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Comparing AnyEvent/README (file contents):
Revision 1.13 by root, Fri Nov 23 10:42:00 2007 UTC vs.
Revision 1.22 by root, Sat May 24 17:58:33 2008 UTC

…		…
1	NAME	1	=> NAME
2	AnyEvent - provide framework for multiple event loops	2	AnyEvent - provide framework for multiple event loops
3		3
4	Event, Coro, Glib, Tk, Perl - various supported event loops	4	EV, Event, Glib, Tk, Perl, Event::Lib, Qt, POE - various supported event
		5	loops
5		6
6	SYNOPSIS	7	SYNOPSIS
7	use AnyEvent;	8	use AnyEvent;
8		9
9	my $w = AnyEvent->io (fh => $fh, poll => "r\|w", cb => sub {	10	my $w = AnyEvent->io (fh => $fh, poll => "r\|w", cb => sub {
12		13
13	my $w = AnyEvent->timer (after => $seconds, cb => sub {	14	my $w = AnyEvent->timer (after => $seconds, cb => sub {
14	...	15	...
15	});	16	});
16		17
17	my $w = AnyEvent->condvar; # stores wether a condition was flagged	18	my $w = AnyEvent->condvar; # stores whether a condition was flagged
		19	$w->send; # wake up current and all future recv's
18	$w->wait; # enters "main loop" till $condvar gets ->broadcast	20	$w->recv; # enters "main loop" till $condvar gets ->send
19	$w->broadcast; # wake up current and all future wait's	21
		22	WHY YOU SHOULD USE THIS MODULE (OR NOT)
		23	Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
		24	nowadays. So what is different about AnyEvent?
		25
		26	Executive Summary: AnyEvent is compatible, AnyEvent is *free of
		27	policy* and AnyEvent is small and efficient.
		28
		29	First and foremost, AnyEvent is not an event model itself, it only
		30	interfaces to whatever event model the main program happens to use in a
		31	pragmatic way. For event models and certain classes of immortals alike,
		32	the statement "there can only be one" is a bitter reality: In general,
		33	only one event loop can be active at the same time in a process.
		34	AnyEvent helps hiding the differences between those event loops.
		35
		36	The goal of AnyEvent is to offer module authors the ability to do event
		37	programming (waiting for I/O or timer events) without subscribing to a
		38	religion, a way of living, and most importantly: without forcing your
		39	module users into the same thing by forcing them to use the same event
		40	model you use.
		41
		42	For modules like POE or IO::Async (which is a total misnomer as it is
		43	actually doing all I/O synchronously...), using them in your module is
		44	like joining a cult: After you joined, you are dependent on them and you
		45	cannot use anything else, as it is simply incompatible to everything
		46	that isn't itself. What's worse, all the potential users of your module
		47	are also forced to use the same event loop you use.
		48
		49	AnyEvent is different: AnyEvent + POE works fine. AnyEvent + Glib works
		50	fine. AnyEvent + Tk works fine etc. etc. but none of these work together
		51	with the rest: POE + IO::Async? no go. Tk + Event? no go. Again: if your
		52	module uses one of those, every user of your module has to use it, too.
		53	But if your module uses AnyEvent, it works transparently with all event
		54	models it supports (including stuff like POE and IO::Async, as long as
		55	those use one of the supported event loops. It is trivial to add new
		56	event loops to AnyEvent, too, so it is future-proof).
		57
		58	In addition to being free of having to use *the one and only true event
		59	model*, AnyEvent also is free of bloat and policy: with POE or similar
		60	modules, you get an enormous amount of code and strict rules you have to
		61	follow. AnyEvent, on the other hand, is lean and up to the point, by
		62	only offering the functionality that is necessary, in as thin as a
		63	wrapper as technically possible.
		64
		65	Of course, if you want lots of policy (this can arguably be somewhat
		66	useful) and you want to force your users to use the one and only event
		67	model, you should not use this module.
20		68
21	DESCRIPTION	69	DESCRIPTION
22	AnyEvent provides an identical interface to multiple event loops. This	70	AnyEvent provides an identical interface to multiple event loops. This
23	allows module authors to utilise an event loop without forcing module	71	allows module authors to utilise an event loop without forcing module
24	users to use the same event loop (as only a single event loop can	72	users to use the same event loop (as only a single event loop can
25	coexist peacefully at any one time).	73	coexist peacefully at any one time).
26		74
27	The interface itself is vaguely similar but not identical to the Event	75	The interface itself is vaguely similar, but not identical to the Event
28	module.	76	module.
29		77
30	On the first call of any method, the module tries to detect the	78	During the first call of any watcher-creation method, the module tries
31	currently loaded event loop by probing wether any of the following	79	to detect the currently loaded event loop by probing whether one of the
32	modules is loaded: Coro::Event, Event, Glib, Tk. The first one found is	80	following modules is already loaded: EV, Event, Glib,
		81	AnyEvent::Impl::Perl, Tk, Event::Lib, Qt, POE. The first one found is
33	used. If none is found, the module tries to load these modules in the	82	used. If none are found, the module tries to load these modules
34	order given. The first one that could be successfully loaded will be	83	(excluding Tk, Event::Lib, Qt and POE as the pure perl adaptor should
35	used. If still none could be found, AnyEvent will fall back to a	84	always succeed) in the order given. The first one that can be
36	pure-perl event loop, which is also not very efficient.	85	successfully loaded will be used. If, after this, still none could be
		86	found, AnyEvent will fall back to a pure-perl event loop, which is not
		87	very efficient, but should work everywhere.
37		88
38	Because AnyEvent first checks for modules that are already loaded,	89	Because AnyEvent first checks for modules that are already loaded,
39	loading an Event model explicitly before first using AnyEvent will	90	loading an event model explicitly before first using AnyEvent will
40	likely make that model the default. For example:	91	likely make that model the default. For example:
41		92
42	use Tk;	93	use Tk;
43	use AnyEvent;	94	use AnyEvent;
44		95
45	# .. AnyEvent will likely default to Tk	96	# .. AnyEvent will likely default to Tk
		97
		98	The likely means that, if any module loads another event model and
		99	starts using it, all bets are off. Maybe you should tell their authors
		100	to use AnyEvent so their modules work together with others seamlessly...
46		101
47	The pure-perl implementation of AnyEvent is called	102	The pure-perl implementation of AnyEvent is called
48	"AnyEvent::Impl::Perl". Like other event modules you can load it	103	"AnyEvent::Impl::Perl". Like other event modules you can load it
49	explicitly.	104	explicitly.
50		105
51	WATCHERS	106	WATCHERS
52	AnyEvent has the central concept of a watcher, which is an object that	107	AnyEvent has the central concept of a watcher, which is an object that
53	stores relevant data for each kind of event you are waiting for, such as	108	stores relevant data for each kind of event you are waiting for, such as
54	the callback to call, the filehandle to watch, etc.	109	the callback to call, the file handle to watch, etc.
55		110
56	These watchers are normal Perl objects with normal Perl lifetime. After	111	These watchers are normal Perl objects with normal Perl lifetime. After
57	creating a watcher it will immediately "watch" for events and invoke the	112	creating a watcher it will immediately "watch" for events and invoke the
		113	callback when the event occurs (of course, only when the event model is
		114	in control).
		115
58	callback. To disable the watcher you have to destroy it (e.g. by setting	116	To disable the watcher you have to destroy it (e.g. by setting the
59	the variable that stores it to "undef" or otherwise deleting all	117	variable you store it in to "undef" or otherwise deleting all references
60	references to it).	118	to it).
