AnyEvent
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NAME
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.
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
An FAQ document is available as AnyEvent::FAQ.
There also is a mailinglist for discussing all things AnyEvent, and an
IRC channel, too.
See the AnyEvent project page at the Schmorpforge Ta-Sa Software
Repository, at <http://anyevent.schmorp.de>, for more info.
WHY YOU SHOULD USE THIS MODULE (OR NOT)
Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
nowadays. So what is different about AnyEvent?
Executive Summary: AnyEvent is *compatible*, AnyEvent is *free of
policy* and AnyEvent is *small and efficient*.
First and foremost, *AnyEvent is not an event model* itself, it only
interfaces to whatever event model the main program happens to use, in a
pragmatic way. For event models and certain classes of immortals alike,
the statement "there can only be one" is a bitter reality: In general,
only one event loop can be active at the same time in a process.
AnyEvent cannot change this, but it can hide the differences between
those event loops.
The goal of AnyEvent is to offer module authors the ability to do event
programming (waiting for I/O or timer events) without subscribing to a
religion, a way of living, and most importantly: without forcing your
module users into the same thing by forcing them to use the same event
model you use.
For modules like POE or IO::Async (which is a total misnomer as it is
actually doing all I/O *synchronously*...), using them in your module is
like joining a cult: After you join, you are dependent on them and you
cannot use anything else, as they are simply incompatible to everything
that isn't them. What's worse, all the potential users of your module
are *also* forced to use the same event loop you use.
AnyEvent is different: AnyEvent + POE works fine. AnyEvent + Glib works
fine. AnyEvent + Tk works fine etc. etc. but none of these work together
with the rest: POE + EV? No go. Tk + Event? No go. Again: if your module
uses one of those, every user of your module has to use it, too. But if
your module uses AnyEvent, it works transparently with all event models
it supports (including stuff like IO::Async, as long as those use one of
the supported event loops. It is easy to add new event loops to
AnyEvent, too, so it is future-proof).
In addition to being free of having to use *the one and only true event
model*, AnyEvent also is free of bloat and policy: with POE or similar
modules, you get an enormous amount of code and strict rules you have to
follow. AnyEvent, on the other hand, is lean and to the point, by only
offering the functionality that is necessary, in as thin as a wrapper as
technically possible.
Of course, AnyEvent comes with a big (and fully optional!) toolbox of
useful functionality, such as an asynchronous DNS resolver, 100%
non-blocking connects (even with TLS/SSL, IPv6 and on broken platforms
such as Windows) and lots of real-world knowledge and workarounds for
platform bugs and differences.
Now, if you *do want* lots of policy (this can arguably be somewhat
useful) and you want to force your users to use the one and only event
model, you should *not* use this module.
DESCRIPTION
AnyEvent provides a uniform interface to various event loops. This
allows module authors to use event loop functionality without forcing
module users to use a specific event loop implementation (since more
than one event loop cannot coexist peacefully).
The interface itself is vaguely similar, but not identical to the Event
module.
During the first call of any watcher-creation method, the module tries
to detect the currently loaded event loop by probing whether one of the
following modules is already loaded: EV, AnyEvent::Loop, Event, Glib,
Tk, Event::Lib, Qt, POE. The first one found is used. If none are
detected, the module tries to load the first four modules in the order
given; but note that if EV is not available, the pure-perl
AnyEvent::Loop should always work, so the other two are not normally
tried.
Because AnyEvent first checks for modules that are already loaded,
loading an event model explicitly before first using AnyEvent will
likely make that model the default. For example:
use Tk;
use AnyEvent;
# .. AnyEvent will likely default to Tk
The *likely* means that, if any module loads another event model and
starts using it, all bets are off - this case should be very rare
though, as very few modules hardcode event loops without announcing this
very loudly.
The pure-perl implementation of AnyEvent is called "AnyEvent::Loop".
Like other event modules you can load it explicitly and enjoy the high
availability of that event loop :)
WATCHERS
AnyEvent has the central concept of a *watcher*, which is an object that
stores relevant data for each kind of event you are waiting for, such as
the callback to call, the file handle to watch, etc.
These watchers are normal Perl objects with normal Perl lifetime. After
creating a watcher it will immediately "watch" for events and invoke the
callback when the event occurs (of course, only when the event model is
in control).
