AnyEvent - provide framework for multiple event loops EV, Event, Glib, Tk, Perl, Event::Lib, Qt, POE - various supported event loops use AnyEvent; my $w = AnyEvent->io (fh => $fh, poll => "r|w", cb => sub { ... }); my $w = AnyEvent->timer (after => $seconds, cb => sub { ... }); my $w = AnyEvent->condvar; # stores whether a condition was flagged $w->send; # wake up current and all future recv's $w->recv; # enters "main loop" till $condvar gets ->send This manpage is mainly a reference manual. If you are interested in a tutorial or some gentle introduction, have a look at the AnyEvent::Intro manpage. Glib, POE, IO::Async, Event... CPAN offers event models by the dozen nowadays. So what is different about AnyEvent? Executive Summary: AnyEvent is compatible, AnyEvent is free of policy and AnyEvent is small and efficient. First and foremost, AnyEvent is not an event model itself, it only interfaces to whatever event model the main program happens to use, in a pragmatic way. For event models and certain classes of immortals alike, the statement "there can only be one" is a bitter reality: In general, only one event loop can be active at the same time in a process. AnyEvent cannot change this, but it can hide the differences between those event loops. The goal of AnyEvent is to offer module authors the ability to do event programming (waiting for I/O or timer events) without subscribing to a religion, a way of living, and most importantly: without forcing your module users into the same thing by forcing them to use the same event model you use. For modules like POE or IO::Async (which is a total misnomer as it is actually doing all I/O synchronously...), using them in your module is like joining a cult: After you joined, you are dependent on them and you cannot use anything else, as they are simply incompatible to everything that isn't them. What's worse, all the potential users of your module are also forced to use the same event loop you use. AnyEvent is different: AnyEvent + POE works fine. AnyEvent + Glib works fine. AnyEvent + Tk works fine etc. etc. but none of these work together with the rest: POE + IO::Async? No go. Tk + Event? No go. Again: if your module uses one of those, every user of your module has to use it, too. But if your module uses AnyEvent, it works transparently with all event models it supports (including stuff like IO::Async, as long as those use one of the supported event loops. It is trivial to add new event loops to AnyEvent, too, so it is future-proof). In addition to being free of having to use the one and only true event model, AnyEvent also is free of bloat and policy: with POE or similar modules, you get an enormous amount of code and strict rules you have to follow. AnyEvent, on the other hand, is lean and up to the point, by only offering the functionality that is necessary, in as thin as a wrapper as technically possible. Of course, AnyEvent comes with a big (and fully optional!) toolbox of useful functionality, such as an asynchronous DNS resolver, 100% non-blocking connects (even with TLS/SSL, IPv6 and on broken platforms such as Windows) and lots of real-world knowledge and workarounds for platform bugs and differences. Now, if you do want lots of policy (this can arguably be somewhat useful) and you want to force your users to use the one and only event model, you should not use this module. AnyEvent provides an identical interface to multiple event loops. This allows module authors to utilise an event loop without forcing module users to use the same event loop (as only a single event loop can coexist peacefully at any one time). The interface itself is vaguely similar, but not identical to the Event module. During the first call of any watcher-creation method, the module tries to detect the currently loaded event loop by probing whether one of the following modules is already loaded: EV, Event, Glib, AnyEvent::Impl::Perl, Tk, Event::Lib, Qt, POE. The first one found is used. If none are found, the module tries to load these modules (excluding Tk, Event::Lib, Qt and POE as the pure perl adaptor should always succeed) in the order given. The first one that can be successfully loaded will be used. If, after this, still none could be found, AnyEvent will fall back to a pure-perl event loop, which is not very efficient, but should work everywhere. Because AnyEvent first checks for modules that are already loaded, loading an event model explicitly before first using AnyEvent will likely make that model the default. For example: use Tk; use AnyEvent; # .. AnyEvent will likely default to Tk The likely means that, if any module loads another event model and starts using it, all bets are off. Maybe you should tell their authors to use AnyEvent so their modules work together with others seamlessly... The pure-perl implementation of AnyEvent is called "AnyEvent::Impl::Perl". Like other event modules you can load it explicitly and enjoy the high availability of that event loop :) AnyEvent has the central concept of a watcher, which is an object that stores relevant data for each kind of event you are waiting for, such as the callback to call, the file handle to watch, etc. These watchers are normal Perl objects with normal Perl lifetime. After creating a watcher it will immediately "watch" for events and invoke the callback when the event occurs (of course, only when the event model is in control). To disable the watcher you have to destroy it (e.g. by setting the variable you store it in to "undef" or otherwise deleting all references to it). All watchers are created by calling a method on the "AnyEvent" class. Many watchers either are used with "recursion" (repeating timers for example), or need to refer to their watcher object in other ways. An any way to achieve that is this pattern: my $w; $w = AnyEvent->type (arg => value ..., cb => sub { # you can use $w here, for example to undef it undef $w; }); Note that "my $w; $w =" combination. This is necessary because in Perl, my variables are only visible after the statement in which they are declared.

