	[Event => Event::],	708	[Event:: => AnyEvent::Impl::Event::],
24	[Glib => Glib::],	709	[Tk:: => AnyEvent::Impl::Tk::],
25	[Tk => Tk::],	710	[Wx:: => AnyEvent::Impl::POE::],
		711	[Prima:: => AnyEvent::Impl::POE::],
		712	[AnyEvent::Impl::Perl:: => AnyEvent::Impl::Perl::],
		713	# everything below here will not be autoprobed as the pureperl backend should work everywhere
		714	[Glib:: => AnyEvent::Impl::Glib::],
		715	[Event::Lib:: => AnyEvent::Impl::EventLib::], # too buggy
		716	[Qt:: => AnyEvent::Impl::Qt::], # requires special main program
		717	[POE::Kernel:: => AnyEvent::Impl::POE::], # lasciate ogni speranza
26	);	718	);
27		719
		720	our %method = map +($_ => 1), qw(io timer signal child condvar one_event DESTROY);
		721
		722	our @post_detect;
		723
		724	sub post_detect(&) {
		725	my ($cb) = @_;
		726
		727	if ($MODEL) {
		728	$cb->();
		729
		730	1
		731	} else {
		732	push @post_detect, $cb;
		733
		734	defined wantarray
		735	? bless \$cb, "AnyEvent::Util::Guard"
		736	: ()
		737	}
		738	}
		739
		740	sub AnyEvent::Util::Guard::DESTROY {
		741	@post_detect = grep $_ != ${$_[0]}, @post_detect;
		742	}
		743
		744	sub detect() {
		745	unless ($MODEL) {
		746	no strict 'refs';
		747
		748	if ($ENV{PERL_ANYEVENT_MODEL} =~ /^([a-zA-Z]+)$/) {
		749	my $model = "AnyEvent::Impl::$1";
		750	if (eval "require $model") {
		751	$MODEL = $model;
		752	warn "AnyEvent: loaded model '$model' (forced by \$PERL_ANYEVENT_MODEL), using it.\n" if $verbose > 1;
		753	} else {
		754	warn "AnyEvent: unable to load model '$model' (from \$PERL_ANYEVENT_MODEL):\n$@" if $verbose;
		755	}
		756	}
		757
		758	# check for already loaded models
		759	unless ($MODEL) {
		760	for (@REGISTRY, @models) {
		761	my ($package, $model) = @$_;
		762	if (${"$package\::VERSION"} > 0) {
		763	if (eval "require $model") {
		764	$MODEL = $model;
		765	warn "AnyEvent: autodetected model '$model', using it.\n" if $verbose > 1;
		766	last;
		767	}
		768	}
		769	}
		770
		771	unless ($MODEL) {
		772	# try to load a model
		773
		774	for (@REGISTRY, @models) {
		775	my ($package, $model) = @$_;
		776	if (eval "require $package"
		777	and ${"$package\::VERSION"} > 0
		778	and eval "require $model") {
		779	$MODEL = $model;
		780	warn "AnyEvent: autoprobed model '$model', using it.\n" if $verbose > 1;
		781	last;
		782	}
		783	}
		784
		785	$MODEL
		786	or die "No event module selected for AnyEvent and autodetect failed. Install any one of these modules: EV, Event or Glib.";
		787	}
		788	}
		789
		790	unshift @ISA, $MODEL;
		791	push @{"$MODEL\::ISA"}, "AnyEvent::Base";
		792
		793	(shift @post_detect)->() while @post_detect;
		794	}
		795
		796	$MODEL
		797	}
		798
28	sub AUTOLOAD {	799	sub AUTOLOAD {
29	$AUTOLOAD =~ s/.*://;	800	(my $func = $AUTOLOAD) =~ s/.*://;
30		801
31	for (@models) {	802	$method{$func}
32	my ($model, $package) = @$_;	803	or croak "$func: not a valid method for AnyEvent objects";
33	if (defined ${"$package\::VERSION"}) {	804
34	$EVENT = "AnyEvent::Impl::$model";	805	detect unless $MODEL;
35	eval "require $EVENT"; die if $@;	806