Note that callbacks must not permanently change global variables
potentially in use by the event loop (such as $_ or $[) and that
callbacks must not "die". The former is good programming practice in
Perl and the latter stems from the fact that exception handling differs
widely between event loops.
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.
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 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;
});
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 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.
SIGNAL WATCHERS
$w = AnyEvent->signal (signal => <uppercase_signal_name>, cb => <callback>);
You can watch for signals using a signal watcher, "signal" is the signal
*name* in uppercase and without any "SIG" prefix, "cb" is the Perl
callback to be invoked whenever a signal occurs.
Although the callback might get passed parameters, their value and
presence is undefined and you cannot rely on them. Portable AnyEvent
callbacks cannot use arguments passed to signal watcher callbacks.
Multiple signal occurrences can be clumped together into one callback
invocation, and callback invocation will be synchronous. Synchronous
means that it might take a while until the signal gets handled by the
process, but it is guaranteed not to interrupt any other callbacks.
The main advantage of using these watchers is that you can share a
signal between multiple watchers, and AnyEvent will ensure that signals
will not interrupt your program at bad times.
This watcher might use %SIG (depending on the event loop used), so
programs overwriting those signals directly will likely not work
correctly.
Example: exit on SIGINT
my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });
Restart Behaviour
While restart behaviour is up to the event loop implementation, most
will not restart syscalls (that includes Async::Interrupt and AnyEvent's
pure perl implementation).
Safe/Unsafe Signals
Perl signals can be either "safe" (synchronous to opcode handling) or
"unsafe" (asynchronous) - the former might delay signal delivery
indefinitely, the latter might corrupt your memory.
AnyEvent signal handlers are, in addition, synchronous to the event
loop, i.e. they will not interrupt your running perl program but will
only be called as part of the normal event handling (just like timer,
I/O etc. callbacks, too).
Signal Races, Delays and Workarounds
Many event loops (e.g. Glib, Tk, Qt, IO::Async) do not support attaching
callbacks to signals in a generic way, which is a pity, as you cannot do
race-free signal handling in perl, requiring C libraries for this.
AnyEvent will try to do its best, which means in some cases, signals
will be delayed. The maximum time a signal might be delayed is 10
seconds by default, but can be overriden via
$ENV{PERL_ANYEVENT_MAX_SIGNAL_LATENCY} or $AnyEvent::MAX_SIGNAL_LATENCY
- see the "ENVIRONMENT VARIABLES" section for details.
All these problems can be avoided by installing the optional
Async::Interrupt module, which works with most event loops. It will not
work with inherently broken event loops such as Event or Event::Lib (and
not with POE currently). For those, you just have to suffer the delays.
CHILD PROCESS WATCHERS
$w = AnyEvent->child (pid => <process id>, cb => <callback>);
You can also watch for a child process exit and catch its exit status.
The child process is specified by the "pid" argument (on some backends,
using 0 watches for any child process exit, on others this will croak).
The watcher will be triggered only when the child process has finished
and an exit status is available, not on any trace events
(stopped/continued).
The callback will be called with the pid and exit status (as returned by
waitpid), so unlike other watcher types, you *can* rely on child watcher
callback arguments.
This watcher type works by installing a signal handler for "SIGCHLD",
and since it cannot be shared, nothing else should use SIGCHLD or reap
random child processes (waiting for specific child processes, e.g.
inside "system", is just fine).
There is a slight catch to child watchers, however: you usually start
them *after* the child process was created, and this means the process
could have exited already (and no SIGCHLD will be sent anymore).
Not all event models handle this correctly (neither POE nor IO::Async
do, see their AnyEvent::Impl manpages for details), but even for event
models that *do* handle this correctly, they usually need to be loaded
before the process exits (i.e. before you fork in the first place).
AnyEvent's pure perl event loop handles all cases correctly regardless
of when you start the watcher.
This means you cannot create a child watcher as the very first thing in
an AnyEvent program, you *have* to create at least one watcher before
you "fork" the child (alternatively, you can call "AnyEvent::detect").
As most event loops do not support waiting for child events, they will
be emulated by AnyEvent in most cases, in which case the latency and
race problems mentioned in the description of signal watchers apply.
Example: fork a process and wait for it
my $done = AnyEvent->condvar;
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
more.
AnyEvent::Handle (part of the AnyEvent distribution)
Provide read and write buffers, manages watchers for reads and
writes, supports raw and formatted I/O, I/O queued and fully
transparent and non-blocking SSL/TLS (via AnyEvent::TLS).
