AnyEvent(3) User Contributed Perl Documentation AnyEvent(3)NAMEAnyEvent - the DBI of event loop programming
EV, Event, Glib, Tk, Perl, Event::Lib, Irssi, rxvt-unicode, IO::Async,
Qt and POE are various supported event loops/environments.
SYNOPSIS
use AnyEvent;
# if you prefer function calls, look at the AE manpage for
# an alternative API.
# file handle or descriptor readable
my $w = AnyEvent->io (fh => $fh, poll => "r", cb => sub { ... });
# one-shot or repeating timers
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
There 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 joined, you are dependent on them and
you cannot use anything else, as they are simply incompatible to
everything that isn't them. What's worse, all the potential users of
your module are also forced to use the same event loop you use.
AnyEvent is different: AnyEvent + POE works fine. AnyEvent + Glib works
fine. AnyEvent + Tk works fine etc. etc. but none of these work
together with the rest: POE + IO::Async? No go. Tk + Event? No go.
Again: if your module uses one of those, every user of your module has
to use it, too. But if your module uses AnyEvent, it works
transparently with all event models it supports (including stuff like
IO::Async, as long as those use one of the supported event loops. It is
trivial to add new event loops to AnyEvent, too, so it is future-
proof).
In addition to being free of having to use the one and only true event
model, AnyEvent also is free of bloat and policy: with POE or similar
modules, you get an enormous amount of code and strict rules you have
to follow. AnyEvent, on the other hand, is lean and up to the point, by
only offering the functionality that is necessary, in as thin as a
wrapper as technically possible.
Of course, AnyEvent comes with a big (and fully optional!) toolbox of
useful functionality, such as an asynchronous DNS resolver, 100% non-
blocking connects (even with TLS/SSL, IPv6 and on broken platforms such
as Windows) and lots of real-world knowledge and workarounds for
platform bugs and differences.
Now, if you do want lots of policy (this can arguably be somewhat
useful) and you want to force your users to use the one and only event
model, you should not use this module.
DESCRIPTIONAnyEvent provides an identical interface to multiple event loops. This
allows module authors to utilise an event loop without forcing module
users to use the same event loop (as only a single event loop can
coexist peacefully at any one time).
The interface itself is vaguely similar, but not identical to the Event
module.
During the first call of any watcher-creation method, the module tries
to detect the currently loaded event loop by probing whether one of the
following modules is already loaded: EV, Event, Glib,
AnyEvent::Impl::Perl, Tk, Event::Lib, Qt, POE. The first one found is
used. If none are found, the module tries to load these modules
(excluding Tk, Event::Lib, Qt and POE as the pure perl adaptor should
always succeed) in the order given. The first one that can be
successfully loaded will be used. If, after this, still none could be
found, AnyEvent will fall back to a pure-perl event loop, which is not
very efficient, but should work everywhere.
Because AnyEvent first checks for modules that are already loaded,
loading an event model explicitly before first using AnyEvent will
likely make that model the default. For example:
use Tk;
use AnyEvent;
# .. AnyEvent will likely default to Tk
The likely means that, if any module loads another event model and
starts using it, all bets are off. Maybe you should tell their authors
to use AnyEvent so their modules work together with others
seamlessly...
The pure-perl implementation of AnyEvent is called
"AnyEvent::Impl::Perl". Like other event modules you can load it
explicitly and enjoy the high availability of that event loop :)
WATCHERSAnyEvent 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 practise in
Perl and the latter stems from the fact that exception handling differs
widely between event loops.
To disable the watcher you have to destroy it (e.g. by setting the
variable you store it in to "undef" or otherwise deleting all
references to it).
All watchers are created by calling a method on the "AnyEvent" class.
Many watchers either are used with "recursion" (repeating timers for
example), or need to refer to their watcher object in other ways.
An any way to achieve that is this pattern:
my $w; $w = AnyEvent->type (arg => value ..., cb => sub {
# you can use $w here, for example to undef it
undef $w;
});
Note that "my $w; $w =" combination. This is necessary because in Perl,
my variables are only visible after the statement in which they are
declared.
I/O WATCHERS
$w = AnyEvent->io (
fh => <filehandle_or_fileno>,
poll => <"r" or "w">,
cb => <callback>,
);
You can create an I/O watcher by calling the "AnyEvent->io" method with
the following mandatory key-value pairs as arguments:
"fh" is the Perl file handle (or a naked file descriptor) to watch for
events (AnyEvent might or might not keep a reference to this file
handle). Note that only file handles pointing to things for which non-
blocking operation makes sense are allowed. This includes sockets, most
character devices, pipes, fifos and so on, but not for example files or
block devices.
"poll" must be a string that is either "r" or "w", which creates a
watcher waiting for "r"eadable or "w"ritable events, respectively.
"cb" is the callback to invoke each time the file handle becomes ready.
Although the callback might get passed parameters, their value and
presence is undefined and you cannot rely on them. Portable AnyEvent
callbacks cannot use arguments passed to I/O watcher callbacks.
The I/O watcher might use the underlying file descriptor or a copy of
it. You must not close a file handle as long as any watcher is active
on the underlying file descriptor.
