AnyEvent
view release on metacpan or search on metacpan
my $w = AnyEvent->timer (after => $seconds, cb => sub { ... });
my $w = AnyEvent->timer (after => $seconds, interval => $seconds, cb => ...);
print AnyEvent->now; # prints current event loop time
print AnyEvent->time; # think Time::HiRes::time or simply CORE::time.
# POSIX signal
my $w = AnyEvent->signal (signal => "TERM", cb => sub { ... });
# child process exit
my $w = AnyEvent->child (pid => $pid, cb => sub {
my ($pid, $status) = @_;
...
});
# called when event loop idle (if applicable)
my $w = AnyEvent->idle (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
# use a condvar in callback mode:
$w->cb (sub { $_[0]->recv });
INTRODUCTION/TUTORIAL
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.
SUPPORT
An FAQ document is available as AnyEvent::FAQ.
There also is a mailinglist for discussing all things AnyEvent, and an
IRC channel, too.
See the AnyEvent project page at the Schmorpforge Ta-Sa Software
Repository, at <http://anyevent.schmorp.de>, for more info.
WHY YOU SHOULD USE THIS MODULE (OR NOT)
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 join, 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 + EV? 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 easy 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 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.
DESCRIPTION
AnyEvent provides a uniform interface to various event loops. This
allows module authors to use event loop functionality without forcing
module users to use a specific event loop implementation (since more
than one event loop cannot coexist peacefully).
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, AnyEvent::Loop, Event, Glib,
Tk, Event::Lib, Qt, POE. The first one found is used. If none are
detected, the module tries to load the first four modules in the order
given; but note that if EV is not available, the pure-perl
AnyEvent::Loop should always work, so the other two are not normally
tried.
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 - this case should be very rare
though, as very few modules hardcode event loops without announcing this
very loudly.
The pure-perl implementation of AnyEvent is called "AnyEvent::Loop".
Like other event modules you can load it explicitly and enjoy the high
availability of that event loop :)
WATCHERS
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).
Note that callbacks must not permanently change global variables
potentially in use by the event loop (such as $_ or $[) and that
callbacks must not "die". The former is good programming practice in
Perl and the latter stems from the fact that exception handling differs
widely between event loops.
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.
AnyEvent->now_update
Some event loops (such as EV or AnyEvent::Loop) cache the current
time for each loop iteration (see the discussion of AnyEvent->now,
above).
When a callback runs for a long time (or when the process sleeps),
then this "current" time will differ substantially from the real
time, which might affect timers and time-outs.
When this is the case, you can call this method, which will update
the event loop's idea of "current time".
A typical example would be a script in a web server (e.g.
"mod_perl") - when mod_perl executes the script, then the event loop
will have the wrong idea about the "current time" (being potentially
far in the past, when the script ran the last time). In that case
you should arrange a call to "AnyEvent->now_update" each time the
web server process wakes up again (e.g. at the start of your script,
or in a handler).
Note that updating the time *might* cause some events to be handled.
SIGNAL WATCHERS
$w = AnyEvent->signal (signal => <uppercase_signal_name>, cb => <callback>);
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, and AnyEvent will ensure that signals
will not interrupt your program at bad times.
This watcher might use %SIG (depending on the event loop used), 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 });
Restart Behaviour
While restart behaviour is up to the event loop implementation, most
will not restart syscalls (that includes Async::Interrupt and AnyEvent's
pure perl implementation).
Safe/Unsafe Signals
Perl signals can be either "safe" (synchronous to opcode handling) or
"unsafe" (asynchronous) - the former might delay signal delivery
indefinitely, the latter might corrupt your memory.
AnyEvent signal handlers are, in addition, synchronous to the event
loop, i.e. they will not interrupt your running perl program but will
only be called as part of the normal event handling (just like timer,
I/O etc. callbacks, too).
Signal Races, Delays and Workarounds
Many event loops (e.g. Glib, Tk, Qt, IO::Async) do not support attaching
callbacks to signals in a generic way, which is a pity, as you cannot do
race-free signal handling in perl, requiring C libraries for this.