36	goto &{"$EVENT\::$AUTOLOAD"};	807	my $class = shift;
37	}	808	$class->$func (@_);
		809	}
		810
		811	package AnyEvent::Base;
		812
		813	# default implementation for ->condvar, ->wait, ->broadcast
		814
		815	sub condvar {
		816	bless \my $flag, "AnyEvent::Base::CondVar"
		817	}
		818
		819	sub AnyEvent::Base::CondVar::broadcast {
		820	${$_[0]}++;
		821	}
		822
		823	sub AnyEvent::Base::CondVar::wait {
		824	AnyEvent->one_event while !${$_[0]};
		825	}
		826
		827	# default implementation for ->signal
		828
		829	our %SIG_CB;
		830
		831	sub signal {
		832	my (undef, %arg) = @_;
		833
		834	my $signal = uc $arg{signal}
		835	or Carp::croak "required option 'signal' is missing";
		836
		837	$SIG_CB{$signal}{$arg{cb}} = $arg{cb};
		838	$SIG{$signal} ||= sub {
		839	$_->() for values %{ $SIG_CB{$signal} || {} };
		840	};
		841
		842	bless [$signal, $arg{cb}], "AnyEvent::Base::Signal"
		843	}
		844
		845	sub AnyEvent::Base::Signal::DESTROY {
		846	my ($signal, $cb) = @{$_[0]};
		847
		848	delete $SIG_CB{$signal}{$cb};
		849
		850	$SIG{$signal} = 'DEFAULT' unless keys %{ $SIG_CB{$signal} };
		851	}
		852
		853	# default implementation for ->child
		854
		855	our %PID_CB;
		856	our $CHLD_W;
		857	our $CHLD_DELAY_W;
		858	our $PID_IDLE;
		859	our $WNOHANG;
		860
		861	sub _child_wait {
		862	while (0 < (my $pid = waitpid -1, $WNOHANG)) {
		863	$_->($pid, $?) for (values %{ $PID_CB{$pid} || {} }),
		864	(values %{ $PID_CB{0} || {} });
38	}	865	}
39		866
40	for (@models) {	867	undef $PID_IDLE;
41	my ($model, $package) = @$_;	868	}
42	$EVENT = "AnyEvent::Impl::$model";	869
43	if (eval "require $EVENT") {	870	sub _sigchld {
44	goto &{"$EVENT\::$AUTOLOAD"};	871	# make sure we deliver these changes "synchronous" with the event loop.
45	}	872	$CHLD_DELAY_W ||= AnyEvent->timer (after => 0, cb => sub {
		873	undef $CHLD_DELAY_W;
		874	&_child_wait;
		875	});
		876	}
		877
		878	sub child {
		879	my (undef, %arg) = @_;
		880
		881	defined (my $pid = $arg{pid} + 0)
		882	or Carp::croak "required option 'pid' is missing";
		883
		884	$PID_CB{$pid}{$arg{cb}} = $arg{cb};
		885
		886	unless ($WNOHANG) {
		887	$WNOHANG = eval { require POSIX; &POSIX::WNOHANG } || 1;
46	}	888	}
47		889
48	die "No event module selected for AnyEvent and autodetect failed. Install any of these: Coro, Event, Glib or Tk.";	890	unless ($CHLD_W) {
		891	$CHLD_W = AnyEvent->signal (signal => 'CHLD', cb => \&_sigchld);
		892	# child could be a zombie already, so make at least one round
		893	&_sigchld;
		894	}
		895
		896	bless [$pid, $arg{cb}], "AnyEvent::Base::Child"
49	}	897	}
50		898
51	1;	899	sub AnyEvent::Base::Child::DESTROY {
		900	my ($pid, $cb) = @{$_[0]};
52		901
		902	delete $PID_CB{$pid}{$cb};
		903	delete $PID_CB{$pid} unless keys %{ $PID_CB{$pid} };
		904
		905	undef $CHLD_W unless keys %PID_CB;
		906	}
		907
		908	=head1 SUPPLYING YOUR OWN EVENT MODEL INTERFACE
		909
		910	This is an advanced topic that you do not normally need to use AnyEvent in
		911	a module. This section is only of use to event loop authors who want to
		912	provide AnyEvent compatibility.