Some event loops issue spurious readyness notifications, so you should
always use non-blocking calls when reading/writing from/to your file
handles.
Example: wait for readability of STDIN, then read a line and disable
the watcher.
my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
chomp (my $input = <STDIN>);
warn "read: $input\n";
undef $w;
});
TIME WATCHERS
$w = AnyEvent->timer (after => <seconds>, cb => <callback>);
$w = AnyEvent->timer (
after => <fractional_seconds>,
interval => <fractional_seconds>,
cb => <callback>,
);
You can create a time watcher by calling the "AnyEvent->timer" method
with the following mandatory arguments:
"after" specifies after how many seconds (fractional values are
supported) the callback should be invoked. "cb" is the callback to
invoke in that case.
Although the callback might get passed parameters, their value and
presence is undefined and you cannot rely on them. Portable AnyEvent
callbacks cannot use arguments passed to time watcher callbacks.
The callback will normally be invoked once only. If you specify another
parameter, "interval", as a strictly positive number (> 0), then the
callback will be invoked regularly at that interval (in fractional
seconds) after the first invocation. If "interval" is specified with a
false value, then it is treated as if it were missing.
The callback will be rescheduled before invoking the callback, but no
attempt is done to avoid timer drift in most backends, so the interval
is only approximate.
Example: fire an event after 7.7 seconds.
my $w = AnyEvent->timer (after => 7.7, cb => sub {
warn "timeout\n";
});
# to cancel the timer:
undef $w;
Example 2: fire an event after 0.5 seconds, then roughly every second.
my $w = AnyEvent->timer (after => 0.5, interval => 1, cb => sub {
warn "timeout\n";
};
TIMING ISSUES
There are two ways to handle timers: based on real time (relative,
"fire in 10 seconds") and based on wallclock time (absolute, "fire at
12 o'clock").
While most event loops expect timers to specified in a relative way,
they use absolute time internally. This makes a difference when your
clock "jumps", for example, when ntp decides to set your clock
backwards from the wrong date of 2014-01-01 to 2008-01-01, a watcher
that is supposed to fire "after" a second might actually take six years
to finally fire.
AnyEvent cannot compensate for this. The only event loop that is
conscious about these issues is EV, which offers both relative
(ev_timer, based on true relative time) and absolute (ev_periodic,
based on wallclock time) timers.
AnyEvent always prefers relative timers, if available, matching the
AnyEvent API.
AnyEvent has two additional methods that return the "current time":
AnyEvent->time
This returns the "current wallclock time" as a fractional number of
seconds since the Epoch (the same thing as "time" or
"Time::HiRes::time" return, and the result is guaranteed to be
compatible with those).
It progresses independently of any event loop processing, i.e. each
call will check the system clock, which usually gets updated
frequently.
AnyEvent->now
This also returns the "current wallclock time", but unlike "time",
above, this value might change only once per event loop iteration,
depending on the event loop (most return the same time as "time",
above). This is the time that AnyEvent's timers get scheduled
against.
In almost all cases (in all cases if you don't care), this is the
function to call when you want to know the current time.
This function is also often faster then "AnyEvent->time", and thus
the preferred method if you want some timestamp (for example,
AnyEvent::Handle uses this to update it's activity timeouts).
The rest of this section is only of relevance if you try to be very
exact with your timing, you can skip it without bad conscience.
For a practical example of when these times differ, consider
Event::Lib and EV and the following set-up:
The event loop is running and has just invoked one of your callback
at time=500 (assume no other callbacks delay processing). In your
callback, you wait a second by executing "sleep 1" (blocking the
process for a second) and then (at time=501) you create a relative
timer that fires after three seconds.
With Event::Lib, "AnyEvent->time" and "AnyEvent->now" will both
return 501, because that is the current time, and the timer will be
scheduled to fire at time=504 (501 + 3).
With EV, "AnyEvent->time" returns 501 (as that is the current
time), but "AnyEvent->now" returns 500, as that is the time the
last event processing phase started. With EV, your timer gets
scheduled to run at time=503 (500 + 3).
In one sense, Event::Lib is more exact, as it uses the current time
regardless of any delays introduced by event processing. However,
most callbacks do not expect large delays in processing, so this
causes a higher drift (and a lot more system calls to get the
current time).
In another sense, EV is more exact, as your timer will be scheduled
at the same time, regardless of how long event processing actually
took.
In either case, if you care (and in most cases, you don't), then
you can get whatever behaviour you want with any event loop, by
taking the difference between "AnyEvent->time" and "AnyEvent->now"
into account.
AnyEvent->now_update
Some event loops (such as EV or AnyEvent::Impl::Perl) 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 get delayed 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 it's best, which means in some cases,
signals will be delayed. The maximum time a signal might be delayed is
specified in $AnyEvent::MAX_SIGNAL_LATENCY (default: 10 seconds). This
variable can be changed only before the first signal watcher is
created, and should be left alone otherwise. This variable determines
how often AnyEvent polls for signals (in case a wake-up was missed).
Higher values will cause fewer spurious wake-ups, which is better for
power and CPU saving.