AnyEvent will try to do its best, which means in some cases, signals
will be delayed. The maximum time a signal might be delayed is 10
seconds by default, but can be overriden via
$ENV{PERL_ANYEVENT_MAX_SIGNAL_LATENCY} or $AnyEvent::MAX_SIGNAL_LATENCY
- see the "ENVIRONMENT VARIABLES" section for details.
All these problems can be avoided by installing the optional
Async::Interrupt module, which works with most event loops. It will not
work with inherently broken event loops such as Event or Event::Lib (and
not with POE currently). For those, you just have to suffer the delays.
CHILD PROCESS WATCHERS
$w = AnyEvent->child (pid => <process id>, cb => <callback>);
You can also watch for a child process exit and catch its exit status.
The child process is specified by the "pid" argument (on some backends,
using 0 watches for any child process exit, on others this will croak).
The watcher will be triggered only when the child process has finished
and an exit status is available, not on any trace events
(stopped/continued).
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.
This watcher type works by installing a signal handler for "SIGCHLD",
and since it cannot be shared, nothing else should use SIGCHLD or reap
random child processes (waiting for specific child processes, e.g.
inside "system", is just fine).
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 (neither POE nor IO::Async
do, see their AnyEvent::Impl manpages for details), 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).
AnyEvent's pure perl event loop handles all cases correctly regardless
of when you start the watcher.
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").
As most event loops do not support waiting for child events, they will
be emulated by AnyEvent in most cases, in which case the latency and
race problems mentioned in the description of signal watchers apply.
Example: fork a process and wait for it
} else {
# otherwise disable the idle watcher again
undef $idle_w;
}
});
});
CONDITION VARIABLES
$cv = AnyEvent->condvar;
$cv->send (<list>);
my @res = $cv->recv;
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 slightly different: it expects somebody else to run the
event loop and will only block when necessary (usually when told by the
user).
The tool to do that is called a "condition variable", so called because
they represent a condition that must become true.
Now is probably a good time to look at the examples further below.
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, with the condition variable as the first argument
(but not the results).
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).
Since condition variables are the most complex part of the AnyEvent API,
here are some different mental models of what they are - pick the ones
you can connect to:
* Condition variables are like callbacks - you can call them (and pass
them instead of callbacks). Unlike callbacks however, you can also
wait for them to be called.
* Condition variables are signals - one side can emit or send them,
the other side can wait for them, or install a handler that is
called when the signal fires.
* Condition variables are like "Merge Points" - points in your program
where you merge multiple independent results/control flows into one.
* Condition variables represent a transaction - functions that start
some kind of transaction can return them, leaving the caller the
choice between waiting in a blocking fashion, or setting a callback.
* Condition variables represent future values, or promises to deliver
some result, long before the result is available.
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
its "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.
# condition: "wait till the timer is fired"
my $timer_fired = AnyEvent->condvar;
# create the timer - we could wait for, say
# a handle becomign ready, or even an
# AnyEvent::HTTP request to finish, but
# in this case, we simply use a timer:
my $w = AnyEvent->timer (
after => 1,
cb => sub { $timer_fired->send },
);
# this "blocks" (while handling events) till the callback
# calls ->send
$timer_fired->recv;
Example: wait for a timer, but take advantage of the fact that condition
variables are also callable directly.
my $done = AnyEvent->condvar;
my $delay = AnyEvent->timer (after => 5, cb => $done);
$done->recv;
Example: Imagine an API that returns a condvar and doesn't support
callbacks. This is how you make a synchronous call, for example from the
main program:
use AnyEvent::CouchDB;
...
my @info = $couchdb->info->recv;
And this is how you would just set a callback to be called whenever the
results are available:
$couchdb->info->cb (sub {
my @info = $_[0]->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.
$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.
Condition variables are overloaded so one can call them directly (as
if they were a code reference). Calling them directly is the same as
calling "send".
$cv->croak ($error)
Similar to send, but causes all calls 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. Doing it this way instead of calling "croak" directly
delays the error detection, but has the overwhelming advantage that
it diagnoses the error at the place where the result is expected,
and not deep in some event callback with no connection to the actual
code causing the problem.
