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Rocco Caputo

NAME

POE - multitasking and networking framework for perl

SYNOPSIS

  #!/usr/bin/perl -w
  use strict;

  # Use POE!
  use POE;

  sub handler_start {
    my ($kernel, $heap, $session) = @_[KERNEL, HEAP, SESSION];
    print "Session ", $session->ID, " has started.\n";
    $heap->{count} = 0;
    $kernel->yield('increment');
  }

  sub handler_increment {
    my ($kernel, $heap, $session) = @_[KERNEL, HEAP, SESSION];
    print "Session ", $session->ID, " counted to ", ++$heap->{count}, ".\n";
    $kernel->yield('increment') if $heap->{count} < 10;
  }

  sub handler_stop {
    print "Session ", $_[SESSION]->ID, " has stopped.\n";
  }

  for (0..9) {
    POE::Session->create(
      inline_states =>
        { _start    => \&handler_start,
          increment => \&handler_increment,
          _stop     => \&handler_stop,
        }
    );
  }

  $poe_kernel->run();
  exit;

QUICK LINKS

Please see the SEE ALSO section for conceptual summary of all POE's documentation.

POE's distribution comes with a directory full of examples, but they are gradually being phased out in favor of an online "cookbook". Please see the samples directory, and also look at http://poe.perl.org/?POE_Cookbook for more contemporary examples.

Please see http://www.perl.com/pub/2001/01/poe.html for an excellent, and more importantly: gradual, introduction to POE. If this manpage doesn't make sense, perhaps the introduction will.

Please see POE's web site at http://poe.perl.org/ for FAQs, cookbook examples, and modules using POE.

DESCRIPTION

POE is an acronym of "Perl Object Environment". It originally was designed as the core of a persistent object server and runtime environment, but it's evolved into a general purpose multitasking and networking framework.

POE's core contains two types of modules. First there's POE::Kernel; this is the main resource manager and event loop. Second are the sessions or state machines which implement the actual threads. The sessions are POE::Session (not quite a proper state machine) and POE::NFA.

The remainder of this distribution consists of convenience and helper modules, most of which aren't required to begin using POE.

USING POE

Using POE can be pretty tedious. Consider this example, which pulls in the necessary modules for a line-based TCP server:

  use POE::Kernel;
  use POE::Session;
  use POE::Wheel::SocketFactory;
  use POE::Wheel::ReadWrite;
  use POE::Filter::Line;
  use POE::Driver::SysRW;

The POE.pm module fixes some of this tedium. When POE.pm is used directly, it automatically includes POE::Kernel and POE::Session. It also includes each of the use statement's parameters, first prepending "POE::" to them. An example is in order:

This use statement is equivalent to the previous six.

  use POE qw( Wheel::SocketFactory Wheel::ReadWrite
              Filter::Line Driver::SysRW
            );

WRITING POE PROGRAMS

Basic POE programs have four parts.

Preliminary program setup

This is the usual overhead for writing a Perl program: a shebang line, perhaps some use statements to import things, and maybe some global variables or configuration constants. It's all pretty standard stuff.

  #!/usr/bin/perl -w
  use strict;
  use POE;
Define the program's event handlers or states

Declare functions which will handle events here. This is deceptive, since the functions can be declared anywhere, including as anonymous subroutines in a session constructor call.

  sub handler_start {
    ...
  }

  sub handler_increment {
    ...
  }

  sub handler_stop {
    ...
  }
Start initial sessions or machines

The Kernel only runs as long as there is something for it to do. It's main loop returns after the last session has stopped. The obvious corolary to this rule is that the main loop will return immediately if nothing is set up when it's called.

  for (0..9) {
    POE::Session->create(
      inline_states =>
        { _start    => \&handler_start,
          increment => \&handler_increment,
          _stop     => \&handler_stop,
        }
    );
  }
Start the kernel's main loop

_start handlers are invoked immediately when sessions are instantiated. Everything else happens because the kernel makes it so, and the kernel can't do that 'til it's started. Most programs exit afterwards since the kernel only returns after everything is done.

  $poe_kernel->run();
  exit 0;

POE's ARCHITECTURE

POE is built in separate layers. Each layer requires the ones beneath it, but no low-level layer requires a higher one.

Events layer

The events layer consists of an event dispatcher, POE::Kernel, and the sessions or state machines it runs: POE::Session (a generic event driven thread) and POE::NFA (an event driven nondeterministic finite automaton).