		913
		914	If you need to support another event library which isn't directly
		915	supported by AnyEvent, you can supply your own interface to it by
		916	pushing, before the first watcher gets created, the package name of
		917	the event module and the package name of the interface to use onto
		918	C<@AnyEvent::REGISTRY>. You can do that before and even without loading
		919	AnyEvent, so it is reasonably cheap.
		920
		921	Example:
		922
		923	push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];
		924
		925	This tells AnyEvent to (literally) use the C<urxvt::anyevent::>
		926	package/class when it finds the C<urxvt> package/module is already loaded.
		927
		928	When AnyEvent is loaded and asked to find a suitable event model, it
		929	will first check for the presence of urxvt by trying to C<use> the
		930	C<urxvt::anyevent> module.
		931
		932	The class should provide implementations for all watcher types. See
		933	L<AnyEvent::Impl::EV> (source code), L<AnyEvent::Impl::Glib> (Source code)
		934	and so on for actual examples. Use C<perldoc -m AnyEvent::Impl::Glib> to
		935	see the sources.
		936
		937	If you don't provide C<signal> and C<child> watchers than AnyEvent will
		938	provide suitable (hopefully) replacements.
		939
		940	The above example isn't fictitious, the I<rxvt-unicode> (a.k.a. urxvt)
		941	terminal emulator uses the above line as-is. An interface isn't included
		942	in AnyEvent because it doesn't make sense outside the embedded interpreter
		943	inside I<rxvt-unicode>, and it is updated and maintained as part of the
		944	I<rxvt-unicode> distribution.
		945
		946	I<rxvt-unicode> also cheats a bit by not providing blocking access to
		947	condition variables: code blocking while waiting for a condition will
		948	C<die>. This still works with most modules/usages, and blocking calls must
		949	not be done in an interactive application, so it makes sense.
		950
		951	=head1 ENVIRONMENT VARIABLES
		952
		953	The following environment variables are used by this module:
		954
		955	=over 4
		956
		957	=item C<PERL_ANYEVENT_VERBOSE>
		958
		959	By default, AnyEvent will be completely silent except in fatal
		960	conditions. You can set this environment variable to make AnyEvent more
		961	talkative.
		962
		963	When set to C<1> or higher, causes AnyEvent to warn about unexpected
		964	conditions, such as not being able to load the event model specified by
		965	C<PERL_ANYEVENT_MODEL>.
		966
		967	When set to C<2> or higher, cause AnyEvent to report to STDERR which event
		968	model it chooses.
		969
		970	=item C<PERL_ANYEVENT_MODEL>
		971
		972	This can be used to specify the event model to be used by AnyEvent, before
		973	autodetection and -probing kicks in. It must be a string consisting
		974	entirely of ASCII letters. The string C<AnyEvent::Impl::> gets prepended
		975	and the resulting module name is loaded and if the load was successful,
		976	used as event model. If it fails to load AnyEvent will proceed with
		977	autodetection and -probing.
		978
		979	This functionality might change in future versions.
		980
		981	For example, to force the pure perl model (L<AnyEvent::Impl::Perl>) you
		982	could start your program like this:
