AnyEvent - the DBI of event loop programming
EV, Event, Glib, Tk, UV, Perl, Event::Lib, Irssi, rxvt-unicode, IO::Async, Qt,
FLTK and POE are various supported event loops/environments.
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 });
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.
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.
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.
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 :)
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.
To disable a 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.
One 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.
$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 readiness 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;
});
$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 only once. 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 not specified at all.
The callback will be rescheduled before invoking the callback, but no attempt is
made 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 of
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 its 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 a 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 callbacks 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::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.
$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 overridden 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.
$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
my $done = AnyEvent->condvar;
# this forks and immediately calls exit in the child. this
# normally has all sorts of bad consequences for your parent,
# so take this as an example only. always fork and exec,
# or call POSIX::_exit, in real code.
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;
$w = AnyEvent->idle (cb => <callback>);
This will 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;
}
});
});
$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])
- $cv->end
- These two methods can be used to combine many
transactions/events into one. For example, a function that pings many
hosts in parallel might want to use a condition variable for the whole
process.
Every call to "->begin" will increment a counter, and every
call to "->end" will decrement it. If the counter reaches 0
in "->end", the (last) callback passed to "begin"
will be executed, passing the condvar as first argument. That callback is
supposed to call "->send", but that is not required.
If no group callback was set, "send" will be called without any
arguments.
You can think of "$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 passed back, and the number of tasks that are begun 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;
...
my $results = $cv->recv;
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 zero) 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 $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 requirement can be dropped by relying on
Coro::AnyEvent , which allows you to do a blocking "->recv"
from any thread that doesn't run the event loop itself. Coro::AnyEvent is
loaded automatically when Coro is used with AnyEvent, so code does not
need to do anything special to take advantage of that: any code that would
normally block your program because it calls "recv", be executed
in an "async" thread instead without blocking other threads.
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
(or "undef" if not) and optionally replaces it before doing so.
The callback will be called when the condition becomes "true",
i.e. when "send" or "croak" are called, with the only
argument being the condition variable itself. If the condition is already
true, the callback is called immediately when it is set. Calling
"recv" inside the callback or at any later time is guaranteed
not to block.
Additionally, when the callback is invoked, it is also removed from the
condvar (reset to "undef"), so the condvar does not keep a
reference to the callback after invocation.
The following backend classes are part of the AnyEvent distribution (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 AnyEvent::Loop, fast and portable.
- Backends that are transparently being picked up when they
are used.
- These will be used if they are already 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::UV based on UV, innovated square wheels.
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.
AnyEvent::Impl::IOAsync based on IO::Async.
AnyEvent::Impl::Cocoa based on Cocoa::EventLoop.
AnyEvent::Impl::FLTK based on FLTK (fltk 2 binding).
- 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.
- 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.
- Known event loops outside the AnyEvent distribution
- The following event loops or programs support AnyEvent by
providing their own AnyEvent backend. They will be picked up
automatically.
urxvt::anyevent available to rxvt-unicode extensions
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. during initialisation of your module.
The effect of calling this function is as if a watcher had been created
(specifically, actions that happen "when the first watcher is
created" happen when calling detetc as well).
If you need to do some initialisation before AnyEvent watchers are created,
use "post_detect".
- $guard = AnyEvent::post_detect { BLOCK }
- Arranges for the code block to be executed as soon as the
event model is autodetected (or immediately if that has already happened).
The block will be executed after the actual backend has been detected
($AnyEvent::MODEL is set), so it is possible to do some initialisation
only when AnyEvent is actually initialised - see the sources of
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,
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 enormously.
- AnyEvent::fh_block $filehandle
- AnyEvent::fh_unblock $filehandle
- Sets blocking or non-blocking behaviour for the given
filehandle.
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 may call
"->recv" freely, as the user of your module knows what she is
doing. Always).
There will always be a single main program - the only place that should dictate
which event model to use.
If 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.
Sometimes (often for short test scripts, or even standalone programs who only
want to use AnyEvent), you do not want to run a specific event loop.
In that case, you can use a condition variable like this:
AnyEvent->condvar->recv;
This has the effect of entering the event loop and looping forever.
Note that usually your program has some exit condition, in which case it is
better to use the "traditional" approach of storing a condition
variable somewhere, waiting for it, and sending it when the program should
exit cleanly.
The following is a non-exhaustive list of additional modules that use AnyEvent
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, $hdr) = Coro::rouse_wait;
};
Starting with version 5.0, AnyEvent officially supports a second, much simpler,
API that is designed to reduce the calling, typing and memory overhead by
using function call syntax and a fixed number of parameters.
See the AE manpage for details.
In general, AnyEvent does not do any error handling - it relies on the caller to
do that if required. The AnyEvent::Strict module (see also the
"PERL_ANYEVENT_STRICT" environment variable, below) provides strict
checking of all AnyEvent methods, however, which is highly useful during
development.
As for exception handling (i.e. runtime errors and exceptions thrown while
executing a callback), this is not only highly event-loop specific, but also
not in any way wrapped by this module, as this is the job of the main program.
The pure perl event loop simply re-throws the exception (usually within
"condvar->recv"), the Event and EV modules call
"$Event/EV::DIED->()", Glib uses
"install_exception_handler" and so on.
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).
