AnyEvent
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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;
# 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;
IDLE WATCHERS
$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;
}
});
});
CONDITION VARIABLES
$cv = AnyEvent->condvar;
$cv->send (<list>);
my @res = $cv->recv;
If you are familiar with some event loops you will know that all of them
require you to run some blocking "loop", "run" or similar function that
will actively watch for new events and call your callbacks.
AnyEvent is slightly different: it expects somebody else to run the
event loop and will only block when necessary (usually when told by the
user).
The tool to do that is called a "condition variable", so called because
they represent a condition that must become true.
Now is probably a good time to look at the examples further below.
Condition variables can be created by calling the "AnyEvent->condvar"
method, usually without arguments. The only argument pair allowed is
"cb", which specifies a callback to be called when the condition
variable becomes true, with the condition variable as the first argument
(but not the results).
After creation, the condition variable is "false" until it becomes
"true" by calling the "send" method (or calling the condition variable
as if it were a callback, read about the caveats in the description for
the "->send" method).
Since condition variables are the most complex part of the AnyEvent API,
here are some different mental models of what they are - pick the ones
you can connect to:
* Condition variables are like callbacks - you can call them (and pass
them instead of callbacks). Unlike callbacks however, you can also
wait for them to be called.
* Condition variables are signals - one side can emit or send them,
the other side can wait for them, or install a handler that is
called when the signal fires.
* Condition variables are like "Merge Points" - points in your program
where you merge multiple independent results/control flows into one.
* Condition variables represent a transaction - functions that start
some kind of transaction can return them, leaving the caller the
choice between waiting in a blocking fashion, or setting a callback.
* Condition variables represent future values, or promises to deliver
some result, long before the result is available.
Condition variables are very useful to signal that something has
finished, for example, if you write a module that does asynchronous http
requests, then a condition variable would be the ideal candidate to
signal the availability of results. The user can either act when the
callback is called or can synchronously "->recv" for the results.
You can also use them to simulate traditional event loops - for example,
you can block your main program until an event occurs - for example, you
could "->recv" in your main program until the user clicks the Quit
button of your app, which would "->send" the "quit" event.
Note that condition variables recurse into the event loop - if you have
two pieces of code that call "->recv" in a round-robin fashion, you
lose. Therefore, condition variables are good to export to your caller,
but you should avoid making a blocking wait yourself, at least in
callbacks, as this asks for trouble.
Condition variables are represented by hash refs in perl, and the keys
used by AnyEvent itself are all named "_ae_XXX" to make subclassing easy
(it is often useful to build your own transaction class on top of
AnyEvent). To subclass, use "AnyEvent::CondVar" as base class and call
its "new" method in your own "new" method.
There are two "sides" to a condition variable - the "producer side"
which eventually calls "-> send", and the "consumer side", which waits
for the send to occur.
Example: wait for a timer.
# condition: "wait till the timer is fired"
my $timer_fired = AnyEvent->condvar;
# create the timer - we could wait for, say
# a handle becomign ready, or even an
# AnyEvent::HTTP request to finish, but
# in this case, we simply use a timer:
my $w = AnyEvent->timer (
after => 1,
cb => sub { $timer_fired->send },
);
# this "blocks" (while handling events) till the callback
# calls ->send
$timer_fired->recv;
Example: wait for a timer, but take advantage of the fact that condition
variables are also callable directly.
my $done = AnyEvent->condvar;
my $delay = AnyEvent->timer (after => 5, cb => $done);
$done->recv;
Example: Imagine an API that returns a condvar and doesn't support
callbacks. This is how you make a synchronous call, for example from the
main program:
use AnyEvent::CouchDB;
...
my @info = $couchdb->info->recv;
And this is how you would just set a callback to be called whenever the
results are available:
$couchdb->info->cb (sub {
my @info = $_[0]->recv;
});
METHODS FOR PRODUCERS
These methods should only be used by the producing side, i.e. the
code/module that eventually sends the signal. Note that it is also the
producer side which creates the condvar in most cases, but it isn't
uncommon for the consumer to create it as well.
$cv->send (...)
Flag the condition as ready - a running "->recv" and all further
calls to "recv" will (eventually) return after this method has been
called. If nobody is waiting the send will be remembered.
If a callback has been set on the condition variable, it is called
immediately from within send.
Any arguments passed to the "send" call will be returned by all
future "->recv" calls.
Condition variables are overloaded so one can call them directly (as
if they were a code reference). Calling them directly is the same as
calling "send".
$cv->croak ($error)
Similar to send, but causes all calls to "->recv" to invoke
"Carp::croak" with the given error message/object/scalar.
This can be used to signal any errors to the condition variable
user/consumer. Doing it this way instead of calling "croak" directly
delays the error detection, but has the overwhelming advantage that
it diagnoses the error at the place where the result is expected,
and not deep in some event callback with no connection to the actual
code causing the problem.
$cv->begin ([group callback])
$cv->end
These two methods can be used to combine many transactions/events
into one. For example, a function that pings many hosts in parallel
might want to use a condition variable for the whole process.
Every call to "->begin" will increment a counter, and every call to
"->end" will decrement it. If the counter reaches 0 in "->end", the
(last) callback passed to "begin" will be executed, passing the
condvar as first argument. That callback is *supposed* to call
"->send", but that is not required. If no group callback was set,
"send" will be called without any arguments.
$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.
SUPPORTED EVENT LOOPS/BACKENDS
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
( run in 1.249 second using v1.01-cache-2.11-cpan-39bf76dae61 )