One or more I/O layers

I/O layers are built upon the event layer, and that allows them to coexist in the same program. POE only includes one I/O layer: Wheels. "Wheels" is a whimsical name for interlocking cogs that together make things go. They're also reinvented a lot, and this is no exception.

POE comes with six wheels.

POE::Wheel::Curses

The Curses wheel handles non-blocking input for programs using the curses text interface. It requires the Curses perl module and a familiarity with curses programming.q

POE::Wheel::FollowTail

FollowTail follows the tail of an ever-growing file. It's useful for watching logs and things of that nature.

POE::Wheel::ListenAccept

ListenAccept performs ye olde non-blocking socket listen and accept. It's great for programs that can't use SocketFactory and instead must listen and accept connections from sockets created elsewhere.

POE::Wheel::ReadLine

The ReadLine wheel accepts console input as lines only. It handles many of the common shell command editing keystrokes, making it pretty easy to input things. It's event driven, unlike Term::ReadLine, and it cooperates nicely with the rest of POE.

POE::Wheel::ReadWrite

ReadWrite is the star of POE's default I/O layer. It performs buffered, flow-controlled I/O on non-blocking, unbuffered filehandles. It almost acts like a Unix stream which can't stack protocol layers, but that may change.

ReadWrite uses two other classes to do its dirty work: Driver and Filter. Drivers do the actual work of reading and writing filehandles. Filters translate between raw streams and cooked chunks of tasty dada.

D comes before F, so Drivers go first.

POE::Driver::SysRW

Nobody has needed another driver yet, so this is the only one currently available. It performs sysread and syswrite in a generic way so that ReadWrite can use it and future drivers interchangeably.

Other drivers will use the same interface, should they ever be written.

Filters next. There are a few.

POE::Filter::Block

This filter parses input as fixed-length blocks. On the output side, it merely passes data through unscathed.

POE::Filter::HTTPD

The HTTPD filter parses input as HTTP requests and translates them into HTTP::Request objects. On the output side, it takes HTTP::Response objects and turns them into something suitable to be sent to a web client/user-agent.

POE::Filter::Line

The Line filter parses incoming streams into lines and turns outgoing lines into streams. It used to be very basic, but recent improvements have added interesting features like newline autodetection.

POE::Filter::Reference

The Reference filter is used to send Perl structures between POE programs or between POE and other Perl programs. On the input side, frozen data (via Storable, FreezeThaw, YAML, or some other data mechanism) is thawed into Perl data structures. On output, references given to the filter are frozen. Data may also be compressed on request if Compress::Zlib is installed.

POE::Filter::Stream

The stream filter does nothing. It merely passes data through without any change.

POE::Wheel::Run

The Run wheel provides a way to run functions or other programs in child processes. It encapsulates the necessary pipe() and fork() code, and sometimes exec(). Internally, it handles reading from and writing to child processes without further intervention. Child output arrives in the Wheel's owner as events.

POE::Wheel::SocketFactory

SocketFactory creates all manner of connectionless and connected network sockets. It also listens on TCP server sockets, only returning accepted client connections as they arrive.

POE COMPONENTS

Components consist of one or more sessions or state machines that encapsulate a very high level procedure. For example, POE::Component::IRC (not included) performs nearly all the functions of a full-featured IRC client. POE::Component::UserBase (not included) is a user authentication and data persistence servlet.

Components tend to be highly reusable libraries that handle tedious tasks, freeing programmers to focus on more interesting things. This should be true for any library, though.

A list of released POE::Component modules is at: http://search.cpan.org/search?mode=module&query=POE%3A%3AComponent

POE::Component::Client::TCP

The Client::TCP component provides the most common core features for writing TCP clients.

POE::Component::Server::TCP

The Server::TCP component provides the most common core features for writing TCP servers. A simple echo server is about 20 lines.

Support Modules

Finally, there are some files which POE uses but aren't required elsewhere. These include POE::Preprocessor and the base classes: POE::Component, POE::Driver, POE::Filter and POE::Wheel. There also are some development files in the lib directory.

POE::Preprocessor

This is a macro preprocessor. It also implements plain and enumerated constants. POE::Kernel uses it to inline smaller functions and make the source generally more readable. There seem to be two drawbacks: First, code is more difficult to examine in perl's debugger since it doesn't necessarily look like the original source. Second, programs take longer to start up because every source line must first pass through a perl filter. The compile-time penalty is negligible in long-running programs, and the runtime boost from fewer function calls can make up for it over time.

POE::Component, POE::Driver and POE::Filter exist to document their classes of objects. POE::Wheel contains some base functions for tracking unique wheel IDs.

