Section: User Contributed Perl Documentation (3)
Updated: 2008-05-01
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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



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 helps hiding 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 it is simply incompatible to everything that isn't itself. 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 POE and 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 enourmous 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, if you 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.  


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 filehandle 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.  


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. "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.


   # 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;



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 timer callback will be invoked at most once: if you want a repeating timer you have to create a new watcher (this is a limitation by both Tk and Glib).


   # 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;

   my $cb = sub {
      # cancel the old timer while creating a new one
      $w = AnyEvent->timer (after => 1, cb => $cb);

   # start the "loop" by creating the first watcher
   $w = AnyEvent->timer (after => 0.5, cb => $cb);


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.  


You can watch for signals using a signal watcher, "signal" is the signal name 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 occurances 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 guarenteed 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 });



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";

  # do something else, then wait for process exit



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 conditon variable is ``false'' until it becomes ``true'' by calling 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 outstandign events have been processed. And yet another way to call them is transations - 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-robbin 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.


   # 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


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.

$cv->send (...)
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.

$cv->croak ($error)
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.

$cv->begin ([group callback])
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) {
      ping_host_then_call_callback $host, sub {
         $result{$host} = ...;


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 eahc subrequest you finish, call "end".


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 suppporting 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.

$bool = $cv->ready
Returns true when the condition is ``true'', i.e. whether "send" or "croak" have been called.
$cb = $cv->cb ([new callback])
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. 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.
$guard = AnyEvent::post_detect { BLOCK }
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 rather inefficient pure-perl implementation by loading the "AnyEvent::Impl::Perl" module, which gives you similar behaviour everywhere, but letting AnyEvent chose is generally better.  


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.
Provide read and write buffers and manages watchers for reads and writes.
Provides a simple web application server framework.
Provides asynchronous DNS resolver capabilities, beyond what AnyEvent::Util offers.
The fastest ping in the west.
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.
AnyEvent::AIO, IO::AIO
Truly asynchronous I/O, should be in the toolbox of every event programmer. AnyEvent::AIO transparently fuses IO::AIO and AnyEvent together.
AnyEvent::BDB, BDB
Truly asynchronous Berkeley DB access. AnyEvent::AIO transparently fuses IO::AIO and AnyEvent together.
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.


   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.

This can be used to specify the event model to be used by AnyEvent, before autodetection 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 autodetection 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:




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->broadcast 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->wait; # 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 ahs finished:

   sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};

   if (end-of-file or data complete) {
     $txn->{result} = $txn->{buf};
     $txb->{cb}->($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:

   return $txn->{result};

The actual code goes further and collects all errors ("die"s, exceptions) that occured 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 $fcp->txn_client_get ($_),

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;


3b. The module user could use AnyEvent, too:

   use AnyEvent;

   my $quit = AnyEvent->condvar;

   $fcp->txn_client_get ($url)->cb (sub {




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.  


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 "->broadcast" a condvar once to signal the end of this phase.

destroy is the time, in microseconds, that it takes to destroy a single watcher.


          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


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).


* Using EV through AnyEvent is faster than any other event loop (even when used without AnyEvent), but most event loops have acceptable performance with or without AnyEvent.
* The overhead AnyEvent adds is usually much smaller than the overhead of the actual event loop, only with extremely fast event loops such as EV adds AnyEvent significant overhead.
* You should avoid POE like the plague if you want performance or reasonable memory usage.


This benchmark atcually benchmarks the event loop itself. It works by creating a number of ``servers'': each server consists of a socketpair, 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 iterstaion, 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 socketpairs (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 socketpair (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.


    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


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.


* The pure perl implementation performs extremely well.
* Avoid Glib or POE in large projects where performance matters.


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.


    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


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 noticably 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.


* C-based event loops perform very well with small number of watchers, as the management overhead dominates.


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:


  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).  


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.

Coroutine support: Coro, Coro::AnyEvent, Coro::EV, Coro::Event,

Nontrivial usage examples: Net::FCP, Net::XMPP2.  


 Marc Lehmann <>



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