IO::Async - Asynchronous event-driven programming


       use IO::Async::Stream;
       use IO::Async::Loop;
       my $loop = IO::Async::Loop->new;
          host     => "",
          service  => 12345,
          socktype => 'stream',
          on_stream => sub {
             my ( $stream ) = @_;
                on_read => sub {
                   my ( $self, $buffref, $eof ) = @_;
                   while( $$buffref =~ s/^(.*\n)// ) {
                      print "Received a line $1";
                   return 0;
             $stream->write( "An initial line here\n" );
             $loop->add( $stream );
          on_resolve_error => sub { die "Cannot resolve - $_[-1]\n"; },
          on_connect_error => sub { die "Cannot connect - $_[0] failed $_[-1]\n"; },


    This collection of modules allows programs to be written that perform
    asynchronous filehandle IO operations. A typical program using them
    would consist of a single subclass of IO::Async::Loop to act as a
    container of other objects, which perform the actual IO work required
    by the program. As well as IO handles, the loop also supports timers
    and signal handlers, and includes more higher-level functionality built
    on top of these basic parts.

    Because there are a lot of classes in this collection, the following
    overview gives a brief description of each.


    The base class of all the event handling subclasses is
    IO::Async::Notifier. It does not perform any IO operations itself, but
    instead acts as a base class to build the specific IO functionality
    upon. It can also coordinate a collection of other Notifiers contained
    within it, forming a tree structure.

    The following sections describe particular types of Notifier.

 File Handle IO

    An IO::Async::Handle object is a Notifier that represents a single IO
    handle being managed. While in most cases it will represent a single
    filehandle, such as a socket (for example, an IO::Socket::INET
    connection), it is possible to have separate reading and writing
    handles (most likely for a program's STDIN and STDOUT streams, or a
    pair of pipes connected to a child process).

    The IO::Async::Stream class is a subclass of IO::Async::Handle which
    maintains internal incoming and outgoing data buffers. In this way, it
    implements bidirectional buffering of a byte stream, such as a TCP
    socket. The class automatically handles reading of incoming data into
    the incoming buffer, and writing of the outgoing buffer. Methods or
    callbacks are used to inform when new incoming data is available, or
    when the outgoing buffer is empty.

    While stream-based sockets can be handled using using
    IO::Async::Stream, datagram or raw sockets do not provide a bytestream.
    For these, the IO::Async::Socket class is another subclass of
    IO::Async::Handle which maintains an outgoing packet queue, and informs
    of packet receipt using a callback or method.

    The IO::Async::Listener class is another subclass of IO::Async::Handle
    which facilitates the use of listen(2)-mode sockets. When a new
    connection is available on the socket it will accept(2) it and pass the
    new client socket to its callback function.


    An IO::Async::Timer::Absolute object represents a timer that expires at
    a given absolute time in the future.

    An IO::Async::Timer::Countdown object represents a count time timer,
    which will invoke a callback after a given delay. It can be stopped and

    An IO::Async::Timer::Periodic object invokes a callback at regular
    intervals from its initial start time. It is reliable and will not
    drift due to the time taken to run the callback.

    The IO::Async::Loop also supports methods for managing timed events on
    a lower level. Events may be absolute, or relative in time to the time
    they are installed.


    An IO::Async::Signal object represents a POSIX signal, which will
    invoke a callback when the given signal is received by the process.
    Multiple objects watching the same signal can be used; they will all
    invoke in no particular order.

 Processes Management

    An IO::Async::PID object invokes its event when a given child process
    exits. An IO::Async::Process object can start a new child process
    running either a given block of code, or executing a given command, set
    up pipes on its filehandles, write to or read from these pipes, and
    invoke its event when the child process exits.


    The IO::Async::Loop object class represents an abstract collection of
    IO::Async::Notifier objects, and manages the actual filehandle IO
    watchers, timers, signal handlers, and other functionality. It performs
    all of the abstract collection management tasks, and leaves the actual
    OS interactions to a particular subclass for the purpose.

    IO::Async::Loop::Poll uses an IO::Poll object for this test.

    IO::Async::Loop::Select uses the select(2) syscall.

    Other subclasses of loop may appear on CPAN under their own dists; see
    the "SEE ALSO" section below for more detail.

    As well as these general-purpose classes, the IO::Async::Loop
    constructor also supports looking for OS-specific subclasses, in case a
    more efficient implementation exists for the specific OS it runs on.

 Child Processes

    The IO::Async::Loop object provides a number of methods to facilitate
    the running of child processes. spawn_child is primarily a wrapper
    around the typical fork(2)/exec(2) style of starting child processes,
    and run_child provide a method similar to perl's readpipe (which is
    used to implement backticks ``).