AnyEvent::DNS (part of the AnyEvent distribution)
Provides rich asynchronous DNS resolver capabilities.
AnyEvent::HTTP, AnyEvent::IRC, AnyEvent::XMPP, AnyEvent::GPSD,
AnyEvent::IGS, AnyEvent::FCP
Implement event-based interfaces to the protocols of the same name
(for the curious, IGS is the International Go Server and FCP is the
Freenet Client Protocol).
AnyEvent::AIO (part of the AnyEvent distribution)
Truly asynchronous (as opposed to non-blocking) I/O, should be in
the toolbox of every event programmer. AnyEvent::AIO transparently
fuses IO::AIO and AnyEvent together, giving AnyEvent access to
event-based file I/O, and much more.
AnyEvent::Fork, AnyEvent::Fork::RPC, AnyEvent::Fork::Pool,
AnyEvent::Fork::Remote
These let you safely fork new subprocesses, either locally or
remotely (e.g.v ia ssh), using some RPC protocol or not, without the
limitations normally imposed by fork (AnyEvent works fine for
example). Dynamically-resized worker pools are obviously included as
well.
And they are quite tiny and fast as well - "abusing" AnyEvent::Fork
just to exec external programs can easily beat using "fork" and
"exec" (or even "system") in most programs.
AnyEvent::Filesys::Notify
AnyEvent is good for non-blocking stuff, but it can't detect file or
path changes (e.g. "watch this directory for new files", "watch this
file for changes"). The AnyEvent::Filesys::Notify module promises to
do just that in a portbale fashion, supporting inotify on GNU/Linux
and some weird, without doubt broken, stuff on OS X to monitor
files. It can fall back to blocking scans at regular intervals
transparently on other platforms, so it's about as portable as it
gets.
(I haven't used it myself, but it seems the biggest problem with it
is it quite bad performance).
AnyEvent::DBI
Executes DBI requests asynchronously in a proxy process for you,
notifying you in an event-based way when the operation is finished.
AnyEvent::FastPing
The fastest ping in the west.
Coro
Has special support for AnyEvent via Coro::AnyEvent, which allows
you to simply invert the flow control - don't call us, we will call
you:
async {
Coro::AnyEvent::sleep 5; # creates a 5s timer and waits for it
print "5 seconds later!\n";
Coro::AnyEvent::readable *STDIN; # uses an I/O watcher
my $line = <STDIN>; # works for ttys
AnyEvent::HTTP::http_get "url", Coro::rouse_cb;
my ($body, $hdr) = Coro::rouse_wait;
};
SIMPLIFIED AE API
Starting with version 5.0, AnyEvent officially supports a second, much
simpler, API that is designed to reduce the calling, typing and memory
overhead by using function call syntax and a fixed number of parameters.
See the AE manpage for details.
ERROR AND EXCEPTION HANDLING
In general, AnyEvent does not do any error handling - it relies on the
caller to do that if required. The AnyEvent::Strict module (see also the
"PERL_ANYEVENT_STRICT" environment variable, below) provides strict
checking of all AnyEvent methods, however, which is highly useful during
development.
As for exception handling (i.e. runtime errors and exceptions thrown
while executing a callback), this is not only highly event-loop
specific, but also not in any way wrapped by this module, as this is the
job of the main program.
The pure perl event loop simply re-throws the exception (usually within
"condvar->recv"), the Event and EV modules call "$Event/EV::DIED->()",
Glib uses "install_exception_handler" and so on.
ENVIRONMENT VARIABLES
AnyEvent supports a number of environment variables that tune the
runtime behaviour. They are usually evaluated when AnyEvent is loaded,
initialised, or a submodule that uses them is loaded. Many of them also
cause AnyEvent to load additional modules - for example,
"PERL_ANYEVENT_DEBUG_WRAP" causes the AnyEvent::Debug module to be
loaded.
All the environment variables documented here start with
"PERL_ANYEVENT_", which is what AnyEvent considers its own namespace.
Other modules are encouraged (but by no means required) to use
"PERL_ANYEVENT_SUBMODULE" if they have registered the
AnyEvent::Submodule namespace on CPAN, for any submodule. For example,
AnyEvent::HTTP could be expected to use "PERL_ANYEVENT_HTTP_PROXY" (it
should not access env variables starting with "AE_", see below).