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, as POE does it's own workaround with one-
second latency). For those, you just have to suffer the delays.
CHILD PROCESS WATCHERS
$w = AnyEvent->child (pid => <process id>, cb => <callback>);
You can also watch on a child process exit and catch its exit status.
The child process is specified by the "pid" argument (one 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 the latency and race
problems mentioned in the description of signal watchers apply.
Example: fork a process and wait for it
my $done = AnyEvent->condvar;
my $pid = fork or exit 5;
my $w = AnyEvent->child (
pid => $pid,
cb => sub {
my ($pid, $status) = @_;
warn "pid $pid exited with status $status";
$done->send;
},
);
# do something else, then wait for process exit
$done->recv;
IDLE WATCHERS
$w = AnyEvent->idle (cb => <callback>);
Repeatedly invoke the callback after the process becomes idle, until
either the watcher is destroyed or new events have been detected.
Idle watchers are useful when there is a need to do something, but it
is not so important (or wise) to do it instantly. The callback will be
invoked only when there is "nothing better to do", which is usually
defined as "all outstanding events have been handled and no new events
have been detected". That means that idle watchers ideally get invoked
when the event loop has just polled for new events but none have been
detected. Instead of blocking to wait for more events, the idle
watchers will be invoked.
Unfortunately, most event loops do not really support idle watchers
(only EV, Event and Glib do it in a usable fashion) - for the rest,
AnyEvent will simply call the callback "from time to time".
Example: read lines from STDIN, but only process them when the program
is otherwise idle:
my @lines; # read data
my $idle_w;
my $io_w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
push @lines, scalar <STDIN>;
# start an idle watcher, if not already done
$idle_w ||= AnyEvent->idle (cb => sub {
# handle only one line, when there are lines left
if (my $line = shift @lines) {
print "handled when idle: $line";
} 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 - function 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
it's "new" method in your own "new" method.
There are two "sides" to a condition variable - the "producer side"
which eventually calls "-> send", and the "consumer side", which waits
for the send to occur.
Example: wait for a timer.
# 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 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. Doing it this way instead of calling "croak"
directly delays the error detetcion, but has the overwhelmign
advantage that it diagnoses the error at the place where the result
is expected, and not deep in some event clalback without connection
to the actual code causing the problem.
$cv->begin ([group callback])
$cv->end
These two methods can be used to combine many transactions/events
into one. For example, a function that pings many hosts in parallel
might want to use a condition variable for the whole process.
Every call to "->begin" will increment a counter, and every call to
"->end" will decrement it. If the counter reaches 0 in "->end",
the (last) callback passed to "begin" will be executed, passing the
condvar as first argument. That callback is supposed to call
"->send", but that is not required. If no group callback was set,
"send" will be called without any arguments.
You can think of "$cv->send" giving you an OR condition (one call
sends), while "$cv->begin" and "$cv->end" giving you an AND
condition (all "begin" calls must be "end"'ed before the condvar
sends).
Let's start with a simple example: you have two I/O watchers (for
example, STDOUT and STDERR for a program), and you want to wait for
both streams to close before activating a condvar:
my $cv = AnyEvent->condvar;
$cv->begin; # first watcher
my $w1 = AnyEvent->io (fh => $fh1, cb => sub {
defined sysread $fh1, my $buf, 4096
or $cv->end;
});
$cv->begin; # second watcher
my $w2 = AnyEvent->io (fh => $fh2, cb => sub {
defined sysread $fh2, my $buf, 4096
or $cv->end;
});
$cv->recv;
This works because for every event source (EOF on file handle),
there is one call to "begin", so the condvar waits for all calls to
"end" before sending.
The ping example mentioned above is slightly more complicated, as
the there are results to be passwd back, and the number of tasks
that are begung can potentially be zero:
my $cv = AnyEvent->condvar;
my %result;
$cv->begin (sub { shift->send (\%result) });
for my $host (@list_of_hosts) {
$cv->begin;
ping_host_then_call_callback $host, sub {
$result{$host} = ...;
$cv->end;
};
}
$cv->end;
This code fragment supposedly pings a number of hosts and calls
"send" after results for all then have have been gathered - in any
order. To achieve this, the code issues a call to "begin" when it
starts each ping request and calls "end" when it has received some
result for it. Since "begin" and "end" only maintain a counter, the
order in which results arrive is not relevant.
There is an additional bracketing call to "begin" and "end" outside
the loop, which serves two important purposes: first, it sets the
callback to be called once the counter reaches 0, and second, it
ensures that "send" is called even when "no" hosts are being pinged
(the loop doesn't execute once).
This is the general pattern when you "fan out" into multiple (but
potentially none) subrequests: use an outer "begin"/"end" pair to
set the callback and ensure "end" is called at least once, and
then, for each subrequest you start, call "begin" and for each
subrequest you finish, call "end".
METHODS FOR CONSUMERS
These methods should only be used by the consuming side, i.e. the code
awaits the condition.
$cv->recv
Wait (blocking if necessary) until the "->send" or "->croak"
methods have been called on c<$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.