 File Change Watches

    The IO::Async::File object observes changes to stat(2) properties of a
    file, directory, or other filesystem object. It invokes callbacks when
    properties change. This is used by IO::Async::FileStream which presents
    the same events as a IO::Async::Stream but operates on a regular file
    on the filesystem, observing it for updates.

 Asynchronous Co-routines and Functions

    The IO::Async framework generally provides mechanisms for multiplexing
    IO tasks between different handles, so there aren't many occasions when
    it is necessary to run code in another thread or process. Two cases
    where this does become useful are when:

      * A large amount of computationally-intensive work needs to be

      * An OS or library-level function needs to be called, that will
      block, and no asynchronous version is supplied.

    For these cases, an instance of IO::Async::Function can be used around
    a code block, to execute it in a worker child process or set of
    processes. The code in the sub-process runs isolated from the main
    program, communicating only by function call arguments and return
    values. This can be used to solve problems involving state-less library

    An IO::Async::Routine object wraps a code block running in a separate
    process to form a kind of co-routine. Communication with it happens via
    IO::Async::Channel objects. It can be used to solve any sort of problem
    involving keeping a possibly-stateful co-routine running alongside the
    rest of an asynchronous program.


    An IO::Async::Future object represents a single outstanding action that
    is yet to complete, such as a name resolution operation or a socket
    connection. It stands in contrast to a IO::Async::Notifier, which is an
    object that represents an ongoing source of activity, such as a
    readable filehandle of bytes or a POSIX signal.

    Futures are a recent addition to the IO::Async API and details are
    still subject to change and experimentation.

    In general, methods that support Futures return a new Future object to
    represent the outstanding operation. If callback functions are supplied
    as well, these will be fired in addition to the Future object becoming
    ready. Any failures that are reported will, in general, use the same
    conventions for the Future's fail arguments to relate it to the legacy
    on_error-style callbacks.

       $on_NAME_error->( $message, @argmuents )
       $f->fail( $message, NAME, @arguments )

    where $message is a message intended for humans to read (so that this
    is the message displayed by $f->get if the failure is not otherwise
    caught), NAME is the name of the failing operation. If the failure is
    due to a failed system call, the value of $! will be the final
    argument. The message should not end with a linefeed.


    The IO::Async::Loop provides several methods for performing
    network-based tasks. Primarily, the connect and listen methods allow
    the creation of client or server network sockets. Additionally, the
    resolve method allows the use of the system's name resolvers in an
    asynchronous way, to resolve names into addresses, or vice versa. These
    methods are fully IPv6-capable if the underlying operating system is.


    The IO::Async::Protocol class provides storage for a IO::Async::Handle
    object, to act as a transport for some protocol. It allows a level of
    independence from the actual transport being for that protocol,
    allowing it to be easily reused. The IO::Async::Protocol::Stream
    subclass provides further support for protocols based on stream
    connections, such as TCP sockets.


    This collection of modules is still very much in development. As a
    result, some of the potentially-useful parts or features currently
    missing are:

      * Consider further ideas on Solaris' ports, BSD's Kevents and
      anything that might be useful on Win32.

      * Consider some form of persistent object wrapper in the form of an
      IO::Async::Object, based on IO::Async::Routine.

      * IO::Async::Protocol::Datagram

      * Support for watching filesystem entries for change. Extract logic
      from IO::Async::File and define a Loop watch/unwatch method pair.

      * Define more Future-returning methods. Consider also one-shot
      Futures on things like IO::Async::Process exits, or IO::Async::Handle


    Bugs may be reported via RT at

    Support by IRC may also be found on in the #io-async


    As well as the two loops supplied in this distribution, many more exist
    on CPAN. At the time of writing this includes:

      * IO::Async::Loop::AnyEvent - use IO::Async with AnyEvent

      * IO::Async::Loop::Epoll - use IO::Async with epoll on Linux

      * IO::Async::Loop::Event - use IO::Async with Event

      * IO::Async::Loop::EV - use IO::Async with EV

      * IO::Async::Loop::Glib - use IO::Async with Glib or GTK

      * IO::Async::Loop::KQueue - use IO::Async with kqueue

      * IO::Async::Loop::Mojo - use IO::Async with Mojolicious

      * IO::Async::Loop::POE - use IO::Async with POE

      * IO::Async::Loop::Ppoll - use IO::Async with ppoll(2)

    Additionally, some other event loops or modules also support being run
    on top of IO::Async:

      * AnyEvent::Impl::IOAsync - AnyEvent adapter for IO::Async

      * Gungho::Engine::IO::Async - IO::Async Engine

      * POE::Loop::IO_Async - IO::Async event loop support for POE


    Paul Evans <>