61		119
62	All watchers are created by calling a method on the "AnyEvent" class.	120	All watchers are created by calling a method on the "AnyEvent" class.
63		121
		122	Many watchers either are used with "recursion" (repeating timers for
		123	example), or need to refer to their watcher object in other ways.
		124
		125	An any way to achieve that is this pattern:
		126
		127	my $w; $w = AnyEvent->type (arg => value ..., cb => sub {
		128	# you can use $w here, for example to undef it
		129	undef $w;
		130	});
		131
		132	Note that "my $w; $w =" combination. This is necessary because in Perl,
		133	my variables are only visible after the statement in which they are
		134	declared.
		135
64	IO WATCHERS	136	I/O WATCHERS
65	You can create I/O watcher by calling the "AnyEvent->io" method with the	137	You can create an I/O watcher by calling the "AnyEvent->io" method with
66	following mandatory arguments:	138	the following mandatory key-value pairs as arguments:
67		139
68	"fh" the Perl filehandle (not filedescriptor) to watch for events.	140	"fh" the Perl file handle (not file descriptor) to watch for events.
69	"poll" must be a string that is either "r" or "w", that creates a	141	"poll" must be a string that is either "r" or "w", which creates a
70	watcher waiting for "r"eadable or "w"ritable events. "cb" the callback	142	watcher waiting for "r"eadable or "w"ritable events, respectively. "cb"
71	to invoke everytime the filehandle becomes ready.	143	is the callback to invoke each time the file handle becomes ready.
72		144
73	Only one io watcher per "fh" and "poll" combination is allowed (i.e. on	145	Although the callback might get passed parameters, their value and
74	a socket you can have one r + one w, not any more (limitation comes from	146	presence is undefined and you cannot rely on them. Portable AnyEvent
75	Tk - if you are sure you are not using Tk this limitation is gone).	147	callbacks cannot use arguments passed to I/O watcher callbacks.
76		148
77	Filehandles will be kept alive, so as long as the watcher exists, the	149	The I/O watcher might use the underlying file descriptor or a copy of
78	filehandle exists, too.	150	it. You must not close a file handle as long as any watcher is active on
		151	the underlying file descriptor.
		152
		153	Some event loops issue spurious readyness notifications, so you should
		154	always use non-blocking calls when reading/writing from/to your file
		155	handles.
79		156
80	Example:	157	Example:
81		158
82	# wait for readability of STDIN, then read a line and disable the watcher	159	# wait for readability of STDIN, then read a line and disable the watcher
83	my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {	160	my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
…		…
88		165
89	TIME WATCHERS	166	TIME WATCHERS
90	You can create a time watcher by calling the "AnyEvent->timer" method	167	You can create a time watcher by calling the "AnyEvent->timer" method
91	with the following mandatory arguments:	168	with the following mandatory arguments:
92		169
93	"after" after how many seconds (fractions are supported) should the	170	"after" specifies after how many seconds (fractional values are
94	timer activate. "cb" the callback to invoke.	171	supported) the callback should be invoked. "cb" is the callback to
		172	invoke in that case.
		173
		174	Although the callback might get passed parameters, their value and
		175	presence is undefined and you cannot rely on them. Portable AnyEvent
		176	callbacks cannot use arguments passed to time watcher callbacks.
95		177
96	The timer callback will be invoked at most once: if you want a repeating	178	The timer callback will be invoked at most once: if you want a repeating
97	timer you have to create a new watcher (this is a limitation by both Tk	179	timer you have to create a new watcher (this is a limitation by both Tk
98	and Glib).	180	and Glib).
99		181
…		…
105	});	187	});
106		188
107	# to cancel the timer:	189	# to cancel the timer:
108	undef $w;	190	undef $w;
109		191
110	CONDITION WATCHERS
111	Condition watchers can be created by calling the "AnyEvent->condvar"
112	method without any arguments.
113
114	A condition watcher watches for a condition - precisely that the
115	"->broadcast" method has been called.
116
117	The watcher has only two methods:
118
119	$cv->wait
120	Wait (blocking if necessary) until the "->broadcast" method has been
121	called on c<$cv>, while servicing other watchers normally.
122
123	Not all event models support a blocking wait - some die in that
124	case, so if you are using this from a module, never require a
125	blocking wait, but let the caller decide wether the call will block
126	or not (for example, by coupling condition variables with some kind
127	of request results and supporting callbacks so the caller knows that
128	getting the result will not block, while still suppporting blockign
129	waits if the caller so desires).
130
131	You can only wait once on a condition - additional calls will return
132	immediately.
133
134	$cv->broadcast
135	Flag the condition as ready - a running "->wait" and all further
136	calls to "wait" will return after this method has been called. If
137	nobody is waiting the broadcast will be remembered..
138
139	Example:	192	Example 2:
140		193
141	# wait till the result is ready	194	# fire an event after 0.5 seconds, then roughly every second
142	my $result_ready = AnyEvent->condvar;	195	my $w;
143		196
144	# do something such as adding a timer	197	my $cb = sub {
145	# or socket watcher the calls $result_ready->broadcast	198	# cancel the old timer while creating a new one
146	# when the "result" is ready.	199	$w = AnyEvent->timer (after => 1, cb => $cb);
		200	};
147		201
148	$result_ready->wait;	202	# start the "loop" by creating the first watcher
		203	$w = AnyEvent->timer (after => 0.5, cb => $cb);
		204
		205	TIMING ISSUES
		206	There are two ways to handle timers: based on real time (relative, "fire
		207	in 10 seconds") and based on wallclock time (absolute, "fire at 12
		208	o'clock").
		209
		210	While most event loops expect timers to specified in a relative way,
		211	they use absolute time internally. This makes a difference when your
		212	clock "jumps", for example, when ntp decides to set your clock backwards
		213	from the wrong date of 2014-01-01 to 2008-01-01, a watcher that is
		214	supposed to fire "after" a second might actually take six years to
		215	finally fire.
		216
		217	AnyEvent cannot compensate for this. The only event loop that is
		218	conscious about these issues is EV, which offers both relative
		219	(ev_timer, based on true relative time) and absolute (ev_periodic, based
		220	on wallclock time) timers.
		221
		222	AnyEvent always prefers relative timers, if available, matching the
		223	AnyEvent API.
149		224
150	SIGNAL WATCHERS	225	SIGNAL WATCHERS
151	You can listen for signals using a signal watcher, "signal" is the	226	You can watch for signals using a signal watcher, "signal" is the signal
152	signal name without any "SIG" prefix. Multiple signals events can be	227	name without any "SIG" prefix, "cb" is the Perl callback to be invoked
153	clumped together into one callback invocation, and callback invocation	228	whenever a signal occurs.
154	might or might not be asynchronous.
155		229
		230	Although the callback might get passed parameters, their value and
		231	presence is undefined and you cannot rely on them. Portable AnyEvent
		232	callbacks cannot use arguments passed to signal watcher callbacks.
		233
		234	Multiple signal occurrences can be clumped together into one callback
		235	invocation, and callback invocation will be synchronous. Synchronous
		236	means that it might take a while until the signal gets handled by the
		237	process, but it is guaranteed not to interrupt any other callbacks.
		238
		239	The main advantage of using these watchers is that you can share a
		240	signal between multiple watchers.
		241
156	These watchers might use %SIG, so programs overwriting those signals	242	This watcher might use %SIG, so programs overwriting those signals
157	directly will likely not work correctly.	243	directly will likely not work correctly.
158		244
159	Example: exit on SIGINT	245	Example: exit on SIGINT
160		246
161	my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });	247	my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });
162		248
163	CHILD PROCESS WATCHERS	249	CHILD PROCESS WATCHERS
164	You can also listen for the status of a child process specified by the	250	You can also watch on a child process exit and catch its exit status.
