ActiveState!

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 POE - A Perl Object Environment


NAME

POE - A Perl Object Environment


SUPPORTED PLATFORMS

  • Linux
  • Solaris
  • Windows
This module is not included with the standard ActivePerl distribution. It is available as a separate download using PPM.

SYNOPSIS

  # Basic usage:
  use POE;
  # create initial sessions here
  $poe_kernel->run();
  exit;
  # Typical usage:
  use POE qw( Wheel::SocketFactory Wheel::ReadWrite
              Driver::SysRW Filter::Line
            );
  # create initial sessions here
  $poe_kernel->run();
  exit;


DESCRIPTION

The POE distribution contains a handful of different modules, each doing something different.

When a program uses the POE module, the mandatory POE::Kernel and POE::Session classes are included. Other modules may be included in the parameter to ``use POE''. POE.pm will prepend ``POE::'' to the module names for you.


CONCEPTUAL OVERVIEW

POE's features are separated into three major sections. Sections are called ``layers'' in the documentation because each builds atop others.

  +-----------+ +--------------+
  | I/O Layer | | Object Layer |
  +-----------+ +--------------+
       /|\            /|\         Commands (to events layer)
        |              |
        |              |
       \|/            \|/         Events (from events layer)
  +----------------------------+
  |        Events Layer        |
  +----------------------------+

Events are also used to pass messages between Sessions.

This is a description of each layer, starting with the lowest and working upwards:

Events Layer

POE's events layer consists of two classes. These classes are always included when a program uses POE. They may also be used separately wherever their exported constants are needed.

POE::Kernel contains the state transition event queue and functions to manage resources (including events). Later on, these functions will be referred to as ``resource commands''. The Kernel will generate events to indicate when watched resources (via a resource command) become active.

POE::Session instances are state machines. They consist of bundles of related states. States may be code references, object methods or package subroutines. States are invoked whenever a queued transition event is dispatched. State transitions may be enqueued by states themselves or by active resources.

I/O Layer

The I/O layer contains one or more libraries that abstract file I/O. Currently there is only one abstraction library, fondly known as ``Wheels''. The ``Wheels'' abstraction consists of groups of classes.

One type of object does only low-level file I/O. These are the Driver objects.

A second type of object translates between raw octet streams and protocol packets. These are the Filter objects.

The final type of object provides a functional interface to file I/O, as well as the select logic to glue Drivers and Filters together. These are the Wheel objects.

Here is a rough picture of the Wheels I/O abstraction:

  +----------------------------------------------------------+
  | Session                                                  |
  |                                                          |
  | +------------+  +-------+     +--------+    +--------+   |
  | |States      |  |       |     |        |    |        |   |
  | |            |  |       |     |        |    |        |   |
  | |Command     |  |       |     | Filter |    |        |   |
  | |events    --|->|       |<--->|        |--->|        |   |
  | |            |  | Wheel |     |        |    | Driver |   |
  | |Functions --|->|       |     +--------+    |        |<--|--> File 
  | |            |  |       |                   |        |   |
  | |Response    |  |       |-> Select Events ->|        |   |
  | |events    <-|--|       |                   |        |   |
  | +------------+  +-------+                   +--------+   |
  |   |   /|\         |  /|\                                 |
  |   |    |          |   |                                  |
  +---|----|----------|---|----------------------------------+
      |    |          |   |
      |    |          |   |   Commands (Session -> Kernel)
      |    |          |   |   & Events (Kernel -> Session)
     \|/   |         \|/  |
  +----------------------------------------------------------+
  |                                                          |
  |                          Kernel                          |
  |                                                          |
  +----------------------------------------------------------+

Object Layer

The Object layer consists of one or more libraries that implement code objects. Currently there are two ways code objects can be created.

First, code may exist as plain Perl subroutines, objects and packages. This is the oldest object layer, and it is often the best for most programming tasks.

The second object layer is still in its infancy. Right now it consists of four classes:

Curator. This is the object manager. It embodies inheritance, attribute fetching and storage, method invocation and security.

Repository. This is the object database. It provides a consistent interface between the Curator and whatever database it hides.

Object. This is a Perl representation of a Repository object. It hides the Curator and Repository behind an interface that resembles a plain Perl object.

Runtime. This is a namespace where Object methods are run. It contains the public functions from Curator, Repository and Object, and it may one day run within a Safe compartment.