I/O WATCHERS
You can create an I/O watcher by calling the "AnyEvent->io" method with the following mandatory key-value pairs as arguments: "fh" the Perl file handle (not file descriptor) to watch for events (AnyEvent might or might not keep a reference to this file handle). "poll" must be a string that is either "r" or "w", which creates a watcher waiting for "r"eadable or "w"ritable events, respectively. "cb" is the callback to invoke each time the file handle becomes ready. Although the callback might get passed parameters, their value and presence is undefined and you cannot rely on them. Portable AnyEvent callbacks cannot use arguments passed to I/O watcher callbacks. The I/O watcher might use the underlying file descriptor or a copy of it. You must not close a file handle as long as any watcher is active on the underlying file descriptor. Some event loops issue spurious readyness notifications, so you should always use non-blocking calls when reading/writing from/to your file handles. Example: wait for readability of STDIN, then read a line and disable the watcher. my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub { chomp (my $input = <STDIN>); warn "read: $input\n"; undef $w; });

TIME WATCHERS
You can create a time watcher by calling the "AnyEvent->timer" method with the following mandatory arguments: "after" specifies after how many seconds (fractional values are supported) the callback should be invoked. "cb" is the callback to invoke in that case. Although the callback might get passed parameters, their value and presence is undefined and you cannot rely on them. Portable AnyEvent callbacks cannot use arguments passed to time watcher callbacks. The callback will normally be invoked once only. If you specify another parameter, "interval", as a strictly positive number (> 0), then the callback will be invoked regularly at that interval (in fractional seconds) after the first invocation. If "interval" is specified with a false value, then it is treated as if it were missing. The callback will be rescheduled before invoking the callback, but no attempt is done to avoid timer drift in most backends, so the interval is only approximate. Example: fire an event after 7.7 seconds. my $w = AnyEvent->timer (after => 7.7, cb => sub { warn "timeout\n"; }); # to cancel the timer: undef $w; Example 2: fire an event after 0.5 seconds, then roughly every second. my $w = AnyEvent->timer (after => 0.5, interval => 1, cb => sub { warn "timeout\n"; }; TIMING ISSUES There are two ways to handle timers: based on real time (relative, "fire in 10 seconds") and based on wallclock time (absolute, "fire at 12 o'clock"). While most event loops expect timers to specified in a relative way, they use absolute time internally. This makes a difference when your clock "jumps", for example, when ntp decides to set your clock backwards from the wrong date of 2014-01-01 to 2008-01-01, a watcher that is supposed to fire "after" a second might actually take six years to finally fire. AnyEvent cannot compensate for this. The only event loop that is conscious about these issues is EV, which offers both relative (ev_timer, based on true relative time) and absolute (ev_periodic, based on wallclock time) timers. AnyEvent always prefers relative timers, if available, matching the AnyEvent API. AnyEvent has two additional methods that return the "current time": This returns the "current wallclock time" as a fractional number of seconds since the Epoch (the same thing as "time" or "Time::HiRes::time" return, and the result is guaranteed to be compatible with those). It progresses independently of any event loop processing, i.e. each call will check the system clock, which usually gets updated frequently. This also returns the "current wallclock time", but unlike "time", above, this value might change only once per event loop iteration, depending on the event loop (most return the same time as "time", above). This is the time that AnyEvent's timers get scheduled against. In almost all cases (in all cases if you don't care), this is the function to call when you want to know the current time. This function is also often faster then "AnyEvent->time", and thus the preferred method if you want some timestamp (for example, AnyEvent::Handle uses this to update it's activity timeouts). The rest of this section is only of relevance if you try to be very exact with your timing, you can skip it without bad conscience. For a practical example of when these times differ, consider Event::Lib and EV and the following set-up: The event loop is running and has just invoked one of your callback at time=500 (assume no other callbacks delay processing). In your callback, you wait a second by executing "sleep 1" (blocking the process for a second) and then (at time=501) you create a relative timer that fires after three seconds. With Event::Lib, "AnyEvent->time" and "AnyEvent->now" will both return 501, because that is the current time, and the timer will be scheduled to fire at time=504 (501 + 3). With EV, "AnyEvent->time" returns 501 (as that is the current time), but "AnyEvent->now" returns 500, as that is the time the last event processing phase started. With EV, your timer gets scheduled to run at time=503 (500 + 3). In one sense, Event::Lib is more exact, as it uses the current time regardless of any delays introduced by event processing. However, most callbacks do not expect large delays in processing, so this causes a higher drift (and a lot more system calls to get the current time). In another sense, EV is more exact, as your timer will be scheduled at the same time, regardless of how long event processing actually took. In either case, if you care (and in most cases, you don't), then you can get whatever behaviour you want with any event loop, by taking the difference between "AnyEvent->time" and "AnyEvent->now" into account.

