AnyEvent::Impl::IOAsync - AnyEvent adaptor for IO::Async
# optionally set another event loop
my $loop = new IO::Async::Loop;
This module provides support for IO::Async as AnyEvent backend. It supports I/O,
timers, signals and child process watchers. Idle watchers are emulated. I/O
watchers need to dup their fh because IO::Async only supports IO handles, not
plain file descriptors.
The only user-servicible part in this module is the "set_loop"
function and $LOOP variable:
- AnyEvent::Impl::IOAsync::set_loop $new_loop
- Unfortunately, IO::Async has no concept of a default loop.
Modules using IO::Async must be told by their caller which loop to use,
which makes it impossible to transparently use IO::Async from a module.
This module is no exception. It creates a new IO::Async::Loop object when it
is loaded. This might not be the right loop object, though, and thus you
can replace it by a call to this function with the loop object of your
Note that switching loops while watchers are already initialised can have
unexpected effects, and is not supported unless you can live with the
- This variable always contains the IO::Async::Loop object
used by this AnyEvent backend. See above for more info.
Storing the "default" loop makes this module a possible arbiter
for other modules that want to use IO::Async transparently. It's advised
to directly refer to this variable each time you want to use it, without
making a local copy.
This section had a long list of problems and shortcomings that made it almost
impossible to support IO::Async. With version 0.33 of IO::Async, however, most
of these have been fixed, so IO::Async can now be used as easily as many other
There are a few remaining problems that require emulation or workarounds:
- No support for multiple watchers per event
- In most (all? documentation?) cases you cannot have
multiple watchers for the same event (what's the point of having all these
fancy notifier classes when you cannot have multiple notifiers for the
same event? That's like only allowing one timer per second or so...).
For I/O watchers, AnyEvent has to dup() every file handle, as
IO::Async fails to support the same or different file handles pointing to
the same fd (the good thing is that it is documented, but why not fix it
Apart from these fatal flaws, there are a number of unpleasent properties that
just need some mentioning:
- Confusing and misleading names
- Another rather negative point about this module family is
its name, which is deeply confusing: Despite the "async" in the
name, IO::Async only does synchronous I/O, there is nothing
"asynchronous" about it whatsoever (when I first heard about it,
I thought, "wow, a second async I/O module, what does it do compared
to IO::AIO", and was somehow set back when I learned that the only
"async" aspect of it is the name).
- Inconsistent, incomplete and convoluted API
- Implementing AnyEvent's rather simple timers on top of
IO::Async's timers was a nightmare (try implementing a timer with
configurable interval and delay value...).
The method naming is chaotic: "watch_child" creates a child
watcher, but "watch_io" is an internal method;
"detach_signal" removes a signal watcher, but
"detach_child" forks a subprocess and so on).
- Unpleasant surprises on GNU/Linux
- When you develop your program on FreeBSD and run it on
GNU/Linux, you might have unpleasant surprises, as IO::Async::Loop will by
default use IO::Async::Loop::Epoll, which is incompatible with
"fork", so your network server will run into spurious and very
hard to debug problems under heavy load, as IO::Async forks a lot of
processes, e.g. for DNS resolution. It would be better if IO::Async would
only load "safe" backends by default (or fix the epoll backend
to work in the presence of fork, which admittedly is hard - EV does it for
you, and also does not use unsafe backends by default).
On the positive side, performance with IO::Async is quite good even in my very
Marc Lehmann <firstname.lastname@example.org>
Paul Evans <email@example.com>
Rewrote the backend for IO::Async version 0.33.