$cv->begin ([group callback])
$cv->end
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, passing the
condvar as first argument. That callback is *supposed* to call
"->send", but that is not required. If no group callback was set,
"send" will be called without any arguments.
AnyEvent::AIO for a case where this is useful.
Example: Create a watcher for the IO::AIO module and store it in
$WATCHER, but do so only do so after the event loop is initialised.
our WATCHER;
my $guard = AnyEvent::post_detect {
$WATCHER = AnyEvent->io (fh => IO::AIO::poll_fileno, poll => 'r', cb => \&IO::AIO::poll_cb);
};
# the ||= is important in case post_detect immediately runs the block,
# as to not clobber the newly-created watcher. assigning both watcher and
# post_detect guard to the same variable has the advantage of users being
# able to just C<undef $WATCHER> if the watcher causes them grief.
$WATCHER ||= $guard;
@AnyEvent::post_detect
This is a lower level interface then "AnyEvent::post_detect" (the
function). This variable is mainly useful for modules that can do
something useful when AnyEvent is used and thus want to know when it
is initialised, but do not need to even load it by default. This
array provides the means to hook into AnyEvent passively, without
loading it.
Here is how it works: If there are any code references in this array
(you can "push" to it before or after loading AnyEvent), then they
will be called directly after the event loop has been chosen.
You should check $AnyEvent::MODEL before adding to this array,
though: if it is defined then the event loop has already been
detected, and the array will be ignored.
Best use "AnyEvent::post_detect { BLOCK }" when your application
allows it, as it takes care of these details.
Example: To load Coro::AnyEvent whenever Coro and AnyEvent are used
together, you could put this into Coro (this is the actual code used
by Coro to accomplish this):
if (defined $AnyEvent::MODEL) {
# AnyEvent already initialised, so load Coro::AnyEvent
require Coro::AnyEvent;
} else {
# AnyEvent not yet initialised, so make sure to load Coro::AnyEvent
# as soon as it is
push @AnyEvent::post_detect, sub { require Coro::AnyEvent };
}
AnyEvent::postpone { BLOCK }
Arranges for the block to be executed as soon as possible, but not
before the call itself returns. In practise, the block will be
executed just before the event loop polls for new events, or shortly
afterwards.
This function never returns anything (to make the "return postpone {
... }" idiom more useful.
To understand the usefulness of this function, consider a function
that asynchronously does something for you and returns some
transaction object or guard to let you cancel the operation. For
example, "AnyEvent::Socket::tcp_connect":
# start a connection attempt unless one is active
$self->{connect_guard} ||= AnyEvent::Socket::tcp_connect "www.example.net", 80, sub {
delete $self->{connect_guard};
...
};
Imagine that this function could instantly call the callback, for
example, because it detects an obvious error such as a negative port
number. Invoking the callback before the function returns causes
problems however: the callback will be called and will try to delete
the guard object. But since the function hasn't returned yet, there
is nothing to delete. When the function eventually returns it will
assign the guard object to "$self->{connect_guard}", where it will
likely never be deleted, so the program thinks it is still trying to
connect.
This is where "AnyEvent::postpone" should be used. Instead of
calling the callback directly on error:
$cb->(undef), return # signal error to callback, BAD!
if $some_error_condition;
It should use "postpone":
AnyEvent::postpone { $cb->(undef) }, return # signal error to callback, later
if $some_error_condition;
AnyEvent::log $level, $msg[, @args]
Log the given $msg at the given $level.
If AnyEvent::Log is not loaded then this function makes a simple
test to see whether the message will be logged. If the test succeeds
it will load AnyEvent::Log and call "AnyEvent::Log::log" -
consequently, look at the AnyEvent::Log documentation for details.
If the test fails it will simply return. Right now this happens when
a numerical loglevel is used and it is larger than the level
specified via $ENV{PERL_ANYEVENT_VERBOSE}.