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).
"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.
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 });
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;
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.
The last AnyEvent benchmark additionally uses AnyEvent::Handle, which
offers similar expressive power as POE and IO::Lambda, using
conventional Perl syntax. This means that both the echo server and the
client are 100% non-blocking, further placing it at a disadvantage.
As you can see, the AnyEvent + EV combination even beats the
hand-optimised "raw sockets benchmark", while AnyEvent + its pure perl
backend easily beats IO::Lambda and POE.
And even the 100% non-blocking version written using the high-level (and
slow :) AnyEvent::Handle abstraction beats both POE and IO::Lambda
higher level ("unoptimised") abstractions by a large margin, even though
it does all of DNS, tcp-connect and socket I/O in a non-blocking way.
The two AnyEvent benchmarks programs can be found as eg/ae0.pl and
eg/ae2.pl in the AnyEvent distribution, the remaining benchmarks are
part of the IO::Lambda distribution and were used without any changes.
SIGNALS
AnyEvent currently installs handlers for these signals:
SIGCHLD
A handler for "SIGCHLD" is installed by AnyEvent's child watcher
emulation for event loops that do not support them natively. Also,
some event loops install a similar handler.
Additionally, when AnyEvent is loaded and SIGCHLD is set to IGNORE,
then AnyEvent will reset it to default, to avoid losing child exit
statuses.
SIGPIPE
A no-op handler is installed for "SIGPIPE" when $SIG{PIPE} is
"undef" when AnyEvent gets loaded.
The rationale for this is that AnyEvent users usually do not really
depend on SIGPIPE delivery (which is purely an optimisation for
shell use, or badly-written programs), but "SIGPIPE" can cause
spurious and rare program exits as a lot of people do not expect
"SIGPIPE" when writing to some random socket.
The rationale for installing a no-op handler as opposed to ignoring
it is that this way, the handler will be restored to defaults on
exec.
Feel free to install your own handler, or reset it to defaults.
RECOMMENDED/OPTIONAL MODULES
One of AnyEvent's main goals is to be 100% Pure-Perl(tm): only perl (and
its built-in modules) are required to use it.
That does not mean that AnyEvent won't take advantage of some additional
modules if they are installed.
This section explains which additional modules will be used, and how
they affect AnyEvent's operation.
Async::Interrupt
This slightly arcane module is used to implement fast signal
handling: To my knowledge, there is no way to do completely
race-free and quick signal handling in pure perl. To ensure that
signals still get delivered, AnyEvent will start an interval timer
to wake up perl (and catch the signals) with some delay (default is
10 seconds, look for $AnyEvent::MAX_SIGNAL_LATENCY).
If this module is available, then it will be used to implement
signal catching, which means that signals will not be delayed, and
the event loop will not be interrupted regularly, which is more
efficient (and good for battery life on laptops).
This affects not just the pure-perl event loop, but also other event
loops that have no signal handling on their own (e.g. Glib, Tk, Qt).
Some event loops (POE, Event, Event::Lib) offer signal watchers
natively, and either employ their own workarounds (POE) or use
AnyEvent's workaround (using $AnyEvent::MAX_SIGNAL_LATENCY).
Installing Async::Interrupt does nothing for those backends.
EV This module isn't really "optional", as it is simply one of the
backend event loops that AnyEvent can use. However, it is simply the
best event loop available in terms of features, speed and stability:
It supports the AnyEvent API optimally, implements all the watcher
types in XS, does automatic timer adjustments even when no monotonic
clock is available, can take avdantage of advanced kernel interfaces
such as "epoll" and "kqueue", and is the fastest backend *by far*.
You can even embed Glib/Gtk2 in it (or vice versa, see EV::Glib and
Glib::EV).
If you only use backends that rely on another event loop (e.g.
"Tk"), then this module will do nothing for you.
Guard
The guard module, when used, will be used to implement
"AnyEvent::Util::guard". This speeds up guards considerably (and
uses a lot less memory), but otherwise doesn't affect guard
operation much. It is purely used for performance.
JSON and JSON::XS
One of these modules is required when you want to read or write JSON
data via AnyEvent::Handle. JSON is also written in pure-perl, but
can take advantage of the ultra-high-speed JSON::XS module when it
is installed.