		983
		984	PERL_ANYEVENT_MODEL=Perl perl ...
		985
		986	=back
		987
		988	=head1 EXAMPLE PROGRAM
		989
		990	The following program uses an I/O watcher to read data from STDIN, a timer
		991	to display a message once per second, and a condition variable to quit the
		992	program when the user enters quit:
		993
		994	use AnyEvent;
		995
		996	my $cv = AnyEvent->condvar;
		997
		998	my $io_watcher = AnyEvent->io (
		999	fh => \*STDIN,
		1000	poll => 'r',
		1001	cb => sub {
		1002	warn "io event <$_[0]>\n"; # will always output <r>
		1003	chomp (my $input = <STDIN>); # read a line
		1004	warn "read: $input\n"; # output what has been read
		1005	$cv->broadcast if $input =~ /^q/i; # quit program if /^q/i
		1006	},
		1007	);
		1008
		1009	my $time_watcher; # can only be used once
		1010
		1011	sub new_timer {
		1012	$timer = AnyEvent->timer (after => 1, cb => sub {
		1013	warn "timeout\n"; # print 'timeout' about every second
		1014	&new_timer; # and restart the time
		1015	});
		1016	}
		1017
		1018	new_timer; # create first timer
		1019
		1020	$cv->wait; # wait until user enters /^q/i
		1021
		1022	=head1 REAL-WORLD EXAMPLE
		1023
		1024	Consider the L<Net::FCP> module. It features (among others) the following
		1025	API calls, which are to freenet what HTTP GET requests are to http:
		1026
		1027	my $data = $fcp->client_get ($url); # blocks
		1028
		1029	my $transaction = $fcp->txn_client_get ($url); # does not block
		1030	$transaction->cb ( sub { ... } ); # set optional result callback
		1031	my $data = $transaction->result; # possibly blocks
		1032
		1033	The C<client_get> method works like C<LWP::Simple::get>: it requests the
		1034	given URL and waits till the data has arrived. It is defined to be:
		1035
		1036	sub client_get { $_[0]->txn_client_get ($_[1])->result }
		1037
		1038	And in fact is automatically generated. This is the blocking API of
		1039	L<Net::FCP>, and it works as simple as in any other, similar, module.
		1040
		1041	More complicated is C<txn_client_get>: It only creates a transaction
		1042	(completion, result, ...) object and initiates the transaction.
		1043
		1044	my $txn = bless { }, Net::FCP::Txn::;
		1045
		1046	It also creates a condition variable that is used to signal the completion
		1047	of the request:
		1048
		1049	$txn->{finished} = AnyAvent->condvar;
		1050
		1051	It then creates a socket in non-blocking mode.
		1052
		1053	socket $txn->{fh}, ...;
		1054	fcntl $txn->{fh}, F_SETFL, O_NONBLOCK;
		1055	connect $txn->{fh}, ...
		1056	and !$!{EWOULDBLOCK}
		1057	and !$!{EINPROGRESS}
		1058	and Carp::croak "unable to connect: $!\n";
		1059
		1060	Then it creates a write-watcher which gets called whenever an error occurs
		1061	or the connection succeeds:
		1062
		1063	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'w', cb => sub { $txn->fh_ready_w });
		1064
		1065	And returns this transaction object. The C<fh_ready_w> callback gets
		1066	called as soon as the event loop detects that the socket is ready for
		1067	writing.
		1068
		1069	The C<fh_ready_w> method makes the socket blocking again, writes the
		1070	request data and replaces the watcher by a read watcher (waiting for reply
		1071	data). The actual code is more complicated, but that doesn't matter for
		1072	this example:
		1073
		1074	fcntl $txn->{fh}, F_SETFL, 0;
		1075	syswrite $txn->{fh}, $txn->{request}
		1076	or die "connection or write error";
		1077	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });
		1078
		1079	Again, C<fh_ready_r> waits till all data has arrived, and then stores the
		1080	result and signals any possible waiters that the request ahs finished:
		1081
		1082	sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};
		1083
		1084	if (end-of-file or data complete) {
		1085	$txn->{result} = $txn->{buf};
		1086	$txn->{finished}->broadcast;
		1087	$txb->{cb}->($txn) of $txn->{cb}; # also call callback
		1088	}
		1089
		1090	The C<result> method, finally, just waits for the finished signal (if the
		1091	request was already finished, it doesn't wait, of course, and returns the
		1092	data:
		1093
		1094	$txn->{finished}->wait;
		1095	return $txn->{result};
		1096
		1097	The actual code goes further and collects all errors (C<die>s, exceptions)
		1098	that occured during request processing. The C<result> method detects
		1099	whether an exception as thrown (it is stored inside the $txn object)
		1100	and just throws the exception, which means connection errors and other
		1101	problems get reported tot he code that tries to use the result, not in a
		1102	random callback.