SIGNAL WATCHERS
You can watch for signals using a signal watcher, "signal" is the signal name in uppercase and without any "SIG" prefix, "cb" is the Perl callback to be invoked whenever a signal occurs. Although the callback might get passed parameters, their value and presence is undefined and you cannot rely on them. Portable AnyEvent callbacks cannot use arguments passed to signal watcher callbacks. Multiple signal occurrences can be clumped together into one callback invocation, and callback invocation will be synchronous. Synchronous means that it might take a while until the signal gets handled by the process, but it is guaranteed not to interrupt any other callbacks. The main advantage of using these watchers is that you can share a signal between multiple watchers. This watcher might use %SIG, so programs overwriting those signals directly will likely not work correctly. Example: exit on SIGINT my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });

CHILD PROCESS WATCHERS
You can also watch on a child process exit and catch its exit status. The child process is specified by the "pid" argument (if set to 0, it watches for any child process exit). The watcher will trigger as often as status change for the child are received. This works by installing a signal handler for "SIGCHLD". The callback will be called with the pid and exit status (as returned by waitpid), so unlike other watcher types, you can rely on child watcher callback arguments. There is a slight catch to child watchers, however: you usually start them after the child process was created, and this means the process could have exited already (and no SIGCHLD will be sent anymore). Not all event models handle this correctly (POE doesn't), but even for event models that do handle this correctly, they usually need to be loaded before the process exits (i.e. before you fork in the first place). This means you cannot create a child watcher as the very first thing in an AnyEvent program, you have to create at least one watcher before you "fork" the child (alternatively, you can call "AnyEvent::detect"). Example: fork a process and wait for it my $done = AnyEvent->condvar; my $pid = fork or exit 5; my $w = AnyEvent->child ( pid => $pid, cb => sub { my ($pid, $status) = @_; warn "pid $pid exited with status $status"; $done->send; }, ); # do something else, then wait for process exit $done->recv;

CONDITION VARIABLES
If you are familiar with some event loops you will know that all of them require you to run some blocking "loop", "run" or similar function that will actively watch for new events and call your callbacks. AnyEvent is different, it expects somebody else to run the event loop and will only block when necessary (usually when told by the user). The instrument to do that is called a "condition variable", so called because they represent a condition that must become true. Condition variables can be created by calling the "AnyEvent->condvar" method, usually without arguments. The only argument pair allowed is "cb", which specifies a callback to be called when the condition variable becomes true. After creation, the condition variable is "false" until it becomes "true" by calling the "send" method (or calling the condition variable as if it were a callback, read about the caveats in the description for the "->send" method). Condition variables are similar to callbacks, except that you can optionally wait for them. They can also be called merge points - points in time where multiple outstanding events have been processed. And yet another way to call them is transactions - each condition variable can be used to represent a transaction, which finishes at some point and delivers a result. Condition variables are very useful to signal that something has finished, for example, if you write a module that does asynchronous http requests, then a condition variable would be the ideal candidate to signal the availability of results. The user can either act when the callback is called or can synchronously "->recv" for the results. You can also use them to simulate traditional event loops - for example, you can block your main program until an event occurs - for example, you could "->recv" in your main program until the user clicks the Quit button of your app, which would "->send" the "quit" event. Note that condition variables recurse into the event loop - if you have two pieces of code that call "->recv" in a round-robin fashion, you lose. Therefore, condition variables are good to export to your caller, but you should avoid making a blocking wait yourself, at least in callbacks, as this asks for trouble. Condition variables are represented by hash refs in perl, and the keys used by AnyEvent itself are all named "_ae_XXX" to make subclassing easy (it is often useful to build your own transaction class on top of AnyEvent). To subclass, use "AnyEvent::CondVar" as base class and call it's "new" method in your own "new" method. There are two "sides" to a condition variable - the "producer side" which eventually calls "-> send", and the "consumer side", which waits for the send to occur. Example: wait for a timer. # wait till the result is ready my $result_ready = AnyEvent->condvar; # do something such as adding a timer # or socket watcher the calls $result_ready->send # when the "result" is ready. # in this case, we simply use a timer: my $w = AnyEvent->timer ( after => 1, cb => sub { $result_ready->send }, ); # this "blocks" (while handling events) till the callback # calls send $result_ready->recv; Example: wait for a timer, but take advantage of the fact that condition variables are also code references. my $done = AnyEvent->condvar; my $delay = AnyEvent->timer (after => 5, cb => $done); $done->recv; METHODS FOR PRODUCERS These methods should only be used by the producing side, i.e. the code/module that eventually sends the signal. Note that it is also the producer side which creates the condvar in most cases, but it isn't uncommon for the consumer to create it as well. Flag the condition as ready - a running "->recv" and all further calls to "recv" will (eventually) return after this method has been called. If nobody is waiting the send will be remembered. If a callback has been set on the condition variable, it is called immediately from within send. Any arguments passed to the "send" call will be returned by all future "->recv" calls. Condition variables are overloaded so one can call them directly (as a code reference). Calling them directly is the same as calling "send". Note, however, that many C-based event loops do not handle overloading, so as tempting as it may be, passing a condition variable instead of a callback does not work. Both the pure perl and EV loops support overloading, however, as well as all functions that use perl to invoke a callback (as in AnyEvent::Socket and AnyEvent::DNS for example). Similar to send, but causes all call's to "->recv" to invoke "Carp::croak" with the given error message/object/scalar. This can be used to signal any errors to the condition variable user/consumer. These two methods are EXPERIMENTAL and MIGHT CHANGE. These two methods can be used to combine many transactions/events into one. For example, a function that pings many hosts in parallel might want to use a condition variable for the whole process. Every call to "->begin" will increment a counter, and every call to "->end" will decrement it. If the counter reaches 0 in "->end", the (last) callback passed to "begin" will be executed. That callback is supposed to call "->send", but that is not required. If no callback was set, "send" will