Net::SSLeay
Implementing TLS/SSL in Perl is certainly interesting, but not very
worthwhile: If this module is installed, then AnyEvent::Handle (with
the help of AnyEvent::TLS), gains the ability to do TLS/SSL.
Time::HiRes
This module is part of perl since release 5.008. It will be used
when the chosen event library does not come with a timing source of
its own. The pure-perl event loop (AnyEvent::Loop) will additionally
load it to try to use a monotonic clock for timing stability.
AnyEvent::AIO (and IO::AIO)
The default implementation of AnyEvent::IO is to do I/O
synchronously, stopping programs while they access the disk, which
is fine for a lot of programs.
Installing AnyEvent::AIO (and its IO::AIO dependency) makes it
switch to a true asynchronous implementation, so event processing
can continue even while waiting for disk I/O.
FORK
Most event libraries are not fork-safe. The ones who are usually are
because they rely on inefficient but fork-safe "select" or "poll" calls
- higher performance APIs such as BSD's kqueue or the dreaded Linux
epoll are usually badly thought-out hacks that are incompatible with
fork in one way or another. Only EV is fully fork-aware and ensures that
you continue event-processing in both parent and child (or both, if you
know what you are doing).
This means that, in general, you cannot fork and do event processing in
the child if the event library was initialised before the fork (which
usually happens when the first AnyEvent watcher is created, or the
library is loaded).
If you have to fork, you must either do so *before* creating your first
watcher OR you must not use AnyEvent at all in the child OR you must do
something completely out of the scope of AnyEvent (see below).
The problem of doing event processing in the parent *and* the child is
much more complicated: even for backends that *are* fork-aware or
fork-safe, their behaviour is not usually what you want: fork clones all
watchers, that means all timers, I/O watchers etc. are active in both
parent and child, which is almost never what you want. Using "exec" to
start worker children from some kind of manage prrocess is usually
preferred, because it is much easier and cleaner, at the expense of
having to have another binary.
In addition to logical problems with fork, there are also implementation
problems. For example, on POSIX systems, you cannot fork at all in Perl
code if a thread (I am talking of pthreads here) was ever created in the
process, and this is just the tip of the iceberg. In general, using fork
from Perl is difficult, and attempting to use fork without an exec to
implement some kind of parallel processing is almost certainly doomed.
To safely fork and exec, you should use a module such as Proc::FastSpawn
that let's you safely fork and exec new processes.
If you want to do multiprocessing using processes, you can look at the
AnyEvent::Fork module (and some related modules such as
AnyEvent::Fork::RPC, AnyEvent::Fork::Pool and AnyEvent::Fork::Remote).
This module allows you to safely create subprocesses without any
limitations - you can use X11 toolkits or AnyEvent in the children
created by AnyEvent::Fork safely and without any special precautions.
SECURITY CONSIDERATIONS
AnyEvent can be forced to load any event model via
$ENV{PERL_ANYEVENT_MODEL}. While this cannot (to my knowledge) be used
to execute arbitrary code or directly gain access, it can easily be used
to make the program hang or malfunction in subtle ways, as AnyEvent
watchers will not be active when the program uses a different event
model than specified in the variable.
You can make AnyEvent completely ignore this variable by deleting it
before the first watcher gets created, e.g. with a "BEGIN" block:
BEGIN { delete $ENV{PERL_ANYEVENT_MODEL} }
use AnyEvent;
Similar considerations apply to $ENV{PERL_ANYEVENT_VERBOSE}, as that can
be used to probe what backend is used and gain other information (which
is probably even less useful to an attacker than PERL_ANYEVENT_MODEL),
and $ENV{PERL_ANYEVENT_STRICT}.
Note that AnyEvent will remove *all* environment variables starting with
"PERL_ANYEVENT_" from %ENV when it is loaded while taint mode is
enabled.
BUGS
Perl 5.8 has numerous memleaks that sometimes hit this module and are
hard to work around. If you suffer from memleaks, first upgrade to Perl
5.10 and check wether the leaks still show up. (Perl 5.10.0 has other
annoying memleaks, such as leaking on "map" and "grep" but it is usually
not as pronounced).
SEE ALSO
Tutorial/Introduction: AnyEvent::Intro.
FAQ: AnyEvent::FAQ.
Utility functions: AnyEvent::Util (misc. grab-bag), AnyEvent::Log
(simply logging).
( run in 1.522 second using v1.01-cache-2.11-cpan-9581c071862 )