		1103
		1104	All of this enables the following usage styles:
		1105
		1106	1. Blocking:
		1107
		1108	my $data = $fcp->client_get ($url);
		1109
		1110	2. Blocking, but running in parallel:
		1111
		1112	my @datas = map $_->result,
		1113	map $fcp->txn_client_get ($_),
		1114	@urls;
		1115
		1116	Both blocking examples work without the module user having to know
		1117	anything about events.
		1118
		1119	3a. Event-based in a main program, using any supported event module:
		1120
		1121	use EV;
		1122
		1123	$fcp->txn_client_get ($url)->cb (sub {
		1124	my $txn = shift;
		1125	my $data = $txn->result;
		1126	...
		1127	});
		1128
		1129	EV::loop;
		1130
		1131	3b. The module user could use AnyEvent, too:
		1132
		1133	use AnyEvent;
		1134
		1135	my $quit = AnyEvent->condvar;
		1136
		1137	$fcp->txn_client_get ($url)->cb (sub {
		1138	...
		1139	$quit->broadcast;
		1140	});
		1141
		1142	$quit->wait;
		1143
		1144
		1145	=head1 BENCHMARKS
		1146
		1147	To give you an idea of the performance and overheads that AnyEvent adds
		1148	over the event loops themselves and to give you an impression of the speed
		1149	of various event loops I prepared some benchmarks.
		1150
		1151	=head2 BENCHMARKING ANYEVENT OVERHEAD
		1152
		1153	Here is a benchmark of various supported event models used natively and
		1154	through anyevent. The benchmark creates a lot of timers (with a zero
		1155	timeout) and I/O watchers (watching STDOUT, a pty, to become writable,
		1156	which it is), lets them fire exactly once and destroys them again.
		1157
		1158	Source code for this benchmark is found as F<eg/bench> in the AnyEvent
		1159	distribution.
		1160
		1161	=head3 Explanation of the columns
		1162
		1163	I<watcher> is the number of event watchers created/destroyed. Since
		1164	different event models feature vastly different performances, each event
		1165	loop was given a number of watchers so that overall runtime is acceptable
		1166	and similar between tested event loop (and keep them from crashing): Glib
		1167	would probably take thousands of years if asked to process the same number
		1168	of watchers as EV in this benchmark.
		1169
		1170	I<bytes> is the number of bytes (as measured by the resident set size,
		1171	RSS) consumed by each watcher. This method of measuring captures both C
		1172	and Perl-based overheads.
		1173
		1174	I<create> is the time, in microseconds (millionths of seconds), that it
		1175	takes to create a single watcher. The callback is a closure shared between
		1176	all watchers, to avoid adding memory overhead. That means closure creation
		1177	and memory usage is not included in the figures.
		1178
		1179	I<invoke> is the time, in microseconds, used to invoke a simple
		1180	callback. The callback simply counts down a Perl variable and after it was
		1181	invoked "watcher" times, it would C<< ->broadcast >> a condvar once to
		1182	signal the end of this phase.
		1183
		1184	I<destroy> is the time, in microseconds, that it takes to destroy a single
		1185	watcher.