165	"pid" argument (or any child if the pid argument is 0). The watcher will	251
		252	The child process is specified by the "pid" argument (if set to 0, it
		253	watches for any child process exit). The watcher will trigger as often
166	trigger as often as status change for the child are received. This works	254	as status change for the child are received. This works by installing a
167	by installing a signal handler for "SIGCHLD". The callback will be	255	signal handler for "SIGCHLD". The callback will be called with the pid
168	called with the pid and exit status (as returned by waitpid).	256	and exit status (as returned by waitpid), so unlike other watcher types,
		257	you can rely on child watcher callback arguments.
169		258
		259	There is a slight catch to child watchers, however: you usually start
		260	them after the child process was created, and this means the process
		261	could have exited already (and no SIGCHLD will be sent anymore).
		262
		263	Not all event models handle this correctly (POE doesn't), but even for
		264	event models that do handle this correctly, they usually need to be
		265	loaded before the process exits (i.e. before you fork in the first
		266	place).
		267
		268	This means you cannot create a child watcher as the very first thing in
		269	an AnyEvent program, you have to create at least one watcher before
		270	you "fork" the child (alternatively, you can call "AnyEvent::detect").
		271
		272	Example: fork a process and wait for it
		273
		274	my $done = AnyEvent->condvar;
		275
		276	my $pid = fork or exit 5;
		277
		278	my $w = AnyEvent->child (
		279	pid => $pid,
		280	cb => sub {
		281	my ($pid, $status) = @_;
		282	warn "pid $pid exited with status $status";
		283	$done->send;
		284	},
		285	);
		286
		287	# do something else, then wait for process exit
		288	$done->recv;
		289
		290	CONDITION VARIABLES
		291	If you are familiar with some event loops you will know that all of them
		292	require you to run some blocking "loop", "run" or similar function that
		293	will actively watch for new events and call your callbacks.
		294
		295	AnyEvent is different, it expects somebody else to run the event loop
		296	and will only block when necessary (usually when told by the user).
		297
		298	The instrument to do that is called a "condition variable", so called
		299	because they represent a condition that must become true.
		300
		301	Condition variables can be created by calling the "AnyEvent->condvar"
		302	method, usually without arguments. The only argument pair allowed is
		303	"cb", which specifies a callback to be called when the condition
		304	variable becomes true.
		305
		306	After creation, the condition variable is "false" until it becomes
		307	"true" by calling the "send" method (or calling the condition variable
		308	as if it were a callback).
		309
		310	Condition variables are similar to callbacks, except that you can
		311	optionally wait for them. They can also be called merge points - points
		312	in time where multiple outstanding events have been processed. And yet
		313	another way to call them is transactions - each condition variable can
		314	be used to represent a transaction, which finishes at some point and
		315	delivers a result.
		316
		317	Condition variables are very useful to signal that something has
		318	finished, for example, if you write a module that does asynchronous http
		319	requests, then a condition variable would be the ideal candidate to
		320	signal the availability of results. The user can either act when the
		321	callback is called or can synchronously "->recv" for the results.
		322
		323	You can also use them to simulate traditional event loops - for example,
		324	you can block your main program until an event occurs - for example, you
		325	could "->recv" in your main program until the user clicks the Quit
		326	button of your app, which would "->send" the "quit" event.
		327
		328	Note that condition variables recurse into the event loop - if you have
		329	two pieces of code that call "->recv" in a round-robin fashion, you
		330	lose. Therefore, condition variables are good to export to your caller,
		331	but you should avoid making a blocking wait yourself, at least in
		332	callbacks, as this asks for trouble.
		333
		334	Condition variables are represented by hash refs in perl, and the keys
		335	used by AnyEvent itself are all named "_ae_XXX" to make subclassing easy
		336	(it is often useful to build your own transaction class on top of
		337	AnyEvent). To subclass, use "AnyEvent::CondVar" as base class and call
		338	it's "new" method in your own "new" method.
		339
		340	There are two "sides" to a condition variable - the "producer side"
		341	which eventually calls "-> send", and the "consumer side", which waits
		342	for the send to occur.
		343
170	Example: wait for pid 1333	344	Example: wait for a timer.
171		345
172	my $w = AnyEvent->child (pid => 1333, cb => sub { warn "exit status $?" });	346	# wait till the result is ready
		347	my $result_ready = AnyEvent->condvar;
173		348
174	GLOBALS	349	# do something such as adding a timer
		350	# or socket watcher the calls $result_ready->send
		351	# when the "result" is ready.
		352	# in this case, we simply use a timer:
		353	my $w = AnyEvent->timer (
		354	after => 1,
		355	cb => sub { $result_ready->send },
		356	);
		357
		358	# this "blocks" (while handling events) till the callback
		359	# calls send
		360	$result_ready->recv;
		361
		362	Example: wait for a timer, but take advantage of the fact that condition
		363	variables are also code references.
		364
		365	my $done = AnyEvent->condvar;
		366	my $delay = AnyEvent->timer (after => 5, cb => $done);
		367	$done->recv;
		368
		369	METHODS FOR PRODUCERS
		370	These methods should only be used by the producing side, i.e. the
		371	code/module that eventually sends the signal. Note that it is also the
		372	producer side which creates the condvar in most cases, but it isn't
		373	uncommon for the consumer to create it as well.
		374
		375	$cv->send (...)
		376	Flag the condition as ready - a running "->recv" and all further
		377	calls to "recv" will (eventually) return after this method has been
		378	called. If nobody is waiting the send will be remembered.
		379
		380	If a callback has been set on the condition variable, it is called
		381	immediately from within send.
		382
		383	Any arguments passed to the "send" call will be returned by all
		384	future "->recv" calls.
		385
		386	Condition variables are overloaded so one can call them directly (as
		387	a code reference). Calling them directly is the same as calling
		388	"send".
		389
		390	$cv->croak ($error)
		391	Similar to send, but causes all call's to "->recv" to invoke
		392	"Carp::croak" with the given error message/object/scalar.
		393
		394	This can be used to signal any errors to the condition variable
		395	user/consumer.
		396
		397	$cv->begin ([group callback])
		398	$cv->end
		399	These two methods are EXPERIMENTAL and MIGHT CHANGE.
		400
		401	These two methods can be used to combine many transactions/events
		402	into one. For example, a function that pings many hosts in parallel
		403	might want to use a condition variable for the whole process.
		404
		405	Every call to "->begin" will increment a counter, and every call to
		406	"->end" will decrement it. If the counter reaches 0 in "->end", the
		407	(last) callback passed to "begin" will be executed. That callback is
		408	supposed to call "->send", but that is not required. If no
		409	callback was set, "send" will be called without any arguments.
		410
		411	Let's clarify this with the ping example:
		412
		413	my $cv = AnyEvent->condvar;
		414
		415	my %result;
		416	$cv->begin (sub { $cv->send (\%result) });
		417
		418	for my $host (@list_of_hosts) {
		419	$cv->begin;
		420	ping_host_then_call_callback $host, sub {
		421	$result{$host} = ...;
		422	$cv->end;
		423	};
		424	}
		425
		426	$cv->end;
		427
		428	This code fragment supposedly pings a number of hosts and calls
		429	"send" after results for all then have have been gathered - in any
		430	order. To achieve this, the code issues a call to "begin" when it
		431	starts each ping request and calls "end" when it has received some
		432	result for it. Since "begin" and "end" only maintain a counter, the
		433	order in which results arrive is not relevant.