SAMPLE PROGRAMS

This distribution contains several example and/or tutorial programs in the ./samples directory. Be advised, however, that they are old and may not exhibit the most current coding practices.

The sample programs are scheduled for removal from this distribution in version 0.1301. They are being moved to the web version of the POE Cookbook, which is available at <http://poe.perl.org/?POE_Cookbook>.

The author is always looking for new example ideas.

COMPATIBILITY ISSUES

POE has tested favorably on as many Perl versions as the author can find or harass people into trying. This includes Linux, FreeBSD, OS/2 and at least one unspecified version of Windows. As far as anyone can tell, nobody ever has tried it on any version of MacOS.

POE has been tested with Perl versions as far back as 5.004_03 and as recent as 5.7.2. The CPAN testers are a wonderful bunch of people who have dedicated resources to running new modules on a variety of platforms. The latest POE tests are visible at <http://testers.cpan.org/search?request=dist&dist=POE>. Thanks, people!

Please let the author know of breakage or success that hasn't been covered already. Thanks!

Specific issues:

Various Unices

No known problems.

OS/2

No known problems. POE has no OS/2 tester starting with version 0.1206.

Windows

POE seems to work very nicely with Perl compiled for CygWin. If you must use ActiveState Perl, please use build 631 or newer.

Currently there is nobody maintaining POE for Windows. Rocco will be fixing things as he's able, but he has only limited access to Windows machines for testing and development. If you would like to accelerate POE's Windows support, please contact Rocco or the mailing list.

Thanks to Sean Puckett, Douglas Couch, Hachi, and Dynweb for their help in bringing POE's Windows support so far.

MacOS

POE 0.18 passes all tests on MacOS/X.

Its pre-X support seems like a lost cause unless someone steps forward to make it happen.

SYSTEM REQUIREMENTS

POE requires Filter::Util::Call version 1.18 or newer. All the other modules are optional.

Time::HiRes is recommended.

Some of POE's sample programs use fork(). They won't work wherever fork() isn't available; sorry.

POE relies heavily on constants in the POSIX module. Some of the constants aren't defined on some platforms. POE works around this as best it can.

Some of POE's sample programs require a recent IO bundle, but you get that for free with recent versions of Perl.

If you want to use Filter::Reference to serialize data for transporting over a network, you will need Storable, FreezeThaw, or some other freezer/thawer package installed. If you want your data to be shipped compressed, you will also need Compress::Zlib.

Important Filter::Reference note: If you're using Filter::Reference to pass data to another machine, make sure every machine has the same versions of the same libraries. Subtle differences, even in different versions of modules like Storable, can cause mysterious errors when data is reconstituted at the receiving end. When all else fails, upgrade to the latest versions.

Filter::HTTPD uses a small world of modules including HTTP::Status; HTTP::Request; HTTP::Date and URI::URL. The httpd.perl sample program uses Filter::HTTPD, which uses all that other stuff. If you want to write web servers, you'll need to install libwww-perl, which requires libnet.

Wheel::Curses requires the Curses module, which in turn requires some sort of curses library.

SUPPORT RESOURCES

These are Internet resources where you may find more information about POE.

The POE Mailing List

POE has a mailing list at perl.org. You can receive subscription information by sending e-mail:

  To: poe-help@perl.org
  Subject: (anything will do)

  The message body is ignored.

All forms of feedback are welcome.

The POE Web Site

POE has a web site where the latest development snapshot, along with the Changes file and other stuff may be found: http://poe.perl.org/

SourceForge

POE's development has moved to SourceForge as an experiment in project management. You can reach POE's project summary page at http://sourceforge.net/projects/poe/

SEE ALSO

This is a summary of POE's modules and the things documented in each.

Events Layer

These are POE's core modules.

POE (this document)

The POE manpage includes a sample program and walkthrough of its parts, a summary of the modules which comprise this distribution, POE's general system requirements, how to use POE (literally), and where to get help. It also has a table of contents which you're even now reading.

POE::Kernel

The POE::Kernel manpage includes information about debugging traces and assertions; FIFO events; filehandle watchers; Kernel data accessors; posting events from traditional callbacks (postbacks); redefining sessions' states; resource management; session aliases; signal types, handlers, and pitfalls; signal watchers; synchronous vs. asynchronous events; and timed events (alarms and delays).

POE::NFA

The POE::NFA manpage covers this session's additional predefined events, how NFA differs from Session, state changing methods, and the spawn constructor.