		1186
		1187	=head3 Results
		1188
		1189	name watchers bytes create invoke destroy comment
		1190	EV/EV 400000 244 0.56 0.46 0.31 EV native interface
		1191	EV/Any 100000 244 2.50 0.46 0.29 EV + AnyEvent watchers
		1192	CoroEV/Any 100000 244 2.49 0.44 0.29 coroutines + Coro::Signal
		1193	Perl/Any 100000 513 4.92 0.87 1.12 pure perl implementation
		1194	Event/Event 16000 516 31.88 31.30 0.85 Event native interface
		1195	Event/Any 16000 590 35.75 31.42 1.08 Event + AnyEvent watchers
		1196	Glib/Any 16000 1357 98.22 12.41 54.00 quadratic behaviour
		1197	Tk/Any 2000 1860 26.97 67.98 14.00 SEGV with >> 2000 watchers
		1198	POE/Event 2000 6644 108.64 736.02 14.73 via POE::Loop::Event
		1199	POE/Select 2000 6343 94.13 809.12 565.96 via POE::Loop::Select
		1200
		1201	=head3 Discussion
		1202
		1203	The benchmark does I<not> measure scalability of the event loop very
		1204	well. For example, a select-based event loop (such as the pure perl one)
		1205	can never compete with an event loop that uses epoll when the number of
		1206	file descriptors grows high. In this benchmark, all events become ready at
		1207	the same time, so select/poll-based implementations get an unnatural speed
		1208	boost.
		1209
		1210	Also, note that the number of watchers usually has a nonlinear effect on
		1211	overall speed, that is, creating twice as many watchers doesn't take twice
		1212	the time - usually it takes longer. This puts event loops tested with a
		1213	higher number of watchers at a disadvantage.
		1214
		1215	To put the range of results into perspective, consider that on the
		1216	benchmark machine, handling an event takes roughly 1600 CPU cycles with
		1217	EV, 3100 CPU cycles with AnyEvent's pure perl loop and almost 3000000 CPU
		1218	cycles with POE.
		1219
		1220	C<EV> is the sole leader regarding speed and memory use, which are both
		1221	maximal/minimal, respectively. Even when going through AnyEvent, it uses
		1222	far less memory than any other event loop and is still faster than Event
		1223	natively.
		1224
		1225	The pure perl implementation is hit in a few sweet spots (both the
		1226	constant timeout and the use of a single fd hit optimisations in the perl
		1227	interpreter and the backend itself). Nevertheless this shows that it
		1228	adds very little overhead in itself. Like any select-based backend its
		1229	performance becomes really bad with lots of file descriptors (and few of
		1230	them active), of course, but this was not subject of this benchmark.
		1231
		1232	The C<Event> module has a relatively high setup and callback invocation
		1233	cost, but overall scores in on the third place.
		1234
		1235	C<Glib>'s memory usage is quite a bit higher, but it features a
		1236	faster callback invocation and overall ends up in the same class as
		1237	C<Event>. However, Glib scales extremely badly, doubling the number of
		1238	watchers increases the processing time by more than a factor of four,
		1239	making it completely unusable when using larger numbers of watchers
		1240	(note that only a single file descriptor was used in the benchmark, so
		1241	inefficiencies of C<poll> do not account for this).
		1242
		1243	The C<Tk> adaptor works relatively well. The fact that it crashes with
		1244	more than 2000 watchers is a big setback, however, as correctness takes
		1245	precedence over speed. Nevertheless, its performance is surprising, as the
		1246	file descriptor is dup()ed for each watcher. This shows that the dup()
		1247	employed by some adaptors is not a big performance issue (it does incur a
		1248	hidden memory cost inside the kernel which is not reflected in the figures
		1249	above).
		1250
		1251	C<POE>, regardless of underlying event loop (whether using its pure perl
		1252	select-based backend or the Event module, the POE-EV backend couldn't
		1253	be tested because it wasn't working) shows abysmal performance and
		1254	memory usage with AnyEvent: Watchers use almost 30 times as much memory
		1255	as EV watchers, and 10 times as much memory as Event (the high memory
		1256	requirements are caused by requiring a session for each watcher). Watcher
		1257	invocation speed is almost 900 times slower than with AnyEvent's pure perl
		1258	implementation.
		1259
		1260	The design of the POE adaptor class in AnyEvent can not really account
		1261	for the performance issues, though, as session creation overhead is
		1262	small compared to execution of the state machine, which is coded pretty
		1263	optimally within L<AnyEvent::Impl::POE> (and while everybody agrees that
		1264	using multiple sessions is not a good approach, especially regarding
		1265	memory usage, even the author of POE could not come up with a faster
		1266	design).