		434
		435	There is an additional bracketing call to "begin" and "end" outside
		436	the loop, which serves two important purposes: first, it sets the
		437	callback to be called once the counter reaches 0, and second, it
		438	ensures that "send" is called even when "no" hosts are being pinged
		439	(the loop doesn't execute once).
		440
		441	This is the general pattern when you "fan out" into multiple
		442	subrequests: use an outer "begin"/"end" pair to set the callback and
		443	ensure "end" is called at least once, and then, for each subrequest
		444	you start, call "begin" and for each subrequest you finish, call
		445	"end".
		446
		447	METHODS FOR CONSUMERS
		448	These methods should only be used by the consuming side, i.e. the code
		449	awaits the condition.
		450
		451	$cv->recv
		452	Wait (blocking if necessary) until the "->send" or "->croak" methods
		453	have been called on c<$cv>, while servicing other watchers normally.
		454
		455	You can only wait once on a condition - additional calls are valid
		456	but will return immediately.
		457
		458	If an error condition has been set by calling "->croak", then this
		459	function will call "croak".
		460
		461	In list context, all parameters passed to "send" will be returned,
		462	in scalar context only the first one will be returned.
		463
		464	Not all event models support a blocking wait - some die in that case
		465	(programs might want to do that to stay interactive), so *if you are
		466	using this from a module, never require a blocking wait*, but let
		467	the caller decide whether the call will block or not (for example,
		468	by coupling condition variables with some kind of request results
		469	and supporting callbacks so the caller knows that getting the result
		470	will not block, while still supporting blocking waits if the caller
		471	so desires).
		472
		473	Another reason never to "->recv" in a module is that you cannot
		474	sensibly have two "->recv"'s in parallel, as that would require
		475	multiple interpreters or coroutines/threads, none of which
		476	"AnyEvent" can supply.
		477
		478	The Coro module, however, can and does supply coroutines and, in
		479	fact, Coro::AnyEvent replaces AnyEvent's condvars by coroutine-safe
		480	versions and also integrates coroutines into AnyEvent, making
		481	blocking "->recv" calls perfectly safe as long as they are done from
		482	another coroutine (one that doesn't run the event loop).
		483
		484	You can ensure that "-recv" never blocks by setting a callback and
		485	only calling "->recv" from within that callback (or at a later
		486	time). This will work even when the event loop does not support
		487	blocking waits otherwise.
		488
		489	$bool = $cv->ready
		490	Returns true when the condition is "true", i.e. whether "send" or
		491	"croak" have been called.
		492
		493	$cb = $cv->cb ([new callback])
		494	This is a mutator function that returns the callback set and
		495	optionally replaces it before doing so.
		496
		497	The callback will be called when the condition becomes "true", i.e.
		498	when "send" or "croak" are called. Calling "recv" inside the
		499	callback or at any later time is guaranteed not to block.
		500
		501	GLOBAL VARIABLES AND FUNCTIONS
175	$AnyEvent::MODEL	502	$AnyEvent::MODEL
176	Contains "undef" until the first watcher is being created. Then it	503	Contains "undef" until the first watcher is being created. Then it
177	contains the event model that is being used, which is the name of	504	contains the event model that is being used, which is the name of
178	the Perl class implementing the model. This class is usually one of	505	the Perl class implementing the model. This class is usually one of
179	the "AnyEvent::Impl:xxx" modules, but can be any other class in the	506	the "AnyEvent::Impl:xxx" modules, but can be any other class in the
180	case AnyEvent has been extended at runtime (e.g. in rxvt-unicode).	507	case AnyEvent has been extended at runtime (e.g. in rxvt-unicode).
181		508
182	The known classes so far are:	509	The known classes so far are:
183		510
184	AnyEvent::Impl::CoroEV based on Coro::EV, best choice.
185	AnyEvent::Impl::EV based on EV (an interface to libev, also best choice).	511	AnyEvent::Impl::EV based on EV (an interface to libev, best choice).
186	AnyEvent::Impl::Coro based on Coro::Event, second best choice.
187	AnyEvent::Impl::Event based on Event, also second best choice :)	512	AnyEvent::Impl::Event based on Event, second best choice.
		513	AnyEvent::Impl::Perl pure-perl implementation, fast and portable.
188	AnyEvent::Impl::Glib based on Glib, second-best choice.	514	AnyEvent::Impl::Glib based on Glib, third-best choice.
189	AnyEvent::Impl::Tk based on Tk, very bad choice.	515	AnyEvent::Impl::Tk based on Tk, very bad choice.
190	AnyEvent::Impl::Perl pure-perl implementation, inefficient.	516	AnyEvent::Impl::Qt based on Qt, cannot be autoprobed (see its docs).
		517	AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse.
		518	AnyEvent::Impl::POE based on POE, not generic enough for full support.
		519
		520	There is no support for WxWidgets, as WxWidgets has no support for
		521	watching file handles. However, you can use WxWidgets through the
		522	POE Adaptor, as POE has a Wx backend that simply polls 20 times per
		523	second, which was considered to be too horrible to even consider for
		524	AnyEvent. Likewise, other POE backends can be used by AnyEvent by
		525	using it's adaptor.
		526
		527	AnyEvent knows about Prima and Wx and will try to use POE when
		528	autodetecting them.
191		529
192	AnyEvent::detect	530	AnyEvent::detect
193	Returns $AnyEvent::MODEL, forcing autodetection of the event model	531	Returns $AnyEvent::MODEL, forcing autodetection of the event model
194	if necessary. You should only call this function right before you	532	if necessary. You should only call this function right before you
195	would have created an AnyEvent watcher anyway, that is, very late at	533	would have created an AnyEvent watcher anyway, that is, as late as
196	runtime.	534	possible at runtime.
		535
		536	$guard = AnyEvent::post_detect { BLOCK }
		537	Arranges for the code block to be executed as soon as the event
		538	model is autodetected (or immediately if this has already happened).
		539
		540	If called in scalar or list context, then it creates and returns an
		541	object that automatically removes the callback again when it is
		542	destroyed. See Coro::BDB for a case where this is useful.
		543
		544	@AnyEvent::post_detect
		545	If there are any code references in this array (you can "push" to it
		546	before or after loading AnyEvent), then they will called directly
		547	after the event loop has been chosen.
		548
		549	You should check $AnyEvent::MODEL before adding to this array,
		550	though: if it contains a true value then the event loop has already
		551	been detected, and the array will be ignored.
		552
		553	Best use "AnyEvent::post_detect { BLOCK }" instead.
197		554
198	WHAT TO DO IN A MODULE	555	WHAT TO DO IN A MODULE
199	As a module author, you should "use AnyEvent" and call AnyEvent methods	556	As a module author, you should "use AnyEvent" and call AnyEvent methods
200	freely, but you should not load a specific event module or rely on it.	557	freely, but you should not load a specific event module or rely on it.
201		558
202	Be careful when you create watchers in the module body - Anyevent will	559	Be careful when you create watchers in the module body - AnyEvent will
203	decide which event module to use as soon as the first method is called,	560	decide which event module to use as soon as the first method is called,
204	so by calling AnyEvent in your module body you force the user of your	561	so by calling AnyEvent in your module body you force the user of your
205	module to load the event module first.	562	module to load the event module first.
206		563
		564	Never call "->recv" on a condition variable unless you know that the
		565	"->send" method has been called on it already. This is because it will
		566	stall the whole program, and the whole point of using events is to stay
		567	interactive.
		568
		569	It is fine, however, to call "->recv" when the user of your module
		570	requests it (i.e. if you create a http request object ad have a method
		571	called "results" that returns the results, it should call "->recv"
		572	freely, as the user of your module knows what she is doing. always).