All variables can also be set via the "AE_" prefix, that is, instead
of setting "PERL_ANYEVENT_VERBOSE" you can also set
"AE_VERBOSE". In case there is a clash btween anyevent and another
program that uses "AE_something" you can set the corresponding
"PERL_ANYEVENT_something" variable to the empty string, as those
variables take precedence.
When AnyEvent is first loaded, it copies all "AE_xxx" env variables to
their "PERL_ANYEVENT_xxx" counterpart unless that variable already
exists. If taint mode is on, then AnyEvent will remove
all environment
variables starting with "PERL_ANYEVENT_" from %ENV (or replace them
with "undef" or the empty string, if the corresaponding
"AE_" variable is set).
The exact algorithm is currently:
1. if taint mode enabled, delete all PERL_ANYEVENT_xyz variables from %ENV
2. copy over AE_xyz to PERL_ANYEVENT_xyz unless the latter already exists
3. if taint mode enabled, set all PERL_ANYEVENT_xyz variables to undef.
This ensures that child processes will not see the "AE_" variables.
The following environment variables are currently known to AnyEvent:
- "PERL_ANYEVENT_VERBOSE"
- By default, AnyEvent will log messages with loglevel 4
("error") or higher (see AnyEvent::Log). You can set this
environment variable to a numerical loglevel to make AnyEvent more (or
less) talkative.
If you want to do more than just set the global logging level you should
have a look at "PERL_ANYEVENT_LOG", which allows much more
complex specifications.
When set to 0 ("off"), then no messages whatsoever will be logged
with everything else at defaults.
When set to 5 or higher ("warn"), AnyEvent warns about unexpected
conditions, such as not being able to load the event model specified by
"PERL_ANYEVENT_MODEL", or a guard callback throwing an exception
- this is the minimum recommended level for use during development.
When set to 7 or higher (info), AnyEvent reports which event model it
chooses.
When set to 8 or higher (debug), then AnyEvent will report extra information
on which optional modules it loads and how it implements certain
features.
- "PERL_ANYEVENT_LOG"
- Accepts rather complex logging specifications. For example,
you could log all "debug" messages of some module to stderr,
warnings and above to stderr, and errors and above to syslog, with:
PERL_ANYEVENT_LOG=Some::Module=debug,+log:filter=warn,+%syslog:%syslog=error,syslog
For the rather extensive details, see AnyEvent::Log.
This variable is evaluated when AnyEvent (or AnyEvent::Log) is loaded, so
will take effect even before AnyEvent has initialised itself.
Note that specifying this environment variable causes the AnyEvent::Log
module to be loaded, while "PERL_ANYEVENT_VERBOSE" does not, so
only using the latter saves a few hundred kB of memory unless a module
explicitly needs the extra features of AnyEvent::Log.
- "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 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 occurrence 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 watchers).
Lower values increase CPU (and energy) usage, higher values can introduce
long delays when reaping children or waiting for signals.
The AnyEvent::Async module, if available, will be used to avoid this polling
(with most event loops).
- "PERL_ANYEVENT_RESOLV_CONF"
- The absolute path to a resolv.conf-style file to use
instead of /etc/resolv.conf (or the OS-specific configuration) in
the default resolver, or the empty string to select the default
configuration.
- "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 are nonempty, 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.
This is an advanced topic that you do not normally need to use AnyEvent in a
module. This section is only of use to event loop authors who want to provide
AnyEvent compatibility.
If you need to support another event library which isn't directly supported by
AnyEvent, you can supply your own interface to it by pushing, before the first
watcher gets created, the package name of the event module and the package
name of the interface to use onto @AnyEvent::REGISTRY. You can do that before
and even without loading AnyEvent, so it is reasonably cheap.
Example:
push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];
This tells AnyEvent to (literally) use the "urxvt::anyevent::"
package/class when it finds the "urxvt" package/module is already
loaded.
When AnyEvent is loaded and asked to find a suitable event model, it will first
check for the presence of urxvt by trying to "use" the
"urxvt::anyevent" module.
The class should provide implementations for all watcher types. See
AnyEvent::Impl::EV (source code), AnyEvent::Impl::Glib (Source code) and so on
for actual examples. Use "perldoc -m AnyEvent::Impl::Glib" to see
the sources.
If you don't provide "signal" and "child" watchers than
AnyEvent will provide suitable (hopefully) replacements.
The above example isn't fictitious, the
rxvt-unicode (a.k.a. urxvt)
terminal emulator uses the above line as-is. An interface isn't included in
AnyEvent because it doesn't make sense outside the embedded interpreter inside
rxvt-unicode, and it is updated and maintained as part of the
rxvt-unicode distribution.
rxvt-unicode also cheats a bit by not providing blocking access to
condition variables: code blocking while waiting for a condition will
"die". This still works with most modules/usages, and blocking calls
must not be done in an interactive application, so it makes sense.
The following 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
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::run;
3b. The module user could use AnyEvent, too:
use AnyEvent;
my $quit = AnyEvent->condvar;
$fcp->txn_client_get ($url)->cb (sub {
...
$quit->send;
});
$quit->recv;
To give you an idea of the performance and overheads that AnyEvent adds over the
event loops themselves and to give you an impression of the speed of various
event loops I prepared some benchmarks.
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 does AnyEvent add significant overhead.
- •
- You should avoid POE like the plague if you want
performance or reasonable memory usage.
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.
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.
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.
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.
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.
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
lets 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.
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.
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
whether 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).
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.
Marc Lehmann <[email protected]>
http://anyevent.schmorp.de