be called without any arguments. Let's clarify this with the ping example: my $cv = AnyEvent->condvar; my %result; $cv->begin (sub { $cv->send (\%result) }); for my $host (@list_of_hosts) { $cv->begin; ping_host_then_call_callback $host, sub { $result{$host} = ...; $cv->end; }; } $cv->end; This code fragment supposedly pings a number of hosts and calls "send" after results for all then have have been gathered - in any order. To achieve this, the code issues a call to "begin" when it starts each ping request and calls "end" when it has received some result for it. Since "begin" and "end" only maintain a counter, the order in which results arrive is not relevant. There is an additional bracketing call to "begin" and "end" outside the loop, which serves two important purposes: first, it sets the callback to be called once the counter reaches 0, and second, it ensures that "send" is called even when "no" hosts are being pinged (the loop doesn't execute once). This is the general pattern when you "fan out" into multiple subrequests: use an outer "begin"/"end" pair to set the callback and ensure "end" is called at least once, and then, for each subrequest you start, call "begin" and for each subrequest you finish, call "end". METHODS FOR CONSUMERS These methods should only be used by the consuming side, i.e. the code awaits the condition. Wait (blocking if necessary) until the "->send" or "->croak" methods have been called on c<$cv>, while servicing other watchers normally. You can only wait once on a condition - additional calls are valid but will return immediately. If an error condition has been set by calling "->croak", then this function will call "croak". In list context, all parameters passed to "send" will be returned, in scalar context only the first one will be returned. Not all event models support a blocking wait - some die in that case (programs might want to do that to stay interactive), so if you are using this from a module, never require a blocking wait, but let the caller decide whether the call will block or not (for example, by coupling condition variables with some kind of request results and supporting callbacks so the caller knows that getting the result will not block, while still supporting blocking waits if the caller so desires). Another reason never to "->recv" in a module is that you cannot sensibly have two "->recv"'s in parallel, as that would require multiple interpreters or coroutines/threads, none of which "AnyEvent" can supply. The Coro module, however, can and does supply coroutines and, in fact, Coro::AnyEvent replaces AnyEvent's condvars by coroutine-safe versions and also integrates coroutines into AnyEvent, making blocking "->recv" calls perfectly safe as long as they are done from another coroutine (one that doesn't run the event loop). You can ensure that "-recv" never blocks by setting a callback and only calling "->recv" from within that callback (or at a later time). This will work even when the event loop does not support blocking waits otherwise. Returns true when the condition is "true", i.e. whether "send" or "croak" have been called. This is a mutator function that returns the callback set and optionally replaces it before doing so. The callback will be called when the condition becomes "true", i.e. when "send" or "croak" are called, with the only argument being the condition variable itself. Calling "recv" inside the callback or at any later time is guaranteed not to block. Contains "undef" until the first watcher is being created. Then it contains the event model that is being used, which is the name of the Perl class implementing the model. This class is usually one of the "AnyEvent::Impl:xxx" modules, but can be any other class in the case AnyEvent has been extended at runtime (e.g. in rxvt-unicode). The known classes so far are: AnyEvent::Impl::EV based on EV (an interface to libev, best choice). AnyEvent::Impl::Event based on Event, second best choice. AnyEvent::Impl::Perl pure-perl implementation, fast and portable. AnyEvent::Impl::Glib based on Glib, third-best choice. AnyEvent::Impl::Tk based on Tk, very bad choice. AnyEvent::Impl::Qt based on Qt, cannot be autoprobed (see its docs). AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse. AnyEvent::Impl::POE based on POE, not generic enough for full support. There is no support for WxWidgets, as WxWidgets has no support for watching file handles. However, you can use WxWidgets through the POE Adaptor, as POE has a Wx backend that simply polls 20 times per second, which was considered to be too horrible to even consider for AnyEvent. Likewise, other POE backends can be used by AnyEvent by using it's adaptor. AnyEvent knows about Prima and Wx and will try to use POE when autodetecting them. Returns $AnyEvent::MODEL, forcing autodetection of the event model if necessary. You should only call this function right before you would have created an AnyEvent watcher anyway, that is, as late as possible at runtime. Arranges for the code block to be executed as soon as the event model is autodetected (or immediately if this has already happened). If called in scalar or list context, then it creates and returns an object that automatically removes the callback again when it is destroyed. See Coro::BDB for a case where this is useful. If there are any code references in this array (you can "push" to it before or after loading AnyEvent), then they will called directly after the event loop has been chosen. You should check $AnyEvent::MODEL before adding to this array, though: if it contains a true value then the event loop has already been detected, and the array will be ignored. Best use "AnyEvent::post_detect { BLOCK }" instead. As a module author, you should "use AnyEvent" and call AnyEvent methods freely, but you should not load a specific event module or rely on it. Be careful when you create watchers in the module body - AnyEvent will decide which event module to use as soon as the first method is called, so by calling AnyEvent in your module body you force the user of your module to load the event module first. Never call "->recv" on a condition variable unless you know that the "->send" method has been called on it already. This is because it will stall the whole program, and the whole point of using events is to stay interactive. It is fine, however, to call "->recv" when the user of your module requests it (i.e. if you create a http request object ad have a method called "results" that returns the results, it should call "->recv" freely, as the user of your module knows what she is doing. always). There will always be a single main program - the only place that should dictate which event model to use. If it doesn't care, it can just "use AnyEvent" and use it itself, or not do anything special (it does not need to be event-based) and let AnyEvent decide which implementation to chose if some module relies on it. If the main program relies on a specific event model - for example, in Gtk2 programs you have to rely on the Glib module - you should load the event module before loading AnyEvent or any module that uses it: generally speaking, you should load it as early as possible. The reason is that modules might create watchers when they are loaded, and AnyEvent will decide on the event model to use as soon as it creates watchers, and it might chose the wrong one unless you load the correct one yourself. You can chose to use a pure-perl implementation by loading the "AnyEvent::Impl::Perl" module, which gives you similar behaviour everywhere, but letting AnyEvent chose the model is generally better.