		1267
		1268	=head3 Summary
		1269
		1270	=over 4
		1271
		1272	=item * Using EV through AnyEvent is faster than any other event loop
		1273	(even when used without AnyEvent), but most event loops have acceptable
		1274	performance with or without AnyEvent.
		1275
		1276	=item * The overhead AnyEvent adds is usually much smaller than the overhead of
		1277	the actual event loop, only with extremely fast event loops such as EV
		1278	adds AnyEvent significant overhead.
		1279
		1280	=item * You should avoid POE like the plague if you want performance or
		1281	reasonable memory usage.
		1282
		1283	=back
		1284
		1285	=head2 BENCHMARKING THE LARGE SERVER CASE
		1286
		1287	This benchmark atcually benchmarks the event loop itself. It works by
		1288	creating a number of "servers": each server consists of a socketpair, a
		1289	timeout watcher that gets reset on activity (but never fires), and an I/O
		1290	watcher waiting for input on one side of the socket. Each time the socket
		1291	watcher reads a byte it will write that byte to a random other "server".
		1292
		1293	The effect is that there will be a lot of I/O watchers, only part of which
		1294	are active at any one point (so there is a constant number of active
		1295	fds for each loop iterstaion, but which fds these are is random). The
		1296	timeout is reset each time something is read because that reflects how
		1297	most timeouts work (and puts extra pressure on the event loops).
		1298
		1299	In this benchmark, we use 10000 socketpairs (20000 sockets), of which 100
		1300	(1%) are active. This mirrors the activity of large servers with many
		1301	connections, most of which are idle at any one point in time.
		1302
		1303	Source code for this benchmark is found as F<eg/bench2> in the AnyEvent
		1304	distribution.
		1305
		1306	=head3 Explanation of the columns
		1307
		1308	I<sockets> is the number of sockets, and twice the number of "servers" (as
		1309	each server has a read and write socket end).
		1310
		1311	I<create> is the time it takes to create a socketpair (which is
		1312	nontrivial) and two watchers: an I/O watcher and a timeout watcher.
		1313
		1314	I<request>, the most important value, is the time it takes to handle a
		1315	single "request", that is, reading the token from the pipe and forwarding
		1316	it to another server. This includes deleting the old timeout and creating
		1317	a new one that moves the timeout into the future.
		1318
		1319	=head3 Results
		1320
		1321	name sockets create request
		1322	EV 20000 69.01 11.16
		1323	Perl 20000 73.32 35.87
		1324	Event 20000 212.62 257.32
		1325	Glib 20000 651.16 1896.30
		1326	POE 20000 349.67 12317.24 uses POE::Loop::Event
		1327
		1328	=head3 Discussion
		1329
		1330	This benchmark I<does> measure scalability and overall performance of the
		1331	particular event loop.
		1332
		1333	EV is again fastest. Since it is using epoll on my system, the setup time
		1334	is relatively high, though.
		1335
		1336	Perl surprisingly comes second. It is much faster than the C-based event
		1337	loops Event and Glib.
		1338
		1339	Event suffers from high setup time as well (look at its code and you will
		1340	understand why). Callback invocation also has a high overhead compared to
		1341	the C<< $_->() for .. >>-style loop that the Perl event loop uses. Event
		1342	uses select or poll in basically all documented configurations.
		1343
		1344	Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It
		1345	clearly fails to perform with many filehandles or in busy servers.
		1346
		1347	POE is still completely out of the picture, taking over 1000 times as long
		1348	as EV, and over 100 times as long as the Perl implementation, even though
		1349	it uses a C-based event loop in this case.
		1350
		1351	=head3 Summary
		1352
		1353	=over 4
		1354
		1355	=item * The pure perl implementation performs extremely well.
		1356
		1357	=item * Avoid Glib or POE in large projects where performance matters.