Note that doing a blocking wait in a callback is not supported by
any event loop, that is, recursive invocation of a blocking
"->recv" is not allowed, and the "recv" call will "croak" if such a
condition is detected. This condition can be slightly loosened by
using Coro::AnyEvent, which allows you to do a blocking "->recv"
from any thread that doesn't run the event loop itself.
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.
Instead, let the caller decide whether the call will block or not
(for example, by coupling condition variables with some kind of
request results and supporting callbacks so the caller knows that
getting the result will not block, while still supporting blocking
waits if the caller so desires).
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 = $cv->cb ($cb->($cv))
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 (or already
was) "true", i.e. when "send" or "croak" are called (or were
called), with the only argument being the condition variable
itself. Calling "recv" inside the callback or at any later time is
guaranteed not to block.
SUPPORTED EVENT LOOPS/BACKENDS
The available backend classes are (every class has its own manpage):
Backends that are autoprobed when no other event loop can be found.
EV is the preferred backend when no other event loop seems to be in
use. If EV is not installed, then AnyEvent will fall back to its
own pure-perl implementation, which is available everywhere as it
comes with AnyEvent itself.
AnyEvent::Impl::EV based on EV (interface to libev, best choice).
AnyEvent::Impl::Perl pure-perl implementation, fast and portable.
Backends that are transparently being picked up when they are used.
These will be used when they are currently loaded when the first
watcher is created, in which case it is assumed that the
application is using them. This means that AnyEvent will
automatically pick the right backend when the main program loads an
event module before anything starts to create watchers. Nothing
special needs to be done by the main program.
AnyEvent::Impl::Event based on Event, very stable, few glitches.
AnyEvent::Impl::Glib based on Glib, slow but very stable.
AnyEvent::Impl::Tk based on Tk, very broken.
AnyEvent::Impl::EventLib based on Event::Lib, leaks memory and worse.
AnyEvent::Impl::POE based on POE, very slow, some limitations.
AnyEvent::Impl::Irssi used when running within irssi.
Backends with special needs.
Qt requires the Qt::Application to be instantiated first, but will
otherwise be picked up automatically. As long as the main program
instantiates the application before any AnyEvent watchers are
created, everything should just work.
AnyEvent::Impl::Qt based on Qt.
Support for IO::Async can only be partial, as it is too broken and
architecturally limited to even support the AnyEvent API. It also
is the only event loop that needs the loop to be set explicitly, so
it can only be used by a main program knowing about AnyEvent. See
AnyEvent::Impl::Async for the gory details.
AnyEvent::Impl::IOAsync based on IO::Async, cannot be autoprobed.
Event loops that are indirectly supported via other backends.
Some event loops can be supported via other modules:
There is no direct support for WxWidgets (Wx) or Prima.
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.
Prima is not supported as nobody seems to be using it, but it has a
POE backend, so it can be supported through POE.
AnyEvent knows about both Prima and Wx, however, and will try to
load POE when detecting them, in the hope that POE will pick them
up, in which case everything will be automatic.
GLOBAL VARIABLES AND FUNCTIONS
These are not normally required to use AnyEvent, but can be useful to
write AnyEvent extension modules.
$AnyEvent::MODEL
Contains "undef" until the first watcher is being created, before
the backend has been autodetected.
Afterwards 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 it will be "urxvt::anyevent").
AnyEvent::detect
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, and not e.g. while initialising of your
module.
If you need to do some initialisation before AnyEvent watchers are
created, use "post_detect".
$guard = AnyEvent::post_detect { BLOCK }
Arranges for the code block to be executed as soon as the event
model is autodetected (or immediately if this has already
happened).
The block will be executed after the actual backend has been
detected ($AnyEvent::MODEL is set), but before any watchers have
been created, so it is possible to e.g. patch @AnyEvent::ISA or do
other initialisations - see the sources of AnyEvent::Strict or
AnyEvent::AIO to see how this is used.
The most common usage is to create some global watchers, without
forcing event module detection too early, for example,
AnyEvent::AIO creates and installs the global IO::AIO watcher in a
"post_detect" block to avoid autodetecting the event module at load
time.
If called in scalar or list context, then it creates and returns an
object that automatically removes the callback again when it is
destroyed (or "undef" when the hook was immediately executed). See
AnyEvent::AIO for a case where this is useful.
Example: Create a watcher for the IO::AIO module and store it in
$WATCHER. Only do so after the event loop is initialised, though.
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
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 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.
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.
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 };
}
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.
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).
WHAT TO DO IN THE MAIN PROGRAM
There will always be a single main program - the only place that should
dictate which event model to use.
If it doesn't care, it can just "use AnyEvent" and use it itself, or
not do anything special (it does not need to be event-based) and let
AnyEvent decide which implementation to chose if some module relies on
it.
If the main program relies on a specific event model - for example, in
Gtk2 programs you have to rely on the Glib module - you should load the
event module before loading AnyEvent or any module that uses it:
generally speaking, you should load it as early as possible. The reason
is that modules might create watchers when they are loaded, and
AnyEvent will decide on the event model to use as soon as it creates
watchers, and it might chose the wrong one unless you load the correct
one yourself.