MAINLOOP EMULATION
Sometimes (often for short test scripts, or even standalone programs who only want to use AnyEvent), you do not want to run a specific event loop. In that case, you can use a condition variable like this: AnyEvent->condvar->recv; This has the effect of entering the event loop and looping forever. Note that usually your program has some exit condition, in which case it is better to use the "traditional" approach of storing a condition variable somewhere, waiting for it, and sending it when the program should exit cleanly. The following is a non-exhaustive list of additional modules that use AnyEvent and can therefore be mixed easily with other AnyEvent modules in the same program. Some of the modules come with AnyEvent, some are available via CPAN. Contains various utility functions that replace often-used but blocking functions such as "inet_aton" by event-/callback-based versions. Provides various utility functions for (internet protocol) sockets, addresses and name resolution. Also functions to create non-blocking tcp connections or tcp servers, with IPv6 and SRV record support and more. Provide read and write buffers, manages watchers for reads and writes, supports raw and formatted I/O, I/O queued and fully transparent and non-blocking SSL/TLS. Provides rich asynchronous DNS resolver capabilities. A simple-to-use HTTP library that is capable of making a lot of concurrent HTTP requests. Provides a simple web application server framework. The fastest ping in the west. Executes DBI requests asynchronously in a proxy process. Truly asynchronous I/O, should be in the toolbox of every event programmer. AnyEvent::AIO transparently fuses IO::AIO and AnyEvent together. Truly asynchronous Berkeley DB access. AnyEvent::BDB transparently fuses BDB and AnyEvent together. A non-blocking interface to gpsd, a daemon delivering GPS information. A non-blocking interface to the Internet Go Server protocol (used by App::IGS). AnyEvent based IRC client module family. AnyEvent based XMPP (Jabber protocol) module family. AnyEvent-based implementation of the Freenet Client Protocol, birthplace of AnyEvent. High level API for event-based execution flow control. Has special support for AnyEvent via Coro::AnyEvent. The lambda approach to I/O - don't ask, look there. Can use AnyEvent. This is an advanced topic that you do not normally need to use AnyEvent in a module. This section is only of use to event loop authors who want to provide AnyEvent compatibility. If you need to support another event library which isn't directly supported by AnyEvent, you can supply your own interface to it by pushing, before the first watcher gets created, the package name of the event module and the package name of the interface to use onto @AnyEvent::REGISTRY. You can do that before and even without loading AnyEvent, so it is reasonably cheap. Example: push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::]; This tells AnyEvent to (literally) use the "urxvt::anyevent::" package/class when it finds the "urxvt" package/module is already loaded. When AnyEvent is loaded and asked to find a suitable event model, it will first check for the presence of urxvt by trying to "use" the "urxvt::anyevent" module. The class should provide implementations for all watcher types. See AnyEvent::Impl::EV (source code), AnyEvent::Impl::Glib (Source code) and so on for actual examples. Use "perldoc -m AnyEvent::Impl::Glib" to see the sources. If you don't provide "signal" and "child" watchers than AnyEvent will provide suitable (hopefully) replacements. The above example isn't fictitious, the rxvt-unicode (a.k.a. urxvt) terminal emulator uses the above line as-is. An interface isn't included in AnyEvent because it doesn't make sense outside the embedded interpreter inside rxvt-unicode, and it is updated and maintained as part of the rxvt-unicode distribution. rxvt-unicode also cheats a bit by not providing blocking access to condition variables: code blocking while waiting for a condition will "die". This still works with most modules/usages, and blocking calls must not be done in an interactive application, so it makes sense. The following environment variables are used by this module: By default, AnyEvent will be completely silent except in fatal conditions. You can set this environment variable to make AnyEvent more talkative. When set to 1 or higher, causes AnyEvent to warn about unexpected conditions, such as not being able to load the event model specified by "PERL_ANYEVENT_MODEL". When set to 2 or higher, cause AnyEvent to report to STDERR which event model it chooses. AnyEvent does not do much argument checking by default, as thorough argument checking is very costly. Setting this variable to a true value will cause AnyEvent to load "AnyEvent::Strict" and then to thoroughly check the arguments passed to most method calls. If it finds any problems it will croak. In other words, enables "strict" mode. Unlike "use strict" it is definitely recommended ot keep it off in production. This can be used to specify the event model to be used by AnyEvent, before auto detection and -probing kicks in. It must be a string consisting entirely of ASCII letters. The string "AnyEvent::Impl::" gets prepended and the resulting module name is loaded and if the load was successful, used as event model. If it fails to load AnyEvent will proceed with auto detection and -probing. This functionality might change in future versions. For example, to force the pure perl model (AnyEvent::Impl::Perl) you could start your program like this: PERL_ANYEVENT_MODEL=Perl perl ... Used by both AnyEvent::DNS and AnyEvent::Socket to determine preferences for IPv4 or