		573
207	WHAT TO DO IN THE MAIN PROGRAM	574	WHAT TO DO IN THE MAIN PROGRAM
208	There will always be a single main program - the only place that should	575	There will always be a single main program - the only place that should
209	dictate which event model to use.	576	dictate which event model to use.
210		577
211	If it doesn't care, it can just "use AnyEvent" and use it itself, or not	578	If it doesn't care, it can just "use AnyEvent" and use it itself, or not
212	do anything special and let AnyEvent decide which implementation to	579	do anything special (it does not need to be event-based) and let
213	chose.	580	AnyEvent decide which implementation to chose if some module relies on
		581	it.
214		582
215	If the main program relies on a specific event model (for example, in	583	If the main program relies on a specific event model. For example, in
216	Gtk2 programs you have to rely on either Glib or Glib::Event), you	584	Gtk2 programs you have to rely on the Glib module. You should load the
217	should load it before loading AnyEvent or any module that uses it,	585	event module before loading AnyEvent or any module that uses it:
218	generally, as early as possible. The reason is that modules might create	586	generally speaking, you should load it as early as possible. The reason
219	watchers when they are loaded, and AnyEvent will decide on the event	587	is that modules might create watchers when they are loaded, and AnyEvent
220	model to use as soon as it creates watchers, and it might chose the	588	will decide on the event model to use as soon as it creates watchers,
221	wrong one unless you load the correct one yourself.	589	and it might chose the wrong one unless you load the correct one
		590	yourself.
222		591
223	You can chose to use a rather inefficient pure-perl implementation by	592	You can chose to use a rather inefficient pure-perl implementation by
224	loading the "AnyEvent::Impl::Perl" module, but letting AnyEvent chose is	593	loading the "AnyEvent::Impl::Perl" module, which gives you similar
225	generally better.	594	behaviour everywhere, but letting AnyEvent chose is generally better.
		595
		596	OTHER MODULES
		597	The following is a non-exhaustive list of additional modules that use
		598	AnyEvent and can therefore be mixed easily with other AnyEvent modules
		599	in the same program. Some of the modules come with AnyEvent, some are
		600	available via CPAN.
		601
		602	AnyEvent::Util
		603	Contains various utility functions that replace often-used but
		604	blocking functions such as "inet_aton" by event-/callback-based
		605	versions.
		606
		607	AnyEvent::Handle
		608	Provide read and write buffers and manages watchers for reads and
		609	writes.
		610
		611	AnyEvent::Socket
		612	Provides various utility functions for (internet protocol) sockets,
		613	addresses and name resolution. Also functions to create non-blocking
		614	tcp connections or tcp servers, with IPv6 and SRV record support and
		615	more.
		616
		617	AnyEvent::HTTPD
		618	Provides a simple web application server framework.
		619
		620	AnyEvent::DNS
		621	Provides rich asynchronous DNS resolver capabilities.
		622
		623	AnyEvent::FastPing
		624	The fastest ping in the west.
		625
		626	Net::IRC3
		627	AnyEvent based IRC client module family.
		628
		629	Net::XMPP2
		630	AnyEvent based XMPP (Jabber protocol) module family.
		631
		632	Net::FCP
		633	AnyEvent-based implementation of the Freenet Client Protocol,
		634	birthplace of AnyEvent.
		635
		636	Event::ExecFlow
		637	High level API for event-based execution flow control.
		638
		639	Coro
		640	Has special support for AnyEvent via Coro::AnyEvent.
		641
		642	AnyEvent::AIO, IO::AIO
		643	Truly asynchronous I/O, should be in the toolbox of every event
		644	programmer. AnyEvent::AIO transparently fuses IO::AIO and AnyEvent
		645	together.
		646
		647	AnyEvent::BDB, BDB
		648	Truly asynchronous Berkeley DB access. AnyEvent::AIO transparently
		649	fuses IO::AIO and AnyEvent together.
		650
		651	IO::Lambda
		652	The lambda approach to I/O - don't ask, look there. Can use
		653	AnyEvent.
226		654
227	SUPPLYING YOUR OWN EVENT MODEL INTERFACE	655	SUPPLYING YOUR OWN EVENT MODEL INTERFACE
		656	This is an advanced topic that you do not normally need to use AnyEvent
		657	in a module. This section is only of use to event loop authors who want
		658	to provide AnyEvent compatibility.
		659
228	If you need to support another event library which isn't directly	660	If you need to support another event library which isn't directly
229	supported by AnyEvent, you can supply your own interface to it by	661	supported by AnyEvent, you can supply your own interface to it by
230	pushing, before the first watcher gets created, the package name of the	662	pushing, before the first watcher gets created, the package name of the
231	event module and the package name of the interface to use onto	663	event module and the package name of the interface to use onto
232	@AnyEvent::REGISTRY. You can do that before and even without loading	664	@AnyEvent::REGISTRY. You can do that before and even without loading
233	AnyEvent.	665	AnyEvent, so it is reasonably cheap.
234		666
235	Example:	667	Example:
236		668
237	push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];	669	push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];
238		670
239	This tells AnyEvent to (literally) use the "urxvt::anyevent::"	671	This tells AnyEvent to (literally) use the "urxvt::anyevent::"
240	package/class when it finds the "urxvt" package/module is loaded. When	672	package/class when it finds the "urxvt" package/module is already
		673	loaded.
		674
241	AnyEvent is loaded and asked to find a suitable event model, it will	675	When AnyEvent is loaded and asked to find a suitable event model, it
242	first check for the presence of urxvt.	676	will first check for the presence of urxvt by trying to "use" the
		677	"urxvt::anyevent" module.
243		678
244	The class should provide implementations for all watcher types (see	679	The class should provide implementations for all watcher types. See
245	AnyEvent::Impl::Event (source code), AnyEvent::Impl::Glib (Source code)	680	AnyEvent::Impl::EV (source code), AnyEvent::Impl::Glib (Source code) and
246	and so on for actual examples, use "perldoc -m AnyEvent::Impl::Glib" to	681	so on for actual examples. Use "perldoc -m AnyEvent::Impl::Glib" to see
247	see the sources).	682	the sources.
248		683
		684	If you don't provide "signal" and "child" watchers than AnyEvent will
		685	provide suitable (hopefully) replacements.
		686
249	The above isn't fictitious, the rxvt-unicode (a.k.a. urxvt) uses the	687	The above example isn't fictitious, the rxvt-unicode (a.k.a. urxvt)
250	above line as-is. An interface isn't included in AnyEvent because it	688	terminal emulator uses the above line as-is. An interface isn't included
251	doesn't make sense outside the embedded interpreter inside	689	in AnyEvent because it doesn't make sense outside the embedded
252	rxvt-unicode, and it is updated and maintained as part of the	690	interpreter inside rxvt-unicode, and it is updated and maintained as
253	rxvt-unicode distribution.	691	part of the rxvt-unicode distribution.
254		692
255	rxvt-unicode also cheats a bit by not providing blocking access to	693	rxvt-unicode also cheats a bit by not providing blocking access to
256	condition variables: code blocking while waiting for a condition will	694	condition variables: code blocking while waiting for a condition will
257	"die". This still works with most modules/usages, and blocking calls	695	"die". This still works with most modules/usages, and blocking calls
258	must not be in an interactive application, so it makes sense.	696	must not be done in an interactive application, so it makes sense.
