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Marc A. Lehmann


AnyEvent - provide framework for multiple event loops

EV, Event, Coro::EV, Coro::Event, Glib, Tk, Perl - 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->wait; # enters "main loop" till $condvar gets ->broadcast
   $w->broadcast; # wake up current and all future wait's


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: Coro::EV, Coro::Event, EV, Event, Glib, Tk. The first one found is used. If none are found, the module tries to load these modules in the stated order. 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.

File handles will be kept alive, so as long as the watcher exists, the file handle exists, too.

It is not allowed to 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) should the timer activate. cb the callback to invoke in that case.

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 2014-01-01 to 2008-01-01, a watcher that you created 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) and absolute (ev_periodic) 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.

Multiple signals 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).

Example: wait for pid 1333

  my $w = AnyEvent->child (
     pid => 1333,
     cb  => sub {
        my ($pid, $status) = @_;
        warn "pid $pid exited with status $status";


Condition variables can be created by calling the AnyEvent->condvar method without any arguments.

A condition variable waits for a condition - precisely that the ->broadcast method has been called.

They are very useful to signal that a condition has been fulfilled, 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.

You can also use condition variables to block your main program until an event occurs - for example, you could ->wait in your main program until the user clicks the Quit button in your app, which would ->broadcast the "quit" event.

Note that condition variables recurse into the event loop - if you have two pirces of code that call ->wait 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.

This object has two methods:


Wait (blocking if necessary) until the ->broadcast method has been called on c<$cv>, while servicing other watchers normally.

You can only wait once on a condition - additional calls will return immediately.

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 ->wait in a module is that you cannot sensibly have two ->wait's in parallel, as that would require multiple interpreters or coroutines/threads, none of which AnyEvent can supply (the coroutine-aware backends AnyEvent::Impl::CoroEV and AnyEvent::Impl::CoroEvent explicitly support concurrent ->wait's from different coroutines, however).


Flag the condition as ready - a running ->wait and all further calls to wait will (eventually) return after this method has been called. If nobody is waiting the broadcast will be remembered..


   # 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->broadcast
   # when the "result" is ready.
   # in this case, we simply use a timer:
   my $w = AnyEvent->timer (
      after => 1,
      cb    => sub { $result_ready->broadcast },

   # this "blocks" (while handling events) till the watcher
   # calls broadcast



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::CoroEV    based on Coro::EV, best choice.
   AnyEvent::Impl::CoroEvent based on Coro::Event, second best choice.
   AnyEvent::Impl::EV        based on EV (an interface to libev, also best choice).
   AnyEvent::Impl::Event     based on Event, also second best choice :)
   AnyEvent::Impl::Glib      based on Glib, third-best choice.
   AnyEvent::Impl::Tk        based on Tk, very bad choice.
   AnyEvent::Impl::Perl      pure-perl implementation, inefficient but portable.

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.


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 ->wait on a condition variable unless you know that the ->broadcast 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 ->wait 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 ->wait 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.


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:

PERL_ANYEVENT_VERBOSE when set to 2 or higher, cause AnyEvent to report to STDERR which event model it chooses.


The following program uses an IO 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 (dies, 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 {



Event modules: Coro::EV, EV, EV::Glib, Glib::EV, Coro::Event, Event, Glib::Event, Glib, Coro, Tk.

Implementations: AnyEvent::Impl::CoroEV, AnyEvent::Impl::EV, AnyEvent::Impl::CoroEvent, AnyEvent::Impl::Event, AnyEvent::Impl::Glib, AnyEvent::Impl::Tk, AnyEvent::Impl::Perl.

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


 Marc Lehmann <schmorp@schmorp.de>