IPv6. The default is unspecified (and might change, or be the result of auto probing). Must be set to a comma-separated list of protocols or address families, current supported: "ipv4" and "ipv6". Only protocols mentioned will be used, and preference will be given to protocols mentioned earlier in the list. This variable can effectively be used for denial-of-service attacks against local programs (e.g. when setuid), although the impact is likely small, as the program has to handle connection errors already- Examples: "PERL_ANYEVENT_PROTOCOLS=ipv4,ipv6" - prefer IPv4 over IPv6, but support both and try to use both. "PERL_ANYEVENT_PROTOCOLS=ipv4" - only support IPv4, never try to resolve or contact IPv6 addresses. "PERL_ANYEVENT_PROTOCOLS=ipv6,ipv4" support either IPv4 or IPv6, but prefer IPv6 over IPv4. Used by AnyEvent::DNS to decide whether to use the EDNS0 extension for DNS. This extension is generally useful to reduce DNS traffic, but some (broken) firewalls drop such DNS packets, which is why it is off by default. Setting this variable to 1 will cause AnyEvent::DNS to announce EDNS0 in its DNS requests. The maximum number of child processes that "AnyEvent::Util::fork_call" will create in parallel. The following program uses an I/O watcher to read data from STDIN, a timer to display a message once per second, and a condition variable to quit the program when the user enters quit: use AnyEvent; my $cv = AnyEvent->condvar; my $io_watcher = AnyEvent->io ( fh => \*STDIN, poll => 'r', cb => sub { warn "io event <$_[0]>\n"; # will always output <r> chomp (my $input = <STDIN>); # read a line warn "read: $input\n"; # output what has been read $cv->send if $input =~ /^q/i; # quit program if /^q/i }, ); my $time_watcher; # can only be used once sub new_timer { $timer = AnyEvent->timer (after => 1, cb => sub { warn "timeout\n"; # print 'timeout' about every second &new_timer; # and restart the time }); } new_timer; # create first timer $cv->recv; # wait until user enters /^q/i Consider the Net::FCP module. It features (among others) the following API calls, which are to freenet what HTTP GET requests are to http: my $data = $fcp->client_get ($url); # blocks my $transaction = $fcp->txn_client_get ($url); # does not block $transaction->cb ( sub { ... } ); # set optional result callback my $data = $transaction->result; # possibly blocks The "client_get" method works like "LWP::Simple::get": it requests the given URL and waits till the data has arrived. It is defined to be: sub client_get { $_[0]->txn_client_get ($_[1])->result } And in fact is automatically generated. This is the blocking API of Net::FCP, and it works as simple as in any other, similar, module. More complicated is "txn_client_get": It only creates a transaction (completion, result, ...) object and initiates the transaction. my $txn = bless { }, Net::FCP::Txn::; It also creates a condition variable that is used to signal the completion of the request: $txn->{finished} = AnyAvent->condvar; It then creates a socket in non-blocking mode. socket $txn->{fh}, ...; fcntl $txn->{fh}, F_SETFL, O_NONBLOCK; connect $txn->{fh}, ... and !$!{EWOULDBLOCK} and !$!{EINPROGRESS} and Carp::croak "unable to connect: $!\n"; Then it creates a write-watcher which gets called whenever an error occurs or the connection succeeds: $txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'w', cb => sub { $txn->fh_ready_w }); And returns this transaction object. The "fh_ready_w" callback gets called as soon as the event loop detects that the socket is ready for writing. The "fh_ready_w" method makes the socket blocking again, writes the request data and replaces the watcher by a read watcher (waiting for reply data). The actual code is more complicated, but that doesn't matter for this example: fcntl $txn->{fh}, F_SETFL, 0; syswrite $txn->{fh}, $txn->{request} or die "connection or write error"; $txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r }); Again, "fh_ready_r" waits till all data has arrived, and then stores the result and signals any possible waiters that the request has finished: sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf}; if (end-of-file or data complete) { $txn->{result} = $txn->{buf}; $txn->{finished}->send; $txb->{cb}->($txn) of $txn->{cb}; # also call callback } The "result" method, finally, just waits for the finished signal (if the request was already finished, it doesn't wait, of course, and returns the data: $txn->{finished}->recv; return $txn->{result}; The actual code goes further and collects all errors ("die"s, exceptions) that occurred during request processing. The "result" method detects whether an exception as thrown (it is stored inside the $txn object) and just throws the exception, which means connection errors and other problems get reported tot he code that tries to use the result, not in a random callback. All of this enables the following usage styles: 1. Blocking: my $data = $fcp->client_get ($url); 2. Blocking, but running in parallel: my @datas = map $_->result, map $fcp->txn_client_get ($_), @urls; Both blocking examples work without the module user having to know anything about events. 3a. Event-based in a main program, using any supported event module: use EV; $fcp->txn_client_get ($url)->cb (sub { my $txn = shift; my $data = $txn->result; ... }); EV::loop; 3b. The module user could use AnyEvent, too: use AnyEvent; my $quit = AnyEvent->condvar; $fcp->txn_client_get ($url)->cb (sub { ... $quit->send; }); $quit->recv; To give you an idea of the performance and overheads that AnyEvent adds over the event loops themselves and to give you an impression of the speed of various event loops I prepared some benchmarks.