259		697
260	ENVIRONMENT VARIABLES	698	ENVIRONMENT VARIABLES
261	The following environment variables are used by this module:	699	The following environment variables are used by this module:
262		700
263	"PERL_ANYEVENT_VERBOSE" when set to 2 or higher, reports which event	701	"PERL_ANYEVENT_VERBOSE"
264	model gets used.	702	By default, AnyEvent will be completely silent except in fatal
		703	conditions. You can set this environment variable to make AnyEvent
		704	more talkative.
265		705
266	EXAMPLE	706	When set to 1 or higher, causes AnyEvent to warn about unexpected
		707	conditions, such as not being able to load the event model specified
		708	by "PERL_ANYEVENT_MODEL".
		709
		710	When set to 2 or higher, cause AnyEvent to report to STDERR which
		711	event model it chooses.
		712
		713	"PERL_ANYEVENT_MODEL"
		714	This can be used to specify the event model to be used by AnyEvent,
		715	before auto detection and -probing kicks in. It must be a string
		716	consisting entirely of ASCII letters. The string "AnyEvent::Impl::"
		717	gets prepended and the resulting module name is loaded and if the
		718	load was successful, used as event model. If it fails to load
		719	AnyEvent will proceed with auto detection and -probing.
		720
		721	This functionality might change in future versions.
		722
		723	For example, to force the pure perl model (AnyEvent::Impl::Perl) you
		724	could start your program like this:
		725
		726	PERL_ANYEVENT_MODEL=Perl perl ...
		727
		728	"PERL_ANYEVENT_PROTOCOLS"
		729	Used by both AnyEvent::DNS and AnyEvent::Socket to determine
		730	preferences for IPv4 or IPv6. The default is unspecified (and might
		731	change, or be the result of auto probing).
		732
		733	Must be set to a comma-separated list of protocols or address
		734	families, current supported: "ipv4" and "ipv6". Only protocols
		735	mentioned will be used, and preference will be given to protocols
		736	mentioned earlier in the list.
		737
		738	This variable can effectively be used for denial-of-service attacks
		739	against local programs (e.g. when setuid), although the impact is
		740	likely small, as the program has to handle connection errors
		741	already-
		742
		743	Examples: "PERL_ANYEVENT_PROTOCOLS=ipv4,ipv6" - prefer IPv4 over
		744	IPv6, but support both and try to use both.
		745	"PERL_ANYEVENT_PROTOCOLS=ipv4" - only support IPv4, never try to
		746	resolve or contact IPv6 addresses.
		747	"PERL_ANYEVENT_PROTOCOLS=ipv6,ipv4" support either IPv4 or IPv6, but
		748	prefer IPv6 over IPv4.
		749
		750	"PERL_ANYEVENT_EDNS0"
		751	Used by AnyEvent::DNS to decide whether to use the EDNS0 extension
		752	for DNS. This extension is generally useful to reduce DNS traffic,
		753	but some (broken) firewalls drop such DNS packets, which is why it
		754	is off by default.
		755
		756	Setting this variable to 1 will cause AnyEvent::DNS to announce
		757	EDNS0 in its DNS requests.
		758
		759	EXAMPLE PROGRAM
267	The following program uses an io watcher to read data from stdin, a	760	The following program uses an I/O watcher to read data from STDIN, a
268	timer to display a message once per second, and a condvar to exit the	761	timer to display a message once per second, and a condition variable to
269	program when the user enters quit:	762	quit the program when the user enters quit:
270		763
271	use AnyEvent;	764	use AnyEvent;
272		765
273	my $cv = AnyEvent->condvar;	766	my $cv = AnyEvent->condvar;
274		767
275	my $io_watcher = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {	768	my $io_watcher = AnyEvent->io (
		769	fh => \*STDIN,
		770	poll => 'r',
		771	cb => sub {
276	warn "io event <$_[0]>\n"; # will always output <r>	772	warn "io event <$_[0]>\n"; # will always output <r>
277	chomp (my $input = <STDIN>); # read a line	773	chomp (my $input = <STDIN>); # read a line
278	warn "read: $input\n"; # output what has been read	774	warn "read: $input\n"; # output what has been read
279	$cv->broadcast if $input =~ /^q/i; # quit program if /^q/i	775	$cv->send if $input =~ /^q/i; # quit program if /^q/i
		776	},
280	});	777	);
281		778
282	my $time_watcher; # can only be used once	779	my $time_watcher; # can only be used once
283		780
284	sub new_timer {	781	sub new_timer {
285	$timer = AnyEvent->timer (after => 1, cb => sub {	782	$timer = AnyEvent->timer (after => 1, cb => sub {
…		…
288	});	785	});
289	}	786	}
290		787
291	new_timer; # create first timer	788	new_timer; # create first timer
292		789
293	$cv->wait; # wait until user enters /^q/i	790	$cv->recv; # wait until user enters /^q/i
294		791
295	REAL-WORLD EXAMPLE	792	REAL-WORLD EXAMPLE
296	Consider the Net::FCP module. It features (among others) the following	793	Consider the Net::FCP module. It features (among others) the following
297	API calls, which are to freenet what HTTP GET requests are to http:	794	API calls, which are to freenet what HTTP GET requests are to http:
298		795
…		…
347	syswrite $txn->{fh}, $txn->{request}	844	syswrite $txn->{fh}, $txn->{request}
348	or die "connection or write error";	845	or die "connection or write error";
349	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });	846	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });
350		847
351	Again, "fh_ready_r" waits till all data has arrived, and then stores the	848	Again, "fh_ready_r" waits till all data has arrived, and then stores the
352	result and signals any possible waiters that the request ahs finished:	849	result and signals any possible waiters that the request has finished:
353		850
354	sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};	851	sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};
355		852
356	if (end-of-file or data complete) {	853	if (end-of-file or data complete) {
357	$txn->{result} = $txn->{buf};	854	$txn->{result} = $txn->{buf};
358	$txn->{finished}->broadcast;	855	$txn->{finished}->send;
359	$txb->{cb}->($txn) of $txn->{cb}; # also call callback	856	$txb->{cb}->($txn) of $txn->{cb}; # also call callback
360	}	857	}
361		858
362	The "result" method, finally, just waits for the finished signal (if the	859	The "result" method, finally, just waits for the finished signal (if the
363	request was already finished, it doesn't wait, of course, and returns	860	request was already finished, it doesn't wait, of course, and returns
364	the data:	861	the data:
365		862
366	$txn->{finished}->wait;	863	$txn->{finished}->recv;
367	return $txn->{result};	864	return $txn->{result};
368		865
369	The actual code goes further and collects all errors ("die"s,	866	The actual code goes further and collects all errors ("die"s,
370	exceptions) that occured during request processing. The "result" method	867	exceptions) that occurred during request processing. The "result" method
371	detects wether an exception as thrown (it is stored inside the $txn	868	detects whether an exception as thrown (it is stored inside the $txn
372	object) and just throws the exception, which means connection errors and	869	object) and just throws the exception, which means connection errors and
373	other problems get reported tot he code that tries to use the result,	870	other problems get reported tot he code that tries to use the result,
374	not in a random callback.	871	not in a random callback.
375		872
376	All of this enables the following usage styles:	873	All of this enables the following usage styles:
377		874
378	1. Blocking:	875	1. Blocking:
379		876
380	my $data = $fcp->client_get ($url);	877	my $data = $fcp->client_get ($url);
381		878
382	2. Blocking, but parallelizing:	879	2. Blocking, but running in parallel:
383		880
384	my @datas = map $_->result,	881	my @datas = map $_->result,
385	map $fcp->txn_client_get ($_),	882	map $fcp->txn_client_get ($_),
386	@urls;	883	@urls;
387		884
388	Both blocking examples work without the module user having to know	885	Both blocking examples work without the module user having to know
389	anything about events.	886	anything about events.