POE::Session

The POE::Session manpage covers different kinds of states (inline coderef, object methods, and package methods); postback mechanics; predefined event names and the parameters included with them; resource management and its effects on sessions; session constructors (new and create); session data accessors; synchronous vs. asynchronous events in more detail; why sessions don't stop by themselves, and how to force them to.

I/O Layer

These modules comprise POE's default I/O abstraction.

POE::Driver

The POE::Driver manpage covers drivers in general and their common interface.

POE::Driver::SysRW

The SysRW driver's manpage describes the sysread/syswrite abstraction and covers parameters which can be used to customize a SysRW driver's operation.

POE::Filter

The POE::Filter manpage covers filters in general and their common interface. It discusses the pitfalls involved in switching filters on a running wheel.

POE::Filter::Grep

Grep is part of the family of filters that includes Stackable, Map, and RecordBlock. It applies a regexp filter on data passing through it, before it reaches a Session. It's mainly used in filter stacks (see POE::Filter::Stackable).

POE::Filter::HTTPD

The HTTPD filter's manpage covers using POE as a web server.

POE::Filter::Line

The Line filter's manpage discusses how to read and write data by lines; how to change the newline literal or regular expression; and how to enable newline autodetection when working with strange peers.

POE::Filter::Map

Map is part of the family of filters that includes Stackable, Grep, and RecordBlock. It transforms data passing through it, before it reaches a Session.

The Map filter is designed primarily to act as an interface between filters that deal with different data formats, but it can be used stand-alone to perform unique functions that no other filter does. In this case it's something of a wildcard filter.

If you find yourself reusing the same custom Map filter, you may want to turn it into a full-fledged filter.

POE::Filter::RecordBlock

RecordBlock combines records into groups by count and flattens groups of records back into a record stream. For example, RecordBlock might combine log records into pairs.

POE::Filter::Reference

The Reference filter's manpage talks about marshalling data and passing it between POE programs; and customizing the way data is frozen, thawed and optionally compressed.

POE::Filter::Stackable

Stackable is a meta-filter designed to stack other filters. Stackable manages the filters it contains and passes data between them. In essence, the inner filters are combined into one super filter.

The Map filter can also be used to perform quick and dirty functions that aren't implemented in any single existing filter.

POE::Filter::Stream

The Stream filter's manpage is pretty empty since it doesn't do much of anything.

POE::Wheel

The Wheel's manpage talks about wheels in general and their common interface.

POE::Wheel::FollowTail

The FollowTail wheel's manpage discusses how to watch the end of an ever-growing file (not to be confused with that orb tune) and how to change aspects of the wheel's behavior with constructor parameters.

POE::Wheel::ListenAccept

The ListenAccept wheel's manpage discusses how to listen and accept connections using sockets created from sources other than SocketFactory.

POE::Wheel::ReadWrite

The ReadWrite wheel's manpage covers non-blocking I/O with optional flow control.

POE::Wheel::SocketFactory

The SocketFactory wheel's manpage discusses how socket factories create and manage sockets; the events they emit on connection, acceptance, and failure; and the parameters which govern what they do.

Standard Components

These components are included with POE because they're nearly universally useful.

POE::Component

The POE::Component manpage discusses what components are and why they exist.

POE::Component::Client::TCP

The TCP client component explains how to create TCP clients quickly and easily.

POE::Component::Server::TCP

The TCP server component explains how to create TCP servers with a minimum of fuss.

Supporting Cast

These modules help in the background.

POE::Pipe::OneWay

This creates unbuffered one-way pipes. It tries various methods in the hope that one of them will work on any given platform.

POE::Pipe::TwoWay

This creates unbuffered two-way pipes. It tries various methods in the hope that one of them will work on any given platform. It's preferred over two OneWay pipes because sometimes two-way transports are available and it can save you a couple filehandles.

POE::Preprocessor

POE's preprocessor covers inline constant replacement, enumerated constants, and macro substitutions in perl programs.

BUGS

The t/*.t tests only cover about 70% of POE. The latest numbers are on POE's web site.

AUTHORS & COPYRIGHT

POE is the combined effort of several people. If someone is missing from this list, please let Rocco know.

Ann Barcomb

Ann Barcomb is <kudra@domaintje.com>, aka kudra. Ann contributed large portions of POE::Simple and the code that became the ReadWrite support in POE::Component::Server::TCP. Her ideas also inspired Client::TCP component, introduced in version 0.1702.

Artur Bergman

Artur Bergman is <artur@vogon-solutions.com>. He contributed many hours' work into POE and quite a lot of ideas. Years later, I decide he's right and actually implement them.