BENCHMARKING ANYEVENT OVERHEAD
Here is a benchmark of various supported event models used natively and through AnyEvent. The benchmark creates a lot of timers (with a zero timeout) and I/O watchers (watching STDOUT, a pty, to become writable, which it is), lets them fire exactly once and destroys them again. Source code for this benchmark is found as eg/bench in the AnyEvent distribution. Explanation of the columns watcher is the number of event watchers created/destroyed. Since different event models feature vastly different performances, each event loop was given a number of watchers so that overall runtime is acceptable and similar between tested event loop (and keep them from crashing): Glib would probably take thousands of years if asked to process the same number of watchers as EV in this benchmark. bytes is the number of bytes (as measured by the resident set size, RSS) consumed by each watcher. This method of measuring captures both C and Perl-based overheads. create is the time, in microseconds (millionths of seconds), that it takes to create a single watcher. The callback is a closure shared between all watchers, to avoid adding memory overhead. That means closure creation and memory usage is not included in the figures. invoke is the time, in microseconds, used to invoke a simple callback. The callback simply counts down a Perl variable and after it was invoked "watcher" times, it would "->send" a condvar once to signal the end of this phase. destroy is the time, in microseconds, that it takes to destroy a single watcher. Results name watchers bytes create invoke destroy comment EV/EV 400000 244 0.56 0.46 0.31 EV native interface EV/Any 100000 244 2.50 0.46 0.29 EV + AnyEvent watchers CoroEV/Any 100000 244 2.49 0.44 0.29 coroutines + Coro::Signal Perl/Any 100000 513 4.92 0.87 1.12 pure perl implementation Event/Event 16000 516 31.88 31.30 0.85 Event native interface Event/Any 16000 590 35.75 31.42 1.08 Event + AnyEvent watchers Glib/Any 16000 1357 98.22 12.41 54.00 quadratic behaviour Tk/Any 2000 1860 26.97 67.98 14.00 SEGV with >> 2000 watchers POE/Event 2000 6644 108.64 736.02 14.73 via POE::Loop::Event POE/Select 2000 6343 94.13 809.12 565.96 via POE::Loop::Select Discussion The benchmark does not measure scalability of the event loop very well. For example, a select-based event loop (such as the pure perl one) can never compete with an event loop that uses epoll when the number of file descriptors grows high. In this benchmark, all events become ready at the same time, so select/poll-based implementations get an unnatural speed boost. Also, note that the number of watchers usually has a nonlinear effect on overall speed, that is, creating twice as many watchers doesn't take twice the time - usually it takes longer. This puts event loops tested with a higher number of watchers at a disadvantage. To put the range of results into perspective, consider that on the benchmark machine, handling an event takes roughly 1600 CPU cycles with EV, 3100 CPU cycles with AnyEvent's pure perl loop and almost 3000000 CPU cycles with POE. "EV" is the sole leader regarding speed and memory use, which are both maximal/minimal, respectively. Even when going through AnyEvent, it uses far less memory than any other event loop and is still faster than Event natively. The pure perl implementation is hit in a few sweet spots (both the constant timeout and the use of a single fd hit optimisations in the perl interpreter and the backend itself). Nevertheless this shows that it adds very little overhead in itself. Like any select-based backend its performance becomes really bad with lots of file descriptors (and few of them active), of course, but this was not subject of this benchmark. The "Event" module has a relatively high setup and callback invocation cost, but overall scores in on the third place. "Glib"'s memory usage is quite a bit higher, but it features a faster callback invocation and overall ends up in the same class as "Event". However, Glib scales extremely badly, doubling the number of watchers increases the processing time by more than a factor of four, making it completely unusable when using larger numbers of watchers (note that only a single file descriptor was used in the benchmark, so inefficiencies of "poll" do not account for this). The "Tk" adaptor works relatively well. The fact that it crashes with more than 2000 watchers is a big setback, however, as correctness takes precedence over speed. Nevertheless, its performance is surprising, as the file descriptor is dup()ed for each watcher. This shows that the dup() employed by some adaptors is not a big performance issue (it does incur a hidden memory cost inside the kernel which is not reflected in the figures above). "POE", regardless of underlying event loop (whether using its pure perl select-based backend or the Event module, the POE-EV backend couldn't be tested because it wasn't working) shows abysmal performance and memory usage with AnyEvent: Watchers use almost 30 times as much memory as EV watchers, and 10 times as much memory as Event (the high memory requirements are caused by requiring a session for each watcher). Watcher invocation speed is almost 900 times slower than with AnyEvent's pure perl implementation. The design of the POE adaptor class in AnyEvent can not really account for the performance issues, though, as session creation overhead is small compared to execution of the state machine, which is coded pretty optimally within AnyEvent::Impl::POE (and while everybody agrees that using multiple sessions is not a good approach, especially regarding memory usage, even the author of POE could not come up with a faster design). Summary

BENCHMARKING THE LARGE SERVER CASE
