POEST::Plugin - Details of, and Base Class for Writing a POEST Plugin


Details for writing a POEST Plugin. The interface and prescribed conventions are detailed.


poest plugins are classes. With the exception of the EVENT(), and new() methods listed later in this document, all other methods will be posted from the POE kernel. You'll need to read some documentation on POE if you don't know what I'm talking about.


Plugin naming and purpose

The plugin heirarchy has some naming conventions. It is highly recommended that you follow them. If you find yourself doing something new and unique, it is imperitive you contact the author of poest, Casey West, and let him know what you're doing. There is a good chance it's really cool and you should share, plus, he can help with naming.

The following rules apply.

NOTE: The following is still fuzzy. A clear definition of rules of interaction must be created!


Plugins that accept incomming email. These plugins will usually handle at least the MAIL, RCPT, DATA, and gotDATA events. They should take in the message data and create a Mail::Internet object from it. Next, that object must be handed off to the Queue.


Plugins in this space handle message queues. There is no set way for a queue to handle it's messages, just so long as it tries to get them out the door. By out the door, I mean that at some point messages should be passed to delivery agents.


These plugins deliver the mail. Novel, I know. There is no set place these messages should be delivered to, so have a blast.


Plugins under this category do a lot of checking conditions to see if everything adds up. They're mostly used to determine if everything is OK. When things don't go well, these plugins will intercept the calling stack and return an error or something.


General commands should go here. This includes things such as send_banner, HELO, and so on.


These plugins implement special features that don't really have to do with the straigt forward process of delivering mail.

Base class POEST::Plugin

This class can be used as a base class for other plugins. Granted, if you're sub-classing some higher-level plugin, you know that you're sub-classing POEST::Plugin behind the scenes.

Sub-classing is simple.

  package POEST::Plugin::Queue::mysql;
  use POEST::Plugin;
  @ISA = qw[POEST::Plugin];

This will give you the default constructor for configuring your plugin, as well as import the constants required for POE, and POE::Component::Server::SMTP. It will also import carp and croak from Carp.

Required Methods


This method returns the events that the plugin will handle. The most simple thing to return is a list reference contianing the names of the events. each named event will corrispond directly to the name of a method in the plugin. The other format is to return a hash reference with the name of events as keys and the method names in the plugin as the values.

There are several ways to define this method. The most cost effective is an inline sub.

  sub EVENTS () { [ qw[MAIL RCPT DATA gotDATA] ] }

Another way is using the constant pragma.

  use constant EVENTS => {
    MAIL    => 'smtp_mail',
    RCPT    => 'smtp_rcpt',
    DATA    => 'smtp_data',
    gotDATA => 'smtp_got_data',

Both forms are valid, though the first form will be used as much as possible in the official distributions.

The plugin will not be given any events to handle except the ones it specifies. Event names are case senstive.

A plugin should document all the events it plans to handle, as well as any special cases that may arise in unforseen flow of execution. In other words, anything clever must be documented.

NOTE: There should be a good way of specifying that 'all' events should be handled.


This method returns the configuration parameters required by the plugin. Each parameter you want must be listed, it will be returned as a list reference. By default, this base class provides a CONFIG() method that returns an empty list, indicating that no configuration parameters are needed. Only the configuration parameters requested will be passed to the plugin upon initialization using the new() constructor.

Just as EVENTS(), there are several options for declaring this method. Inline subroutines or the use of the constant pragma are acceptable. Here is an example.

  sub CONFIG() { [ qw[hostname port aliases] ] }

A plugin should document all the configuration parameters it expects to find. The syntax for the value, and any outstanding issues, such as external files, that must be taken into consideration.

NOTE: There should be a good way of specifying that 'all' configuration parameters are required.


This is the constructor for the plugin. Since all plugins are classes, it's important to get an object instance. In most cases, new() will be called when poest is started. new() will be passed a hash (key, value pair) of configuration options and is expeted to return an object.

If your class doesn't supply new(), and you're sub-classing as you should, a generic new() is provided for you by POEST::Plugin. It looks just like the example below.


  sub new {
    my ($self, %args) = @_;
    return bless \%args, $self;

If you want to do the same thing as the example, with just a little more functionality, save yourself some work with this fun trick.

  sub new {
    my $self = shift->SUPER::new( @_ );
    # .. do initialization ..
    return $self;

new() should check the validity of the configuration parameters passed to it. During copious and thourough validation, if something is wrong the plugin should throw an exception. This is a good thing, as it will happen on server startup and may be logged and dealt with appropriatley.


Finally, the only other required methods are the ones corrisponding to the events the plugin reports it can handle, via the EVENTS() method.


  sub HELO {
    my ($kernel, $heap, $self, $session, @args) =
      @_[KERNEL, HEAP, OBJECT, SESSION, ARG0 .. $#_];
    my $client = $heap->{client};
    $client->put( SMTP_OK, "Welcome." );

Life Enhancers


In thinking about Plugins and how people would like to write them, I came to the conclusion that not a lot of people like the POE state interface of taking slices on @_. Even though the current method is the fastest Perl allows, some people want syntactic sugar, enter POE::Sugar::Args. This module exports a single function that should take some of the headache away. Here is example usage.

  sub HELO {
    my $poe = sweet_args;
    $poe->heap->{client}->put( SMTP_OK, "Hi there!" );


Casey West, <>


Copyright 2003

You may distribute this package under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file, with the exception that it may not be placed on physical media for distribution without the prior written approval of the author.


For more information, please visit


perl, POEST::Server, POE, POE::Session::MultiDispatch, POE::Component::Server::SMTP, POE::Session, POE::Kernel.