You can chose to use a pure-perl implementation by loading the
"AnyEvent::Impl::Perl" module, which gives you similar behaviour
everywhere, but letting AnyEvent chose the model is generally better.
MAINLOOP EMULATION
Sometimes (often for short test scripts, or even standalone programs
who only want to use AnyEvent), you do not want to run a specific event
loop.
In that case, you can use a condition variable like this:
AnyEvent->condvar->recv;
This has the effect of entering the event loop and looping forever.
Note that usually your program has some exit condition, in which case
it is better to use the "traditional" approach of storing a condition
variable somewhere, waiting for it, and sending it when the program
should exit cleanly.
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.
AnyEvent::Util
Contains various utility functions that replace often-used but
blocking functions such as "inet_aton" by event-/callback-based
versions.
AnyEvent::Socket
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
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
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::Handle::UDP
Here be danger!
As Pauli would put it, "Not only is it not right, it's not even
wrong!" - there are so many things wrong with
AnyEvent::Handle::UDP, most notably it's use of a stream-based API
with a protocol that isn't streamable, that the only way to improve
it is to delete it.
It features data corruption (but typically only under load) and
general confusion. On top, the author is not only clueless about
UDP but also fact-resistant - some gems of his understanding:
"connect doesn't work with UDP", "UDP packets are not IP packets",
"UDP only has datagrams, not packets", "I don't need to implement
proper error checking as UDP doesn't support error checking" and so
on - he doesn't even understand what's wrong with his module when
it is explained to him.
AnyEvent::DBI
Executes DBI requests asynchronously in a proxy process for you,
notifying you in an event-bnased way when the operation is
finished.
AnyEvent::AIO
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::HTTPD
A simple embedded webserver.
AnyEvent::FastPing
The fastest ping in the west.
Coro
Has special support for AnyEvent via Coro::AnyEvent.
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
The following environment variables are used by this module or its
submodules.
Note that AnyEvent will remove all environment variables starting with
"PERL_ANYEVENT_" from %ENV when it is loaded while taint mode is
enabled.
"PERL_ANYEVENT_VERBOSE"
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.
When set to 8 or higher, then AnyEvent will report extra
information on which optional modules it loads and how it
implements certain features.
"PERL_ANYEVENT_STRICT"
AnyEvent does not do much argument checking by default, as thorough
argument checking is very costly. Setting this variable to a true
value will cause AnyEvent to load "AnyEvent::Strict" and then to
thoroughly check the arguments passed to most method calls. If it
finds any problems, it will croak.
In other words, enables "strict" mode.
Unlike "use strict" (or it's 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_MODEL"
This can be used to specify the event model to be used by AnyEvent,
before auto detection and -probing kicks in. It must be a string
consisting entirely of ASCII letters. The string "AnyEvent::Impl::"
gets prepended and the resulting module name is loaded and if the
load was successful, used as event model. If it fails to load
AnyEvent will proceed with auto detection and -probing.
This functionality might change in future versions.
For example, to force the pure perl model (AnyEvent::Impl::Perl)
you could start your program like this:
PERL_ANYEVENT_MODEL=Perl perl ...
"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 conenction 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_EDNS0"
Used by AnyEvent::DNS to decide whether to use the EDNS0 extension
for DNS. This extension is generally useful to reduce DNS traffic,
but some (broken) firewalls drop such DNS packets, which is why it
is off by default.
Setting this variable to 1 will cause AnyEvent::DNS to announce
EDNS0 in its DNS requests.
"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_RESOLV_CONF"
The file to use instead of /etc/resolv.conf (or OS-specific
configuration) in the default resolver. When set to the empty
string, no default config will be used.
"PERL_ANYEVENT_CA_FILE", "PERL_ANYEVENT_CA_PATH".
When neither "ca_file" nor "ca_path" was specified during
AnyEvent::TLS context creation, and either of these environment
variables exist, they will be used to specify CA certificate
locations instead of a system-dependent default.
"PERL_ANYEVENT_AVOID_GUARD" and "PERL_ANYEVENT_AVOID_ASYNC_INTERRUPT"
When these are set to 1, then the respective modules are not
loaded. Mostly good for testing AnyEvent itself.
SUPPLYING YOUR OWN EVENT MODEL INTERFACE
This is an advanced topic that you do not normally need to use AnyEvent
in a module. This section is only of use to event loop authors who want
to provide AnyEvent compatibility.
If you need to support another event library which isn't directly
supported by AnyEvent, you can supply your own interface to it by
pushing, before the first watcher gets created, the package name of the
event module and the package name of the interface to use onto
@AnyEvent::REGISTRY. You can do that before and even without loading
AnyEvent, so it is reasonably cheap.
Example:
push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];
This tells AnyEvent to (literally) use the "urxvt::anyevent::"
package/class when it finds the "urxvt" package/module is already
loaded.
When AnyEvent is loaded and asked to find a suitable event model, it
will first check for the presence of urxvt by trying to "use" the
"urxvt::anyevent" module.