390		887
391	3a. Event-based in a main program, using any support Event module:	888	3a. Event-based in a main program, using any supported event module:
392		889
393	use Event;	890	use EV;
394		891
395	$fcp->txn_client_get ($url)->cb (sub {	892	$fcp->txn_client_get ($url)->cb (sub {
396	my $txn = shift;	893	my $txn = shift;
397	my $data = $txn->result;	894	my $data = $txn->result;
398	...	895	...
399	});	896	});
400		897
401	Event::loop;	898	EV::loop;
402		899
403	3b. The module user could use AnyEvent, too:	900	3b. The module user could use AnyEvent, too:
404		901
405	use AnyEvent;	902	use AnyEvent;
406		903
407	my $quit = AnyEvent->condvar;	904	my $quit = AnyEvent->condvar;
408		905
409	$fcp->txn_client_get ($url)->cb (sub {	906	$fcp->txn_client_get ($url)->cb (sub {
410	...	907	...
411	$quit->broadcast;	908	$quit->send;
412	});	909	});
413		910
414	$quit->wait;	911	$quit->recv;
		912
		913	BENCHMARKS
		914	To give you an idea of the performance and overheads that AnyEvent adds
		915	over the event loops themselves and to give you an impression of the
		916	speed of various event loops I prepared some benchmarks.
		917
		918	BENCHMARKING ANYEVENT OVERHEAD
		919	Here is a benchmark of various supported event models used natively and
		920	through AnyEvent. The benchmark creates a lot of timers (with a zero
		921	timeout) and I/O watchers (watching STDOUT, a pty, to become writable,
		922	which it is), lets them fire exactly once and destroys them again.
		923
		924	Source code for this benchmark is found as eg/bench in the AnyEvent
		925	distribution.
		926
		927	Explanation of the columns
		928	watcher is the number of event watchers created/destroyed. Since
		929	different event models feature vastly different performances, each event
		930	loop was given a number of watchers so that overall runtime is
		931	acceptable and similar between tested event loop (and keep them from
		932	crashing): Glib would probably take thousands of years if asked to
		933	process the same number of watchers as EV in this benchmark.
		934
		935	bytes is the number of bytes (as measured by the resident set size,
		936	RSS) consumed by each watcher. This method of measuring captures both C
		937	and Perl-based overheads.
		938
		939	create is the time, in microseconds (millionths of seconds), that it
		940	takes to create a single watcher. The callback is a closure shared
		941	between all watchers, to avoid adding memory overhead. That means
		942	closure creation and memory usage is not included in the figures.
		943
		944	invoke is the time, in microseconds, used to invoke a simple callback.
		945	The callback simply counts down a Perl variable and after it was invoked
		946	"watcher" times, it would "->send" a condvar once to signal the end of
		947	this phase.
		948
		949	destroy is the time, in microseconds, that it takes to destroy a
		950	single watcher.
		951
		952	Results
		953	name watchers bytes create invoke destroy comment
		954	EV/EV 400000 244 0.56 0.46 0.31 EV native interface
		955	EV/Any 100000 244 2.50 0.46 0.29 EV + AnyEvent watchers
		956	CoroEV/Any 100000 244 2.49 0.44 0.29 coroutines + Coro::Signal
		957	Perl/Any 100000 513 4.92 0.87 1.12 pure perl implementation
		958	Event/Event 16000 516 31.88 31.30 0.85 Event native interface
		959	Event/Any 16000 590 35.75 31.42 1.08 Event + AnyEvent watchers
		960	Glib/Any 16000 1357 98.22 12.41 54.00 quadratic behaviour
		961	Tk/Any 2000 1860 26.97 67.98 14.00 SEGV with >> 2000 watchers
		962	POE/Event 2000 6644 108.64 736.02 14.73 via POE::Loop::Event
		963	POE/Select 2000 6343 94.13 809.12 565.96 via POE::Loop::Select
		964
		965	Discussion
		966	The benchmark does not measure scalability of the event loop very
		967	well. For example, a select-based event loop (such as the pure perl one)
		968	can never compete with an event loop that uses epoll when the number of
		969	file descriptors grows high. In this benchmark, all events become ready
		970	at the same time, so select/poll-based implementations get an unnatural
		971	speed boost.
		972
		973	Also, note that the number of watchers usually has a nonlinear effect on
		974	overall speed, that is, creating twice as many watchers doesn't take
		975	twice the time - usually it takes longer. This puts event loops tested
		976	with a higher number of watchers at a disadvantage.
		977
		978	To put the range of results into perspective, consider that on the
		979	benchmark machine, handling an event takes roughly 1600 CPU cycles with
		980	EV, 3100 CPU cycles with AnyEvent's pure perl loop and almost 3000000
		981	CPU cycles with POE.
		982
		983	"EV" is the sole leader regarding speed and memory use, which are both
		984	maximal/minimal, respectively. Even when going through AnyEvent, it uses
		985	far less memory than any other event loop and is still faster than Event
		986	natively.
		987
		988	The pure perl implementation is hit in a few sweet spots (both the
		989	constant timeout and the use of a single fd hit optimisations in the
		990	perl interpreter and the backend itself). Nevertheless this shows that
		991	it adds very little overhead in itself. Like any select-based backend
		992	its performance becomes really bad with lots of file descriptors (and
		993	few of them active), of course, but this was not subject of this
		994	benchmark.
		995
		996	The "Event" module has a relatively high setup and callback invocation
		997	cost, but overall scores in on the third place.
		998
		999	"Glib"'s memory usage is quite a bit higher, but it features a faster
		1000	callback invocation and overall ends up in the same class as "Event".
		1001	However, Glib scales extremely badly, doubling the number of watchers
		1002	increases the processing time by more than a factor of four, making it
		1003	completely unusable when using larger numbers of watchers (note that
		1004	only a single file descriptor was used in the benchmark, so
		1005	inefficiencies of "poll" do not account for this).
		1006
		1007	The "Tk" adaptor works relatively well. The fact that it crashes with
		1008	more than 2000 watchers is a big setback, however, as correctness takes
		1009	precedence over speed. Nevertheless, its performance is surprising, as
		1010	the file descriptor is dup()ed for each watcher. This shows that the
		1011	dup() employed by some adaptors is not a big performance issue (it does
		1012	incur a hidden memory cost inside the kernel which is not reflected in
		1013	the figures above).
		1014
		1015	"POE", regardless of underlying event loop (whether using its pure perl
		1016	select-based backend or the Event module, the POE-EV backend couldn't be
		1017	tested because it wasn't working) shows abysmal performance and memory
		1018	usage with AnyEvent: Watchers use almost 30 times as much memory as EV
		1019	watchers, and 10 times as much memory as Event (the high memory
		1020	requirements are caused by requiring a session for each watcher).
		1021	Watcher invocation speed is almost 900 times slower than with AnyEvent's
		1022	pure perl implementation.
		1023
		1024	The design of the POE adaptor class in AnyEvent can not really account
		1025	for the performance issues, though, as session creation overhead is
		1026	small compared to execution of the state machine, which is coded pretty
		1027	optimally within AnyEvent::Impl::POE (and while everybody agrees that
		1028	using multiple sessions is not a good approach, especially regarding
		1029	memory usage, even the author of POE could not come up with a faster
		1030	design).
		1031
		1032	Summary
		1033	* Using EV through AnyEvent is faster than any other event loop (even
		1034	when used without AnyEvent), but most event loops have acceptable
		1035	performance with or without AnyEvent.