The obligatory ASCII art:

  +--------------------------------------------------+
  |                     Runtime                      |
  | +----------------+                               |
  | | Object Methods |-------> Public Functions      |
  | +----------------+                               |
  |   /|\                          |                 |
  +----|---------------------------|-----------------+
       |                           |
       | Events                    |  Commands
       |                          \|/
  +--------------------------------------------------+
  |                                                  |
  |  +------------+     Curator                      |
  |  |            |                                  |
  |  |  Sessions  |  +-------------------------------+
  |  |            |  |
  |  +------------+  |   +------------+   +--======--+
  |    /|\     |     |<->| Repository |<->| Database |
  +-----|------|-----+   +------------+   +--======--+
        |      |
        |      |   Events & Commands
        |     \|/
  +--------------------------------------------------+
  |                                                  |
  |                      Kernel                      |
  |                                                  |
  +--------------------------------------------------+


EXAMPLES

As of this writing there are 24 sample programs. Each illustrates and tests some aspect of POE use. They are included in the POE distribution archive, but they are not installed. If POE was installed via the CPAN shell, then you should be able to find them in your .cpan/build/POE-(version) directory.

Events Layer Examples

These sample programs demonstrate and exercise POE's events layer and resource management functions.

  • create.perl

    This program is essentially the same as sessions.perl, but it uses the newer &POE::Session::create constructor rather than the original &POE::Session::new constructor.

  • forkbomb.perl

    This program is an extensive test of Session construction and destruction in the kernel. Despite the name, it does not use fork(2). By default, this program will stop after about 200 sessions, so it shouldn't run away with machines it's run on.

    Stopping forkbomb.perl with SIGINT is a good way to test signal propagation.

  • names.perl

    This program demonstrates the use of session aliases as a method of ``daemonizing'' sessions and communicating between them by name. It also shows how to do non-blocking inter-session communication with callback states.

  • objmaps.perl

    This is a version of objsessions.perl that maps states to differently named object methods.

  • objsessions.perl

    This program is essentially the same as sessions.perl, but it uses object methods as states instead of inline code references.

  • packagesessions.perl

    This program is essentially the same as sessions.perl, but it uses package functions as states instead of inline code references.

  • piong.perl

    This is a quick and dirty multiple-host icmp ping program. It requires two common vt100 escape codes (``\e[2J'' to clear the screen and ``\e[0;0H\'' to home the cursor). It needs to be run by root, since it expects to open a raw socket.

    I thank Russell Mosemann <mose@ccsn.edu> for the Net::Ping module, which I ``borrowed'' heavily from. Net::Ping is the route of choice if you don't need parallel ping capability.

  • selects.perl

    This program exercises the POE::Kernel interface to select(2). It creates a simple chargen server, and a simple client to visit it. The client will disconnect after receiving a few lines from the server. The server will remain active, and it will accept telnet connections.

  • sessions.perl

    This program is a basic test of Session construction, destruction and maintenance in the Kernel. It is much more friendly than forkbomb.perl. People who are new to POE may want to look at this test first.

  • signals.perl

    This program is a basic test of the POE::Kernel interface to system and Session signals. It creates two sessions that wait for signals and periodically send signals to themselves.

I/O Layer Examples

These sample programs demonstrate and exercise POE's default I/O layer.

  • fakelogin.perl

    This program tests the ability for POE::Wheel instances to change the events they emit. The port it listens on can be specified on the command line. Its default listen port is 23.

  • filterchange.perl

    This program tests the ability for POE::Wheel instances to change the filters they use to process information.

  • followtail.perl

    This program tests POE::Wheel::FollowTail, a read-only wheel that follows the end of an ever-growing file.

    It creates 21 sessions: 10 log writers, 10 log followers, and one loop to make sure none of the other 20 are blocking. SIGINT should stop the program and clean up its /tmp files.

  • httpd.perl

    This program tests POE::Filter::HTTPD by implementing a very basic web server. It will try to bind to port 80 of every available interface, and it will not run if something has already bound to port 80. It will accept a new port number on the command line:

      ./httpd.perl 8080

  • ref-type.perl

    This program tests the ability for POE::Filter::Reference to use specified serialization methods. It is part of Philip Gwyn's work on XML based RPC.

  • refsender.perl and refserver.perl

    These two programs test POE::Filter::Reference's ability to pass blessed and unblessed references between processes. The standard Storable caveats (such as the inability to freeze and thaw CODE references) apply.

    To run this test, first start refserver, then run refsender. Check refserver's STDOUT to see if it received some data.

  • socketfactory.perl

    This program tests POE::Wheel::SocetFactory, a high level wheel that creates listening and connecting sockets. It creates a server and client for each socket type it currently supports. The clients visit the servers and process some sample transactions.