The class should provide implementations for all watcher types. See
AnyEvent::Impl::EV (source code), AnyEvent::Impl::Glib (Source code)
and so on for actual examples. Use "perldoc -m AnyEvent::Impl::Glib" to
see the sources.
If you don't provide "signal" and "child" watchers than AnyEvent will
provide suitable (hopefully) replacements.
The above example isn't fictitious, the rxvt-unicode (a.k.a. urxvt)
terminal emulator uses the above line as-is. An interface isn't
included in AnyEvent because it doesn't make sense outside the embedded
interpreter inside rxvt-unicode, and it is updated and maintained as
part of the rxvt-unicode distribution.
rxvt-unicode also cheats a bit by not providing blocking access to
condition variables: code blocking while waiting for a condition will
"die". This still works with most modules/usages, and blocking calls
must not be done in an interactive application, so it makes sense.
EXAMPLE PROGRAM
The following program uses an I/O watcher to read data from STDIN, a
timer to display a message once per second, and a condition variable to
quit the program when the user enters quit:
use AnyEvent;
my $cv = AnyEvent->condvar;
my $io_watcher = AnyEvent->io (
fh => \*STDIN,
poll => 'r',
cb => sub {
warn "io event <$_[0]>\n"; # will always output <r>
chomp (my $input = <STDIN>); # read a line
warn "read: $input\n"; # output what has been read
$cv->send if $input =~ /^q/i; # quit program if /^q/i
},
);
my $time_watcher = AnyEvent->timer (after => 1, interval => 1, cb => sub {
warn "timeout\n"; # print 'timeout' at most every second
});
$cv->recv; # wait until user enters /^q/i
REAL-WORLD EXAMPLE
Consider the Net::FCP module. It features (among others) the following
API calls, which are to freenet what HTTP GET requests are to http:
my $data = $fcp->client_get ($url); # blocks
my $transaction = $fcp->txn_client_get ($url); # does not block
$transaction->cb ( sub { ... } ); # set optional result callback
my $data = $transaction->result; # possibly blocks
The "client_get" method works like "LWP::Simple::get": it requests the
given URL and waits till the data has arrived. It is defined to be:
sub client_get { $_[0]->txn_client_get ($_[1])->result }
And in fact is automatically generated. This is the blocking API of
Net::FCP, and it works as simple as in any other, similar, module.
More complicated is "txn_client_get": It only creates a transaction
(completion, result, ...) object and initiates the transaction.
my $txn = bless { }, Net::FCP::Txn::;
It also creates a condition variable that is used to signal the
completion of the request:
$txn->{finished} = AnyAvent->condvar;
It then creates a socket in non-blocking mode.
socket $txn->{fh}, ...;
fcntl $txn->{fh}, F_SETFL, O_NONBLOCK;
connect $txn->{fh}, ...
and !$!{EWOULDBLOCK}
and !$!{EINPROGRESS}
and Carp::croak "unable to connect: $!\n";
Then it creates a write-watcher which gets called whenever an error
occurs or the connection succeeds:
$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'w', cb => sub { $txn->fh_ready_w });
And returns this transaction object. The "fh_ready_w" callback gets
called as soon as the event loop detects that the socket is ready for
writing.
The "fh_ready_w" method makes the socket blocking again, writes the
request data and replaces the watcher by a read watcher (waiting for
reply data). The actual code is more complicated, but that doesn't
matter for this example:
fcntl $txn->{fh}, F_SETFL, 0;
syswrite $txn->{fh}, $txn->{request}
or die "connection or write error";
$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });
Again, "fh_ready_r" waits till all data has arrived, and then stores
the result and signals any possible waiters that the request has
finished:
sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};
if (end-of-file or data complete) {
$txn->{result} = $txn->{buf};
$txn->{finished}->send;
$txb->{cb}->($txn) of $txn->{cb}; # also call callback
}
The "result" method, finally, just waits for the finished signal (if
the request was already finished, it doesn't wait, of course, and
returns the data:
$txn->{finished}->recv;
return $txn->{result};
The actual code goes further and collects all errors ("die"s,
exceptions) that occurred during request processing. The "result"
method detects whether an exception as thrown (it is stored inside the
$txn object) and just throws the exception, which means connection
errors and other problems get reported to the code that tries to use
the result, not in a random callback.
All of this enables the following usage styles:
1. Blocking:
my $data = $fcp->client_get ($url);
2. Blocking, but running in parallel:
my @datas = map $_->result,
map $fcp->txn_client_get ($_),
@urls;
Both blocking examples work without the module user having to know
anything about events.
3a. Event-based in a main program, using any supported event module:
use EV;
$fcp->txn_client_get ($url)->cb (sub {
my $txn = shift;
my $data = $txn->result;
...
});
EV::loop;
3b. The module user could use AnyEvent, too:
use AnyEvent;
my $quit = AnyEvent->condvar;
$fcp->txn_client_get ($url)->cb (sub {
...
$quit->send;
});
$quit->recv;
BENCHMARKS
To give you an idea of the performance and overheads that AnyEvent adds
over the event loops themselves and to give you an impression of the
speed of various event loops I prepared some benchmarks.