Artur is the author of Filter::HTTPD and Filter::Reference, as well as bits and pieces throughout POE. His intangible contributions include feedback, testing, conceptual planning and inspiration. POE would never have come this far without his support.

Artur is investing his time heavily into perl 5's ithreads at the moment. This project has far-reaching implications for POE's future.

Jos Boumans

Jos Boumans is <boumans@frg.eur.nl>, aka Co-Kane. Jos is a major driving force behind the POE::Simple movement and is one of the POE idea fairies. Jos is working with Ann on POE::Simple.

Matt Cashner

Matt Cashner is <eek@eekeek.org>, aka sungo. Matt is a POE ambassador, or something, between Rocco's point of view and people who haven't had the benefit of knowing the system since its conception. He's spearheaded the movement to smiplify POE for new users, flattening the learning curve and making the system more accessible to everyone. He's almost singlehandedly rewriting POE's documentation. He uses the system in mission critical applications, folding feedback and features back into the distribution for everyone's enjoyment.

Andrew Chen

Andrew Chen is <achen-poe@micropixel.com>. Andrew is the resident POE/Windows guru. He contributes much needed testing for Solaris on the SPARC and Windows on various Intel platforms.

Douglas Couch

Douglas Couch is <dscouch@purdue.edu>. Douglas maintains POE's PPD for Windows as well as up-to-date online documentation at http://poe.sourceforge.net/

Jeffrey Goff

Jeffrey Goff is <jgoff@blackboard.com>. Jeffrey is the author of several POE modules, including a tokenizing filter and a component for managing user information, PoCo::UserBase. He's also co-author of "A Beginner's Introduction to POE" at www.perl.com.

Philip Gwyn

Philip Gwyn is <gwynp@artware.qc.ca>. He extended the Wheels I/O abstraction to support hot-swappable filters, and he eventually convinced Rocco that unique session and kernel IDs were a good thing.

Philip also enhanced Filter::Reference to support different serialization methods. His intangible contributions include the discovery and/or destruction of several bugs (see the Changes file) and a thorough code review around version 0.06.

Arnar M. Hrafnkelsson

Arnar is <addi@umich.edu>. Addi tested POE and POE::Component::IRC on Windows, finding bugs and testing fixes. He appears throughout the Changes file.

Dave Paris

Dave Paris is <dparis@w3works.com>. Dave tested and benchmarked POE around version 0.05, discovering some subtle (and not so subtle) timing problems. The pre-forking server sample was his idea. Versions 0.06 and later should scale to higher loads because of his work. He has contributed a lot of testing and feedback, much of which is tagged in the Changes file as a-mused.

And I do mean *lots* of testing. I go and announce a new development version, and he's, like, "All tests passed!" just a few minutes later. If that wasn't enough, he investigates any bugs that turn up, and often fixes them. The man's scarily good.

Dieter Pearcey

Dieter Pearcey is <dieter@bullfrog.perlhacker.org>. He goes by several Japanese nicknames. Dieter's current area of expertise is in Wheels and Filters. He greatly improved Wheel::FollowTail, and his Filter contributions include the basic Block filter, as well as Stackable, RecordBlock, Grep and Map.

Robert Seifer

Robert Seifer is <e-mail unknown>. He rotates IRC nicknames regularly.

Robert contributed entirely too much time, both his own and his computers, towards the detection and eradication of a memory corruption bug that POE tickled in earlier Perl versions. In the end, his work produced a simple compile-time hack that worked around a problem relating to anonymous subs, scope and @{} processing.

Matt Sergeant

Matt contributed POE::Kernel::Poll, a more efficient way to watch multiple files than select().

Richard Soderberg

Richard Soderberg is <poe@crystalflame.net>, aka coral. Richard is a collaborator on several side projects involving POE. His work provides valuable testing and feedback from a user's point of view.

Dennis Taylor

Dennis Taylor is <dennis@funkplanet.com>. Dennis has been testing, debugging and patching bits here and there, such as Filter::Line which he improved by leaps in 0.1102. He's also the author of POE::Component::IRC.

Others?

Please contact the author if you've been forgotten.

Author

Rocco Caputo

Rocco Caputo is <rcaputo@cpan.org>. POE is his brainchild.

Except where otherwise noted, POE is Copyright 1998-2002 Rocco Caputo. All rights reserved. POE is free software; you may redistribute it and/or modify it under the same terms as Perl itself.

Thank you for reading!