TITLE

Advanced I/O (AIO)

VERSION

  Maintainer: Uri Guttman <uri@sysarch.com>
  Date: 29 Sept 2000
  Mailing List: perl6-language-io@perl.org
  Number: 350
  Version: 1
  Status: Developing

ABSTRACT

This RFC describes a pragma and module which support an advanced I/O subsystem. It is meant to be a centralized subsystem that supports a wide range of I/O requirements, many of which are covered in other RFCs. It doesn't add any new syntax or break any existing code.

DESCRIPTION

There are many way that coders want to do I/O in Perl and many I/O sources as well. RFC 14 discusses ways to make open smarter and to have handlers for each flavor. The goal is good, but it requires new syntax and semantics for open. You have effectively multiple versions of open, each with their own argument order. It also doesn't address asynchronous I/O or events and callbacks. I don't see how to make a socket or http connect and not block on the request.

This RFC addresses those issues in a Perl5 way by creating a new class with attribute based methods. This allows use to add new ways to do open and I/O and not have to learn specialized argument formats. The style is very similar to IO:: now but more generalized and tightly integrated in the core. This is why I choose to call it Advanced I/O (AIO).

The main pragma will be 'use aio' and it will allow the coder to control and load various components. I won't have the time to specify all of them but think along the lines of RFC 14, LWP, libnet, IO::*, LWP::Parallel, etc. Supporting them all under this single pardigm is simple as you don't need to know more then the minimum attributes to get your job done. Also as I have coverd some in RFC 47, many of the attributes will have well chosen defaults which allow for shorter argument lists for common situations. For example, instead of

	$sock = IO::Socket::INET->new(PeerAddr => 'www.perl.org',
                                          PeerPort => 'http(80)',
                                          Proto    => 'tcp');

you would do:

	$sock = AIO::Open( Host => 'www.perl.org',
			   Port => 80 ) ;

TCP would be the default protocol as it is much more common than UDP, and it would know it is a socket connection because of the Host/Port attributes.

Similarly for LWP you would just do:

	$sock = AIO::Open( Url => 'www.perl.org' ) ;

If you refer to a special attribute, it can cause the appropriate module to be loaded at runtime. In the above case the AIO::LWP (or whatever it is called) would get loaded and then it would be passed the open call.

One critical feature of AIO is direct support of asynchronous I/O (including connections, server accepts, and real asynchronous file I/O). There is no special interface required, you just specify a callback attribute. That makes the request automatically non-blocking and registers the event for you. You have to had previously specified an event dispatch method (with use event or use aio) or it is a runtime error.

Assuming an object $foo in package Foo, the above calls can be done with asynchronous callbacks (see RFC 321 for more on callbacks) like this:

	$event = AIO::Open( Host => 'www.perl.org',
			    Port => 80
			    Callback => $obj  ) ;


	$event = AIO::Open( Url => 'www.perl.org
			    Callback => $obj  ) ;


	Package Foo ;

	sub Connected {

		my( $self, $socket ) = @_ ;

		print "i am connected to perl.org\n" ;
	}

	sub Url_gotten {

		my( $self, $socket ) = @_ ;

		print $socket->read( 8096 ) ;
	}

Here is an asynchronous file read :

	$event = AIO::Read( Fd => $fh,
			    Count => 1000,
			    Callback => \&read_handler ) ;

	sub read_handler {

		my( $read_data ) = @_ ;

		print "i read [$read_data]\n" ;
	}

The callback method names are the default ones (we can pick better ones but I am under deadline pressure!) or you can choose your own. Timeouts can also be set with their own method names or defualt ones:

	$event = AIO::Open( Url		=> 'www.perl.org
			    Callback	=> $obj,
			    Method	=> 'Url_Ready'
			    Timeout	=> 10
			    Timeout_Method => 'URL_timeout' ) ;

So you see, the same simple syntax and a consistant API lets you do blocking and non-blocking connect, I/O and timers. No need to learn IO::, LWP and LWP::Parallel and IO::Select. All of them are covered in under this approach and adding new attributes is easy and won't conflict with other code written to this specification.

There is much more than can be desribed and I hope to have a chance to flesh this out but I have no more time before the October 1 deadline. I trust you can extrapolate read/write calls and other related features from this and RFC's 47, 321 and 345.

IMPLEMENTATION

Implementation should be very straightforward. Much of the work is in the event loop and I/O subsystems. Some work has to be done with managing which module supports which attributes. Event.pm does much of this now and its API was a major influence on this.

IMPACT

None. No new syntax or semantics are added. The event loop and I/O subsystem do all the work under the hood. Translation from Perl5 will be very simple (or not even needed)

UNKNOWNS

None.

REFERENCES

Event.pm - XS based event loop module.

RFC 14: Modify open() to support FileObjects and Extensibility

RFC 47: Universal Asynchronous I/O

RFC 60: Safe Signals

RFC 174: Improved parsing and flexibility of indirect object syntax

RFC 321: Common Callback API for all AIO calls.

RFC 345: Putting an Event Loop in the Core

This file is part of the Perl 6 Archive