BENCHMARKING ANYEVENT OVERHEAD
Here is a benchmark of various supported event models used natively and
through AnyEvent. The benchmark creates a lot of timers (with a zero
timeout) and I/O watchers (watching STDOUT, a pty, to become writable,
which it is), lets them fire exactly once and destroys them again.
Source code for this benchmark is found as eg/bench in the AnyEvent
distribution. It uses the AE interface, which makes a real difference
for the EV and Perl backends only.
Explanation of the columns
watcher is the number of event watchers created/destroyed. Since
different event models feature vastly different performances, each
event loop was given a number of watchers so that overall runtime is
acceptable and similar between tested event loop (and keep them from
crashing): Glib would probably take thousands of years if asked to
process the same number of watchers as EV in this benchmark.
bytes is the number of bytes (as measured by the resident set size,
RSS) consumed by each watcher. This method of measuring captures both C
and Perl-based overheads.
create is the time, in microseconds (millionths of seconds), that it
takes to create a single watcher. The callback is a closure shared
between all watchers, to avoid adding memory overhead. That means
closure creation and memory usage is not included in the figures.
invoke is the time, in microseconds, used to invoke a simple callback.
The callback simply counts down a Perl variable and after it was
invoked "watcher" times, it would "->send" a condvar once to signal the
end of this phase.
destroy is the time, in microseconds, that it takes to destroy a single
watcher.
Results
name watchers bytes create invoke destroy comment
EV/EV 100000 223 0.47 0.43 0.27 EV native interface
EV/Any 100000 223 0.48 0.42 0.26 EV + AnyEvent watchers
Coro::EV/Any 100000 223 0.47 0.42 0.26 coroutines + Coro::Signal
Perl/Any 100000 431 2.70 0.74 0.92 pure perl implementation
Event/Event 16000 516 31.16 31.84 0.82 Event native interface
Event/Any 16000 1203 42.61 34.79 1.80 Event + AnyEvent watchers
IOAsync/Any 16000 1911 41.92 27.45 16.81 via IO::Async::Loop::IO_Poll
IOAsync/Any 16000 1726 40.69 26.37 15.25 via IO::Async::Loop::Epoll
Glib/Any 16000 1118 89.00 12.57 51.17 quadratic behaviour
Tk/Any 2000 1346 20.96 10.75 8.00 SEGV with >> 2000 watchers
POE/Any 2000 6951 108.97 795.32 14.24 via POE::Loop::Event
POE/Any 2000 6648 94.79 774.40 575.51 via POE::Loop::Select
Discussion
The benchmark does not measure scalability of the event loop very well.
For example, a select-based event loop (such as the pure perl one) can
never compete with an event loop that uses epoll when the number of
file descriptors grows high. In this benchmark, all events become ready
at the same time, so select/poll-based implementations get an unnatural
speed boost.
Also, note that the number of watchers usually has a nonlinear effect
on overall speed, that is, creating twice as many watchers doesn't take
twice the time - usually it takes longer. This puts event loops tested
with a higher number of watchers at a disadvantage.
To put the range of results into perspective, consider that on the
benchmark machine, handling an event takes roughly 1600 CPU cycles with
EV, 3100 CPU cycles with AnyEvent's pure perl loop and almost 3000000
CPU cycles with POE.
"EV" is the sole leader regarding speed and memory use, which are both
maximal/minimal, respectively. When using the AE API there is zero
overhead (when going through the AnyEvent API create is about 5-6 times
slower, with other times being equal, so still 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.
"IO::Async" performs admirably well, about on par with "Event", even
when using its pure perl backend.
"Glib"'s memory usage is quite a bit higher, but it features a faster
callback invocation and overall ends up in the same class as "Event".
However, Glib scales extremely badly, doubling the number of watchers
increases the processing time by more than a factor of four, making it
completely unusable when using larger numbers of watchers (note that
only a single file descriptor was used in the benchmark, so
inefficiencies of "poll" do not account for this).
The "Tk" adaptor works relatively well. The fact that it crashes with
more than 2000 watchers is a big setback, however, as correctness takes
precedence over speed. Nevertheless, its performance is surprising, as
the file descriptor is dup()ed for each watcher. This shows that the
dup() employed by some adaptors is not a big performance issue (it does
incur a hidden memory cost inside the kernel which is not reflected in
the figures above).
"POE", regardless of underlying event loop (whether using its pure perl
select-based backend or the Event module, the POE-EV backend couldn't
be tested because it wasn't working) shows abysmal performance and
memory usage with AnyEvent: Watchers use almost 30 times as much memory
as EV watchers, and 10 times as much memory as Event (the high memory
requirements are caused by requiring a session for each watcher).
Watcher invocation speed is almost 900 times slower than with
AnyEvent's pure perl implementation.
The design of the POE adaptor class in AnyEvent can not really account
for the performance issues, though, as session creation overhead is
small compared to execution of the state machine, which is coded pretty
optimally within AnyEvent::Impl::POE (and while everybody agrees that
using multiple sessions is not a good approach, especially regarding
memory usage, even the author of POE could not come up with a faster
design).
Summary
· 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.