If you want to sprinkle loads of logging calls around your code,
consider creating a logger callback with the "AnyEvent::Log::logger"
function, which can reduce typing, codesize and can reduce the
logging overhead enourmously.
AnyEvent::fh_block $filehandle
AnyEvent::fh_unblock $filehandle
Sets blocking or non-blocking behaviour for the given filehandle.
WHAT TO DO IN A MODULE
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.
If the program is not event-based, it need not do anything special, even
when it depends on a module that uses an AnyEvent. If the program itself
uses AnyEvent, but does not care which event loop is used, all it needs
to do is "use AnyEvent". In either case, AnyEvent will choose the best
available loop implementation.
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 choose the wrong one unless you load the correct one
yourself.
You can chose to use a pure-perl implementation by loading the
"AnyEvent::Loop" 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.
OTHER MODULES
The following is a non-exhaustive list of additional modules that use
AnyEvent as a client and can therefore be mixed easily with other
AnyEvent modules and other event loops in the same program. Some of the
modules come as part of AnyEvent, the others are available via CPAN (see
<http://search.cpan.org/search?m=module&q=anyevent%3A%3A*> for a longer
non-exhaustive list), and the list is heavily biased towards modules of
the AnyEvent author himself :)
AnyEvent::Util (part of the AnyEvent distribution)
Contains various utility functions that replace often-used blocking
functions such as "inet_aton" with event/callback-based versions.
AnyEvent::Socket (part of the AnyEvent distribution)
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.
AnyEvent::Handle (part of the AnyEvent distribution)
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 (via AnyEvent::TLS).
AnyEvent::DNS (part of the AnyEvent distribution)
Provides rich asynchronous DNS resolver capabilities.
AnyEvent::HTTP, AnyEvent::IRC, AnyEvent::XMPP, AnyEvent::GPSD,
AnyEvent::IGS, AnyEvent::FCP
Implement event-based interfaces to the protocols of the same name
(for the curious, IGS is the International Go Server and FCP is the
Freenet Client Protocol).
AnyEvent::AIO (part of the AnyEvent distribution)
Truly asynchronous (as opposed to non-blocking) I/O, should be in
the toolbox of every event programmer. AnyEvent::AIO transparently
fuses IO::AIO and AnyEvent together, giving AnyEvent access to
event-based file I/O, and much more.
AnyEvent::Fork, AnyEvent::Fork::RPC, AnyEvent::Fork::Pool,
AnyEvent::Fork::Remote
These let you safely fork new subprocesses, either locally or
remotely (e.g.v ia ssh), using some RPC protocol or not, without the
limitations normally imposed by fork (AnyEvent works fine for
example). Dynamically-resized worker pools are obviously included as
well.
And they are quite tiny and fast as well - "abusing" AnyEvent::Fork
just to exec external programs can easily beat using "fork" and
"exec" (or even "system") in most programs.
AnyEvent::Filesys::Notify
AnyEvent is good for non-blocking stuff, but it can't detect file or
path changes (e.g. "watch this directory for new files", "watch this
file for changes"). The AnyEvent::Filesys::Notify module promises to
do just that in a portbale fashion, supporting inotify on GNU/Linux
and some weird, without doubt broken, stuff on OS X to monitor
files. It can fall back to blocking scans at regular intervals
transparently on other platforms, so it's about as portable as it
gets.
(I haven't used it myself, but it seems the biggest problem with it
is it quite bad performance).
AnyEvent::DBI
Executes DBI requests asynchronously in a proxy process for you,
notifying you in an event-based way when the operation is finished.
AnyEvent::FastPing
The fastest ping in the west.
Coro
Has special support for AnyEvent via Coro::AnyEvent, which allows
you to simply invert the flow control - don't call us, we will call
you:
async {
Coro::AnyEvent::sleep 5; # creates a 5s timer and waits for it
print "5 seconds later!\n";
Coro::AnyEvent::readable *STDIN; # uses an I/O watcher
my $line = <STDIN>; # works for ttys
AnyEvent::HTTP::http_get "url", Coro::rouse_cb;
my ($body, $hdr) = Coro::rouse_wait;
};
SIMPLIFIED AE API
Starting with version 5.0, AnyEvent officially supports a second, much
simpler, API that is designed to reduce the calling, typing and memory
overhead by using function call syntax and a fixed number of parameters.