		1358
		1359	=back
		1360
		1361	=head2 BENCHMARKING SMALL SERVERS
		1362
		1363	While event loops should scale (and select-based ones do not...) even to
		1364	large servers, most programs we (or I :) actually write have only a few
		1365	I/O watchers.
		1366
		1367	In this benchmark, I use the same benchmark program as in the large server
		1368	case, but it uses only eight "servers", of which three are active at any
		1369	one time. This should reflect performance for a small server relatively
		1370	well.
		1371
		1372	The columns are identical to the previous table.
		1373
		1374	=head3 Results
		1375
		1376	name sockets create request
		1377	EV 16 20.00 6.54
		1378	Perl 16 25.75 12.62
		1379	Event 16 81.27 35.86
		1380	Glib 16 32.63 15.48
		1381	POE 16 261.87 276.28 uses POE::Loop::Event
		1382
		1383	=head3 Discussion
		1384
		1385	The benchmark tries to test the performance of a typical small
		1386	server. While knowing how various event loops perform is interesting, keep
		1387	in mind that their overhead in this case is usually not as important, due
		1388	to the small absolute number of watchers (that is, you need efficiency and
		1389	speed most when you have lots of watchers, not when you only have a few of
		1390	them).
		1391
		1392	EV is again fastest.
		1393
		1394	Perl again comes second. It is noticably faster than the C-based event
		1395	loops Event and Glib, although the difference is too small to really
		1396	matter.
		1397
		1398	POE also performs much better in this case, but is is still far behind the
		1399	others.
		1400
		1401	=head3 Summary
		1402
		1403	=over 4
		1404
		1405	=item * C-based event loops perform very well with small number of
		1406	watchers, as the management overhead dominates.
		1407
		1408	=back
		1409
		1410
		1411	=head1 FORK
		1412
		1413	Most event libraries are not fork-safe. The ones who are usually are
		1414	because they rely on inefficient but fork-safe C<select> or C<poll>
		1415	calls. Only L<EV> is fully fork-aware.
		1416
		1417	If you have to fork, you must either do so I<before> creating your first
		1418	watcher OR you must not use AnyEvent at all in the child.
		1419
		1420
		1421	=head1 SECURITY CONSIDERATIONS
		1422
		1423	AnyEvent can be forced to load any event model via
		1424	$ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used to
		1425	execute arbitrary code or directly gain access, it can easily be used to
		1426	make the program hang or malfunction in subtle ways, as AnyEvent watchers
		1427	will not be active when the program uses a different event model than
		1428	specified in the variable.
		1429
		1430	You can make AnyEvent completely ignore this variable by deleting it
		1431	before the first watcher gets created, e.g. with a C<BEGIN> block:
		1432
		1433	BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} }
		1434
		1435	use AnyEvent;
		1436
		1437	Similar considerations apply to $ENV{PERL_ANYEVENT_VERBOSE}, as that can
		1438	be used to probe what backend is used and gain other information (which is
		1439	probably even less useful to an attacker than PERL_ANYEVENT_MODEL).
		1440
		1441
		1442	=head1 SEE ALSO
		1443
		1444	Event modules: L<EV>, L<EV::Glib>, L<Glib::EV>, L<Event>, L<Glib::Event>,
		1445	L<Glib>, L<Tk>, L<Event::Lib>, L<Qt>, L<POE>.
		1446
		1447	Implementations: L<AnyEvent::Impl::EV>, L<AnyEvent::Impl::Event>,
		1448	L<AnyEvent::Impl::Glib>, L<AnyEvent::Impl::Tk>, L<AnyEvent::Impl::Perl>,
		1449	L<AnyEvent::Impl::EventLib>, L<AnyEvent::Impl::Qt>,
		1450	L<AnyEvent::Impl::POE>.
		1451
		1452	Coroutine support: L<Coro>, L<Coro::AnyEvent>, L<Coro::EV>, L<Coro::Event>,
		1453
		1454	Nontrivial usage examples: L<Net::FCP>, L<Net::XMPP2>.
		1455
		1456
		1457	=head1 AUTHOR
		1458
		1459	Marc Lehmann <schmorp@schmorp.de>
		1460	http://home.schmorp.de/
		1461
		1462	=cut
		1463
		1464	1
		1465

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