  • thrash.perl

    This program tests the Wheel abstraction's ability to handle heavy loads. It creates a simple TCP daytime server and a pool of 5 clients within the same process. Each client connects to the server, accepts the current time, and destructs. The client pool creates replacements for destroyed clients, and so it goes.

    This program has been known to exhaust some systems' available sockets. On systems that are susceptible to socket exhaustion, netstat will report a lot of sockets in various WAIT states, and thrash.perl will show an abnormally low connections/second rate.

  • udp.perl

    Udp shows how to use UDP sockets with Kernel::select calls.

  • wheels.perl

    This program is a basic rot13 server. It is a basic test of the whole premise of wheels.

  • wheels2.perl

    Wheels2 shows how to use separate input and output filehandles with wheels. It's a simple raw tcp socket client, piping between a client socket and stdio (in cooked mode).

Object Layer Examples

This program illustrates POE's Object Layer, which is still in early development.

  • olayer.perl

    This program demonstrates some of the features of the early Object Layer implementation. It's also something of a reference standard, to make sure that the Object Layer is consistent and usable.

Proofs of Concepts

Proofs of concepts mainly show how to do something with POE. In some cases, they prove that the concept is possible, even though it wasn't considered while POE was being designed.

  • poing.perl

    Poing is a ping program that can check multiple hosts at the same time. Historical information scrolls across the screen in a ``strip chart'' fashion. It's great for listening to the seismology of your local network (no, it's not deliberately a Quake reference).

    Poing's event-driven pinger ``borrows'' heavily from Net::Ping.

  • preforkedserver.perl

    This program demonstrates a way to write pre-forking servers with POE. It tends to dump core after a while. Perl still isn't safe with signals, especially in a long-running daemon process.

    One work-around is to comment out the yield('_stop') calls (there are two). They only exist to cycle the child servers. That idea was borrowed from Apache, which only did it to thwart memory leaks. POE shouldn't leak memory, so churning the children shouldn't be needed.

  • proxy.perl

    This program demonstrates a way to write TCP forwarders with POE.

  • tutorial-chat.perl

    This program is a heavily commented ``chat'' program. It contains a running narrative of what's going on and is intended to be both functional and educational.


SEE ALSO

  • Events Layer

    POE::Kernel; POE::Session

  • I/O Layer

    POE::Driver; POE::Driver::SysRW POE::Filter; POE::Filter::HTTPD; POE::Filter::Line; POE::Filter::Reference; POE::Filter::Stream; POE::Wheel; POE::Wheel::FollowTail; POE::Wheel::ListenAccept; POE::Wheel::ReadWrite; POE::Wheel::SocketFactory

  • Object Layer

    POE::Curator; POE::Object; POE::Repository; POE::Repository::Array; POE::Runtime


BUGS

The Object Layer is still in early design and implementation, so it's not documented yet.

There are no automated regression tests.


AUTHORS & COPYRIGHTS

POE is brought to you by the following people:

Contributors

All contributions are Copyright 1998-1999 by their respective contributors. All rights reserved. Contributions to POE are free software, and they may be redistributed and/or modified under the same terms as Perl itself.

  • Artur Bergman

    Artur Bergman is <vogon-solutions.com!artur>.

    He has contributed Filter::HTTPD and Filter::Reference. His intangible contributions include feedback, testing, conceptual planning and inspiration. POE would not be as far along without his support.

  • Philip Gwyn

    Philip Gwyn is <artware.qc.ca!gwynp>.

    He has extended the Wheels I/O abstraction to allow filters to be changed at runtime. He has enhanced Filter::Reference to support different serialization methods. His intangible contributions include feedback and quality assurance (bug finding). A lot of cleanup between 0.06 and 0.07 is a result of his keen eye. His other eye's not so bad either.

  • Dave Paris

    Dave Paris is <w3works.com!dparis>.

    His contributions include testing and benchmarking. He discovered some subtle (and not so subtle) timing problems in version 0.05. The pre-forking server test was his idea. Versions 0.06 and later should scale to higher loads because of his work.

  • Robert Seifer

    Robert Seifer is <?!?>.

    He contributed entirely too much time, both his own and his computer's, to the detection and eradication of a memory corruption bug that POE tickled in Perl. In the end, his work produced a patch that circumvents problems found relating to anonymous subs, scope and @{} processing.

  • Others?

    Have I forgotten someone? Please let me know.

Author

  • Rocco Caputo

    Rocco Caputo is <netrus.net!troc>. POE is his brainchild.

    Except where otherwise noted, POE is Copyright 1998-1999 Rocco Caputo. All rights reserved. POE is free software; you may redistribute it and/or modify it under the same terms as Perl itself.

     POE - A Perl Object Environment