See the AE manpage for details.
ERROR AND EXCEPTION HANDLING
In general, AnyEvent does not do any error handling - it relies on the
caller to do that if required. The AnyEvent::Strict module (see also the
"PERL_ANYEVENT_STRICT" environment variable, below) provides strict
checking of all AnyEvent methods, however, which is highly useful during
development.
As for exception handling (i.e. runtime errors and exceptions thrown
while executing a callback), this is not only highly event-loop
specific, but also not in any way wrapped by this module, as this is the
job of the main program.
The pure perl event loop simply re-throws the exception (usually within
"condvar->recv"), the Event and EV modules call "$Event/EV::DIED->()",
Glib uses "install_exception_handler" and so on.
ENVIRONMENT VARIABLES
AnyEvent supports a number of environment variables that tune the
runtime behaviour. They are usually evaluated when AnyEvent is loaded,
initialised, or a submodule that uses them is loaded. Many of them also
cause AnyEvent to load additional modules - for example,
"PERL_ANYEVENT_DEBUG_WRAP" causes the AnyEvent::Debug module to be
loaded.
All the environment variables documented here start with
"PERL_ANYEVENT_", which is what AnyEvent considers its own namespace.
Other modules are encouraged (but by no means required) to use
"PERL_ANYEVENT_SUBMODULE" if they have registered the
AnyEvent::Submodule namespace on CPAN, for any submodule. For example,
AnyEvent::HTTP could be expected to use "PERL_ANYEVENT_HTTP_PROXY" (it
should not access env variables starting with "AE_", see below).
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" (or its modern cousin, "use common::sense", it
is definitely recommended to keep it off in production. Keeping
"PERL_ANYEVENT_STRICT=1" in your environment while developing
programs can be very useful, however.
"PERL_ANYEVENT_DEBUG_SHELL"
If this env variable is nonempty, then its contents will be
interpreted by "AnyEvent::Socket::parse_hostport" and
"AnyEvent::Debug::shell" (after replacing every occurance of $$ by
the process pid). The shell object is saved in
$AnyEvent::Debug::SHELL.
This happens when the first watcher is created.
For example, to bind a debug shell on a unix domain socket in
/tmp/debug<pid>.sock, you could use this:
PERL_ANYEVENT_DEBUG_SHELL=/tmp/debug\$\$.sock perlprog
# connect with e.g.: socat readline /tmp/debug123.sock
Or to bind to tcp port 4545 on localhost:
PERL_ANYEVENT_DEBUG_SHELL=127.0.0.1:4545 perlprog
# connect with e.g.: telnet localhost 4545
Note that creating sockets in /tmp or on localhost is very unsafe on
multiuser systems.
"PERL_ANYEVENT_DEBUG_WRAP"
Can be set to 0, 1 or 2 and enables wrapping of all watchers for
debugging purposes. See "AnyEvent::Debug::wrap" for details.
"PERL_ANYEVENT_MODEL"
This can be used to specify the event model to be used by AnyEvent,
before auto detection and -probing kicks in.
It normally is a string consisting entirely of ASCII letters (e.g.
"EV" or "IOAsync"). The string "AnyEvent::Impl::" gets prepended and
the resulting module name is loaded and - if the load was successful
- used as event model backend. If it fails to load then AnyEvent
will proceed with auto detection and -probing.
If the string ends with "::" instead (e.g. "AnyEvent::Impl::EV::")
then nothing gets prepended and the module name is used as-is (hint:
"::" at the end of a string designates a module name and quotes it
appropriately).
For example, to force the pure perl model (AnyEvent::Loop::Perl) you
could start your program like this:
PERL_ANYEVENT_MODEL=Perl perl ...
"PERL_ANYEVENT_IO_MODEL"
The current file I/O model - see AnyEvent::IO for more info.