BENCHMARKING THE LARGE SERVER CASE
This benchmark actually benchmarks the event loop itself. It works by
creating a number of "servers": each server consists of a socket pair,
a timeout watcher that gets reset on activity (but never fires), and an
I/O watcher waiting for input on one side of the socket. Each time the
socket watcher reads a byte it will write that byte to a random other
"server".
The effect is that there will be a lot of I/O watchers, only part of
which are active at any one point (so there is a constant number of
active fds for each loop iteration, but which fds these are is random).
The timeout is reset each time something is read because that reflects
how most timeouts work (and puts extra pressure on the event loops).
In this benchmark, we use 10000 socket pairs (20000 sockets), of which
100 (1%) are active. This mirrors the activity of large servers with
many connections, most of which are idle at any one point in time.
Source code for this benchmark is found as eg/bench2 in the AnyEvent
distribution. It uses the AE interface, which makes a real difference
for the EV and Perl backends only.
Explanation of the columns
sockets is the number of sockets, and twice the number of "servers" (as
each server has a read and write socket end).
create is the time it takes to create a socket pair (which is
nontrivial) and two watchers: an I/O watcher and a timeout watcher.
request, the most important value, is the time it takes to handle a
single "request", that is, reading the token from the pipe and
forwarding it to another server. This includes deleting the old timeout
and creating a new one that moves the timeout into the future.
Results
name sockets create request
EV 20000 62.66 7.99
Perl 20000 68.32 32.64
IOAsync 20000 174.06 101.15 epoll
IOAsync 20000 174.67 610.84 poll
Event 20000 202.69 242.91
Glib 20000 557.01 1689.52
POE 20000 341.54 12086.32 uses POE::Loop::Event
Discussion
This benchmark does measure scalability and overall performance of the
particular event loop.
EV is again fastest. Since it is using epoll on my system, the setup
time is relatively high, though.
Perl surprisingly comes second. It is much faster than the C-based
event loops Event and Glib.
IO::Async performs very well when using its epoll backend, and still
quite good compared to Glib when using its pure perl backend.
Event suffers from high setup time as well (look at its code and you
will understand why). Callback invocation also has a high overhead
compared to the "$_->() for .."-style loop that the Perl event loop
uses. Event uses select or poll in basically all documented
configurations.
Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It
clearly fails to perform with many filehandles or in busy servers.
POE is still completely out of the picture, taking over 1000 times as
long as EV, and over 100 times as long as the Perl implementation, even
though it uses a C-based event loop in this case.
Summary
· The pure perl implementation performs extremely well.
· Avoid Glib or POE in large projects where performance matters.
BENCHMARKING SMALL SERVERS
While event loops should scale (and select-based ones do not...) even
to large servers, most programs we (or I :) actually write have only a
few I/O watchers.
In this benchmark, I use the same benchmark program as in the large
server case, but it uses only eight "servers", of which three are
active at any one time. This should reflect performance for a small
server relatively well.
The columns are identical to the previous table.
Results
name sockets create request
EV 16 20.00 6.54
Perl 16 25.75 12.62
Event 16 81.27 35.86
Glib 16 32.63 15.48
POE 16 261.87 276.28 uses POE::Loop::Event
Discussion
The benchmark tries to test the performance of a typical small server.
While knowing how various event loops perform is interesting, keep in
mind that their overhead in this case is usually not as important, due
to the small absolute number of watchers (that is, you need efficiency
and speed most when you have lots of watchers, not when you only have a
few of them).
EV is again fastest.
Perl again comes second. It is noticeably faster than the C-based event
loops Event and Glib, although the difference is too small to really
matter.
POE also performs much better in this case, but is is still far behind
the others.
Summary
· 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.
SIGNALSAnyEvent 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 it's 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 on
it's own. The pure-perl event loop (AnyEvent::Impl::Perl) will
additionally use it to try to use a monotonic clock for timing
stability.
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.
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 rprocess is usually
preferred, because it is much easier and cleaner, at the expense of
having to have another binary.
SECURITY CONSIDERATIONSAnyEvent 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
Utility functions: AnyEvent::Util.
Event modules: EV, EV::Glib, Glib::EV, Event, Glib::Event, Glib, Tk,
Event::Lib, Qt, POE.
Implementations: AnyEvent::Impl::EV, AnyEvent::Impl::Event,
AnyEvent::Impl::Glib, AnyEvent::Impl::Tk, AnyEvent::Impl::Perl,
AnyEvent::Impl::EventLib, AnyEvent::Impl::Qt, AnyEvent::Impl::POE,
AnyEvent::Impl::IOAsync, Anyevent::Impl::Irssi.
Non-blocking file handles, sockets, TCP clients and servers:
AnyEvent::Handle, AnyEvent::Socket, AnyEvent::TLS.
Asynchronous DNS: AnyEvent::DNS.
Coroutine support: Coro, Coro::AnyEvent, Coro::EV, Coro::Event,
Nontrivial usage examples: AnyEvent::GPSD, AnyEvent::XMPP,
AnyEvent::HTTP.
AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/
perl v5.14.2 2010-06-08 AnyEvent(3)