This benchmark actually benchmarks the event loop itself. It works by creating a number of "servers": each server consists of a socket pair, a timeout watcher that gets reset on activity (but never fires), and an I/O watcher waiting for input on one side of the socket. Each time the socket watcher reads a byte it will write that byte to a random other "server". The effect is that there will be a lot of I/O watchers, only part of which are active at any one point (so there is a constant number of active fds for each loop iteration, but which fds these are is random). The timeout is reset each time something is read because that reflects how most timeouts work (and puts extra pressure on the event loops). In this benchmark, we use 10000 socket pairs (20000 sockets), of which 100 (1%) are active. This mirrors the activity of large servers with many connections, most of which are idle at any one point in time. Source code for this benchmark is found as eg/bench2 in the AnyEvent distribution. Explanation of the columns sockets is the number of sockets, and twice the number of "servers" (as each server has a read and write socket end). create is the time it takes to create a socket pair (which is nontrivial) and two watchers: an I/O watcher and a timeout watcher. request, the most important value, is the time it takes to handle a single "request", that is, reading the token from the pipe and forwarding it to another server. This includes deleting the old timeout and creating a new one that moves the timeout into the future. Results name sockets create request EV 20000 69.01 11.16 Perl 20000 73.32 35.87 Event 20000 212.62 257.32 Glib 20000 651.16 1896.30 POE 20000 349.67 12317.24 uses POE::Loop::Event Discussion This benchmark does measure scalability and overall performance of the particular event loop. EV is again fastest. Since it is using epoll on my system, the setup time is relatively high, though. Perl surprisingly comes second. It is much faster than the C-based event loops Event and Glib. Event suffers from high setup time as well (look at its code and you will understand why). Callback invocation also has a high overhead compared to the "$_->() for .."-style loop that the Perl event loop uses. Event uses select or poll in basically all documented configurations. Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It clearly fails to perform with many filehandles or in busy servers. POE is still completely out of the picture, taking over 1000 times as long as EV, and over 100 times as long as the Perl implementation, even though it uses a C-based event loop in this case. Summary

BENCHMARKING SMALL SERVERS
While event loops should scale (and select-based ones do not...) even to large servers, most programs we (or I :) actually write have only a few I/O watchers. In this benchmark, I use the same benchmark program as in the large server case, but it uses only eight "servers", of which three are active at any one time. This should reflect performance for a small server relatively well. The columns are identical to the previous table. Results name sockets create request EV 16 20.00 6.54 Perl 16 25.75 12.62 Event 16 81.27 35.86 Glib 16 32.63 15.48 POE 16 261.87 276.28 uses POE::Loop::Event Discussion The benchmark tries to test the performance of a typical small server. While knowing how various event loops perform is interesting, keep in mind that their overhead in this case is usually not as important, due to the small absolute number of watchers (that is, you need efficiency and speed most when you have lots of watchers, not when you only have a few of them). EV is again fastest. Perl again comes second. It is noticeably faster than the C-based event loops Event and Glib, although the difference is too small to really matter. POE also performs much better in this case, but is is still far behind the others. Summary Most event libraries are not fork-safe. The ones who are usually are because they rely on inefficient but fork-safe "select" or "poll" calls. Only EV is fully fork-aware. If you have to fork, you must either do so before creating your first watcher OR you must not use AnyEvent at all in the child. AnyEvent can be forced to load any event model via $ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used to execute arbitrary code or directly gain access, it can easily be used to make the program hang or malfunction in subtle ways, as AnyEvent watchers will not be active when the program uses a different event model than specified in the variable. You can make AnyEvent completely ignore this variable by deleting it before the first watcher gets created, e.g. with a "BEGIN" block: BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} } use AnyEvent; Similar considerations apply to $ENV{PERL_ANYEVENT_VERBOSE}, as that can be used to probe what backend is used and gain other information (which is probably even less useful to an attacker than PERL_ANYEVENT_MODEL), and $ENV{PERL_ANYEGENT_STRICT}. Perl 5.8 has numerous memleaks that sometimes hit this module and are hard to work around. If you suffer from memleaks, first upgrade to Perl 5.10 and check wether the leaks still show up. (Perl 5.10.0 has other annoying mamleaks, such as leaking on "map" and "grep" but it is usually not as pronounced). Utility functions: AnyEvent::Util. Event modules: EV, EV::Glib, Glib::EV, Event, Glib::Event, Glib, Tk, Event::Lib, Qt, POE. Implementations: AnyEvent::Impl::EV, AnyEvent::Impl::Event, AnyEvent::Impl::Glib, AnyEvent::Impl::Tk, AnyEvent::Impl::Perl, AnyEvent::Impl::EventLib, AnyEvent::Impl::Qt, AnyEvent::Impl::POE. Non-blocking file handles, sockets, TCP clients and servers: AnyEvent::Handle, AnyEvent::Socket. Asynchronous DNS: AnyEvent::DNS. Coroutine support: Coro, Coro::AnyEvent, Coro::EV, Coro::Event, Nontrivial usage examples: Net::FCP, Net::XMPP2, AnyEvent::DNS. Marc Lehmann <schmorp@schmorp.de> http://home.schmorp.de/