At the moment, only "Perl" (small, pure-perl, synchronous) and
"IOAIO" (truly asynchronous) are supported. The default is "IOAIO"
if AnyEvent::AIO can be loaded, otherwise it is "Perl".
"PERL_ANYEVENT_PROTOCOLS"
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 and other
failures anyways.
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.
"PERL_ANYEVENT_HOSTS"
This variable, if specified, overrides the /etc/hosts file used by
AnyEvent::Socket"::resolve_sockaddr", i.e. hosts aliases will be
read from that file instead.
"PERL_ANYEVENT_EDNS0"
Used by AnyEvent::DNS to decide whether to use the EDNS0 extension
for DNS. This extension is generally useful to reduce DNS traffic,
especially when DNSSEC is involved, 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.
"PERL_ANYEVENT_MAX_FORKS"
The maximum number of child processes that
"AnyEvent::Util::fork_call" will create in parallel.
"PERL_ANYEVENT_MAX_OUTSTANDING_DNS"
The default value for the "max_outstanding" parameter for the
default DNS resolver - this is the maximum number of parallel DNS
requests that are sent to the DNS server.
"PERL_ANYEVENT_MAX_SIGNAL_LATENCY"
Perl has inherently racy signal handling (you can basically choose
between losing signals and memory corruption) - pure perl event
loops (including "AnyEvent::Loop", when "Async::Interrupt" isn't
available) therefore have to poll regularly to avoid losing signals.
Some event loops are racy, but don't poll regularly, and some event
loops are written in C but are still racy. For those event loops,
AnyEvent installs a timer that regularly wakes up the event loop.
By default, the interval for this timer is 10 seconds, but you can
override this delay with this environment variable (or by setting
the $AnyEvent::MAX_SIGNAL_LATENCY variable before creating signal
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
* C-based event loops perform very well with small number of watchers,
as the management overhead dominates.
THE IO::Lambda BENCHMARK
Recently I was told about the benchmark in the IO::Lambda manpage, which
could be misinterpreted to make AnyEvent look bad. In fact, the
benchmark simply compares IO::Lambda with POE, and IO::Lambda looks
better (which shouldn't come as a surprise to anybody). As such, the
benchmark is fine, and mostly shows that the AnyEvent backend from
IO::Lambda isn't very optimal. But how would AnyEvent compare when used
without the extra baggage? To explore this, I wrote the equivalent
benchmark for AnyEvent.
The benchmark itself creates an echo-server, and then, for 500 times,
connects to the echo server, sends a line, waits for the reply, and then
creates the next connection. This is a rather bad benchmark, as it
doesn't test the efficiency of the framework or much non-blocking I/O,
but it is a benchmark nevertheless.
name runtime
Lambda/select 0.330 sec
+ optimized 0.122 sec
Lambda/AnyEvent 0.327 sec
+ optimized 0.138 sec
Raw sockets/select 0.077 sec
POE/select, components 0.662 sec
POE/select, raw sockets 0.226 sec
POE/select, optimized 0.404 sec
AnyEvent/select/nb 0.085 sec
AnyEvent/EV/nb 0.068 sec
+state machine 0.134 sec
The benchmark is also a bit unfair (my fault): the IO::Lambda/POE
benchmarks actually make blocking connects and use 100% blocking I/O,
defeating the purpose of an event-based solution. All of the newly
written AnyEvent benchmarks use 100% non-blocking connects (using
AnyEvent::Socket::tcp_connect and the asynchronous pure perl DNS
resolver), so AnyEvent is at a disadvantage here, as non-blocking
connects generally require a lot more bookkeeping and event handling
than blocking connects (which involve a single syscall only).
The last AnyEvent benchmark additionally uses AnyEvent::Handle, which
offers similar expressive power as POE and IO::Lambda, using
conventional Perl syntax. This means that both the echo server and the
client are 100% non-blocking, further placing it at a disadvantage.
As you can see, the AnyEvent + EV combination even beats the
hand-optimised "raw sockets benchmark", while AnyEvent + its pure perl
backend easily beats IO::Lambda and POE.