		1036
		1037	* The overhead AnyEvent adds is usually much smaller than the overhead
		1038	of the actual event loop, only with extremely fast event loops such
		1039	as EV adds AnyEvent significant overhead.
		1040
		1041	* You should avoid POE like the plague if you want performance or
		1042	reasonable memory usage.
		1043
		1044	BENCHMARKING THE LARGE SERVER CASE
		1045	This benchmark actually benchmarks the event loop itself. It works by
		1046	creating a number of "servers": each server consists of a socket pair, a
		1047	timeout watcher that gets reset on activity (but never fires), and an
		1048	I/O watcher waiting for input on one side of the socket. Each time the
		1049	socket watcher reads a byte it will write that byte to a random other
		1050	"server".
		1051
		1052	The effect is that there will be a lot of I/O watchers, only part of
		1053	which are active at any one point (so there is a constant number of
		1054	active fds for each loop iteration, but which fds these are is random).
		1055	The timeout is reset each time something is read because that reflects
		1056	how most timeouts work (and puts extra pressure on the event loops).
		1057
		1058	In this benchmark, we use 10000 socket pairs (20000 sockets), of which
		1059	100 (1%) are active. This mirrors the activity of large servers with
		1060	many connections, most of which are idle at any one point in time.
		1061
		1062	Source code for this benchmark is found as eg/bench2 in the AnyEvent
		1063	distribution.
		1064
		1065	Explanation of the columns
		1066	sockets is the number of sockets, and twice the number of "servers"
		1067	(as each server has a read and write socket end).
		1068
		1069	create is the time it takes to create a socket pair (which is
		1070	nontrivial) and two watchers: an I/O watcher and a timeout watcher.
		1071
		1072	request, the most important value, is the time it takes to handle a
		1073	single "request", that is, reading the token from the pipe and
		1074	forwarding it to another server. This includes deleting the old timeout
		1075	and creating a new one that moves the timeout into the future.
		1076
		1077	Results
		1078	name sockets create request
		1079	EV 20000 69.01 11.16
		1080	Perl 20000 73.32 35.87
		1081	Event 20000 212.62 257.32
		1082	Glib 20000 651.16 1896.30
		1083	POE 20000 349.67 12317.24 uses POE::Loop::Event
		1084
		1085	Discussion
		1086	This benchmark does measure scalability and overall performance of the
		1087	particular event loop.
		1088
		1089	EV is again fastest. Since it is using epoll on my system, the setup
		1090	time is relatively high, though.
		1091
		1092	Perl surprisingly comes second. It is much faster than the C-based event
		1093	loops Event and Glib.
		1094
		1095	Event suffers from high setup time as well (look at its code and you
		1096	will understand why). Callback invocation also has a high overhead
		1097	compared to the "$_->() for .."-style loop that the Perl event loop
		1098	uses. Event uses select or poll in basically all documented
		1099	configurations.
		1100
		1101	Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It
		1102	clearly fails to perform with many filehandles or in busy servers.
		1103
		1104	POE is still completely out of the picture, taking over 1000 times as
		1105	long as EV, and over 100 times as long as the Perl implementation, even
		1106	though it uses a C-based event loop in this case.
		1107
		1108	Summary
		1109	* The pure perl implementation performs extremely well.
		1110
		1111	* Avoid Glib or POE in large projects where performance matters.
		1112
		1113	BENCHMARKING SMALL SERVERS
		1114	While event loops should scale (and select-based ones do not...) even to
		1115	large servers, most programs we (or I :) actually write have only a few
		1116	I/O watchers.
		1117
		1118	In this benchmark, I use the same benchmark program as in the large
		1119	server case, but it uses only eight "servers", of which three are active
		1120	at any one time. This should reflect performance for a small server
		1121	relatively well.
		1122
		1123	The columns are identical to the previous table.
		1124
		1125	Results
		1126	name sockets create request
		1127	EV 16 20.00 6.54
		1128	Perl 16 25.75 12.62
		1129	Event 16 81.27 35.86
		1130	Glib 16 32.63 15.48
		1131	POE 16 261.87 276.28 uses POE::Loop::Event
		1132
		1133	Discussion
		1134	The benchmark tries to test the performance of a typical small server.
		1135	While knowing how various event loops perform is interesting, keep in
		1136	mind that their overhead in this case is usually not as important, due
		1137	to the small absolute number of watchers (that is, you need efficiency
		1138	and speed most when you have lots of watchers, not when you only have a
		1139	few of them).
		1140
		1141	EV is again fastest.
		1142
		1143	Perl again comes second. It is noticeably faster than the C-based event
		1144	loops Event and Glib, although the difference is too small to really
		1145	matter.
		1146
		1147	POE also performs much better in this case, but is is still far behind
		1148	the others.
		1149
		1150	Summary
		1151	* C-based event loops perform very well with small number of watchers,
		1152	as the management overhead dominates.
		1153
		1154	FORK
		1155	Most event libraries are not fork-safe. The ones who are usually are
		1156	because they rely on inefficient but fork-safe "select" or "poll" calls.
		1157	Only EV is fully fork-aware.
		1158
		1159	If you have to fork, you must either do so before creating your first
		1160	watcher OR you must not use AnyEvent at all in the child.
		1161
		1162	SECURITY CONSIDERATIONS
		1163	AnyEvent can be forced to load any event model via
		1164	$ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used
		1165	to execute arbitrary code or directly gain access, it can easily be used
		1166	to make the program hang or malfunction in subtle ways, as AnyEvent
		1167	watchers will not be active when the program uses a different event
		1168	model than specified in the variable.
		1169
		1170	You can make AnyEvent completely ignore this variable by deleting it
		1171	before the first watcher gets created, e.g. with a "BEGIN" block:
		1172
		1173	BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} }
		1174
		1175	use AnyEvent;
		1176
		1177	Similar considerations apply to $ENV{PERL_ANYEVENT_VERBOSE}, as that can
		1178	be used to probe what backend is used and gain other information (which
		1179	is probably even less useful to an attacker than PERL_ANYEVENT_MODEL).
415		1180
416	SEE ALSO	1181	SEE ALSO
417	Event modules: Coro::Event, Coro, Event, Glib::Event, Glib.	1182	Utility functions: AnyEvent::Util.
418		1183
		1184	Event modules: EV, EV::Glib, Glib::EV, Event, Glib::Event, Glib, Tk,
		1185	Event::Lib, Qt, POE.
		1186
419	Implementations: AnyEvent::Impl::Coro, AnyEvent::Impl::Event,	1187	Implementations: AnyEvent::Impl::EV, AnyEvent::Impl::Event,
420	AnyEvent::Impl::Glib, AnyEvent::Impl::Tk.	1188	AnyEvent::Impl::Glib, AnyEvent::Impl::Tk, AnyEvent::Impl::Perl,
		1189	AnyEvent::Impl::EventLib, AnyEvent::Impl::Qt, AnyEvent::Impl::POE.
421		1190
422	Nontrivial usage example: Net::FCP.	1191	Non-blocking file handles, sockets, TCP clients and servers:
		1192	AnyEvent::Handle, AnyEvent::Socket.
423		1193
		1194	Asynchronous DNS: AnyEvent::DNS.
424		1195
		1196	Coroutine support: Coro, Coro::AnyEvent, Coro::EV, Coro::Event,
		1197
		1198	Nontrivial usage examples: Net::FCP, Net::XMPP2, AnyEvent::DNS.
		1199
		1200	AUTHOR
		1201	Marc Lehmann <schmorp@schmorp.de>
		1202	http://home.schmorp.de/
		1203

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Revision 1.22 by root, Sat May 24 17:58:33 2008 UTC