And even the 100% non-blocking version written using the high-level (and
slow :) AnyEvent::Handle abstraction beats both POE and IO::Lambda
higher level ("unoptimised") abstractions by a large margin, even though
it does all of DNS, tcp-connect and socket I/O in a non-blocking way.
The two AnyEvent benchmarks programs can be found as eg/ae0.pl and
eg/ae2.pl in the AnyEvent distribution, the remaining benchmarks are
part of the IO::Lambda distribution and were used without any changes.
SIGNALS
AnyEvent currently installs handlers for these signals:
SIGCHLD
A handler for "SIGCHLD" is installed by AnyEvent's child watcher
emulation for event loops that do not support them natively. Also,
some event loops install a similar handler.
Additionally, when AnyEvent is loaded and SIGCHLD is set to IGNORE,
then AnyEvent will reset it to default, to avoid losing child exit
statuses.
SIGPIPE
A no-op handler is installed for "SIGPIPE" when $SIG{PIPE} is
"undef" when AnyEvent gets loaded.
The rationale for this is that AnyEvent users usually do not really
depend on SIGPIPE delivery (which is purely an optimisation for
shell use, or badly-written programs), but "SIGPIPE" can cause
spurious and rare program exits as a lot of people do not expect
"SIGPIPE" when writing to some random socket.
The rationale for installing a no-op handler as opposed to ignoring
it is that this way, the handler will be restored to defaults on
exec.
Feel free to install your own handler, or reset it to defaults.
RECOMMENDED/OPTIONAL MODULES
One of AnyEvent's main goals is to be 100% Pure-Perl(tm): only perl (and
its built-in modules) are required to use it.
That does not mean that AnyEvent won't take advantage of some additional
modules if they are installed.
This section explains which additional modules will be used, and how
they affect AnyEvent's operation.
Async::Interrupt
This slightly arcane module is used to implement fast signal
handling: To my knowledge, there is no way to do completely
race-free and quick signal handling in pure perl. To ensure that
signals still get delivered, AnyEvent will start an interval timer
to wake up perl (and catch the signals) with some delay (default is
10 seconds, look for $AnyEvent::MAX_SIGNAL_LATENCY).
If this module is available, then it will be used to implement
signal catching, which means that signals will not be delayed, and
the event loop will not be interrupted regularly, which is more
efficient (and good for battery life on laptops).
This affects not just the pure-perl event loop, but also other event
loops that have no signal handling on their own (e.g. Glib, Tk, Qt).
Some event loops (POE, Event, Event::Lib) offer signal watchers
natively, and either employ their own workarounds (POE) or use
AnyEvent's workaround (using $AnyEvent::MAX_SIGNAL_LATENCY).
Installing Async::Interrupt does nothing for those backends.
EV This module isn't really "optional", as it is simply one of the
backend event loops that AnyEvent can use. However, it is simply the
best event loop available in terms of features, speed and stability:
It supports the AnyEvent API optimally, implements all the watcher
types in XS, does automatic timer adjustments even when no monotonic
clock is available, can take avdantage of advanced kernel interfaces
such as "epoll" and "kqueue", and is the fastest backend *by far*.
You can even embed Glib/Gtk2 in it (or vice versa, see EV::Glib and
Glib::EV).
If you only use backends that rely on another event loop (e.g.
"Tk"), then this module will do nothing for you.
Guard
The guard module, when used, will be used to implement
"AnyEvent::Util::guard". This speeds up guards considerably (and
uses a lot less memory), but otherwise doesn't affect guard
operation much. It is purely used for performance.
JSON and JSON::XS
One of these modules is required when you want to read or write JSON
data via AnyEvent::Handle. JSON is also written in pure-perl, but
can take advantage of the ultra-high-speed JSON::XS module when it
is installed.
Net::SSLeay
Implementing TLS/SSL in Perl is certainly interesting, but not very
worthwhile: If this module is installed, then AnyEvent::Handle (with
the help of AnyEvent::TLS), gains the ability to do TLS/SSL.
Time::HiRes
This module is part of perl since release 5.008. It will be used
when the chosen event library does not come with a timing source of
its own. The pure-perl event loop (AnyEvent::Loop) will additionally
load it to try to use a monotonic clock for timing stability.
AnyEvent::AIO (and IO::AIO)
The default implementation of AnyEvent::IO is to do I/O
synchronously, stopping programs while they access the disk, which
is fine for a lot of programs.
Installing AnyEvent::AIO (and its IO::AIO dependency) makes it
switch to a true asynchronous implementation, so event processing
can continue even while waiting for disk I/O.
FORK
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
- higher performance APIs such as BSD's kqueue or the dreaded Linux
epoll are usually badly thought-out hacks that are incompatible with
fork in one way or another. Only EV is fully fork-aware and ensures that
you continue event-processing in both parent and child (or both, if you
know what you are doing).
This means that, in general, you cannot fork and do event processing in
the child if the event library was initialised before the fork (which
usually happens when the first AnyEvent watcher is created, or the
library is loaded).
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 OR you must do
something completely out of the scope of AnyEvent (see below).
The problem of doing event processing in the parent *and* the child is
much more complicated: even for backends that *are* fork-aware or
fork-safe, their behaviour is not usually what you want: fork clones all
watchers, that means all timers, I/O watchers etc. are active in both
parent and child, which is almost never what you want. Using "exec" to
start worker children from some kind of manage prrocess is usually
preferred, because it is much easier and cleaner, at the expense of
having to have another binary.
In addition to logical problems with fork, there are also implementation
problems. For example, on POSIX systems, you cannot fork at all in Perl
code if a thread (I am talking of pthreads here) was ever created in the
process, and this is just the tip of the iceberg. In general, using fork
from Perl is difficult, and attempting to use fork without an exec to
implement some kind of parallel processing is almost certainly doomed.
To safely fork and exec, you should use a module such as Proc::FastSpawn
that let's you safely fork and exec new processes.
If you want to do multiprocessing using processes, you can look at the
AnyEvent::Fork module (and some related modules such as
AnyEvent::Fork::RPC, AnyEvent::Fork::Pool and AnyEvent::Fork::Remote).
This module allows you to safely create subprocesses without any
limitations - you can use X11 toolkits or AnyEvent in the children
created by AnyEvent::Fork safely and without any special precautions.
SECURITY CONSIDERATIONS
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_ANYEVENT_STRICT}.
Note that AnyEvent will remove *all* environment variables starting with
"PERL_ANYEVENT_" from %ENV when it is loaded while taint mode is
enabled.
BUGS
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 memleaks, such as leaking on "map" and "grep" but it is usually
not as pronounced).
SEE ALSO
Tutorial/Introduction: AnyEvent::Intro.
FAQ: AnyEvent::FAQ.
Utility functions: AnyEvent::Util (misc. grab-bag), AnyEvent::Log
(simply logging).
Development/Debugging: AnyEvent::Strict (stricter checking),
AnyEvent::Debug (interactive shell, watcher tracing).
Supported event modules: AnyEvent::Loop, EV, EV::Glib, Glib::EV, Event,
Glib::Event, Glib, Tk, Event::Lib, Qt, POE, FLTK, Cocoa::EventLoop, UV.
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,
AnyEvent::Impl::IOAsync, AnyEvent::Impl::Irssi, AnyEvent::Impl::FLTK,
AnyEvent::Impl::Cocoa, AnyEvent::Impl::UV.
Non-blocking handles, pipes, stream sockets, TCP clients and servers:
AnyEvent::Handle, AnyEvent::Socket, AnyEvent::TLS.
Asynchronous File I/O: AnyEvent::IO.
Asynchronous DNS: AnyEvent::DNS.
Thread support: Coro, Coro::AnyEvent, Coro::EV, Coro::Event.
Nontrivial usage examples: AnyEvent::GPSD, AnyEvent::IRC,
AnyEvent::HTTP.
AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://anyevent.schmorp.de
( run in 0.651 second using v1.01-cache-2.11-cpan-39bf76dae61 )