
=head1 NAME

Mail::Box-Overview - objects used by Mail::Box

=head1 DESCRIPTION

=head2 Introduction

The MailBox package is a suite of classes for accessing and managing
email folders in a folder-independent manner. This package is an
alternative to the Mail::Folder and MIME::* packages. It abstracts the
details of messages, message storage, and message threads, while
providing better performance than older mail packages. It is meant to
provide an object-oriented toolset for all kinds of e-mail applications,
under which Mail User-Agents (MUA) and mail filtering programs.

This package is modular --parts of it can be used independently of the
rest. For example, the Mail::Box::Manager can automatically determine
that a folder is in Mbox format and return an object of the
Mail::Box::Mbox class, or the user program can bypass the manager and
create Mail::Box::Mbox objects directly. Similarly, if the user program
is only manipulating a single message, a Mail::Message.

The Mail::Box package has special features to help MUA's access folder
data quickly in random order.  You will not really benefit (neither
slower) if you need the full folder sequentially.

This code is beta, which means that there are no serious applications
written with it yet.  Please inform the author when you have, so this
module can be marked as stable.  Read the STATUS file enclosed in the package
for more details.  You may also want to have a look at the sample
scripts in the C<scripts> directory.

=head2 The class relations

Mail::Box::Manager objects play a central role in any program which
is built with MailBox.  Each program will create one manager, and
then open folders via that manager.  Besides folders, the manager can
also be used to discover message threads: sequences of messages with
their follow-ups.

                       <has-a>      Mail::Box::Mbox
  Mail::Box::Manager <---------*    (Mail::Box::MH)
         ^                :         (Mail::Box::Maildir)
         |           (maintains)    (Mail::Box::POP3)
         |                :
         |                :
         `---------------------*  Mail::Box::Thread::Manager
                      (<has-a>)


Each folder maintains a list of messages.  Much effort is made to hide
differences between folder types and kinds of messages.  Your program can
be used for MBOX, MH, Maildir, and POP3 folders with no change at all (as
long as you stick to the rules).

 Mail::Box::Mbox  <-----------* Mail::Box::Mbox::Message
        ^             <has-a>            ^
        | <isa>                          | <isa>
        |                                |
    Mail::Box     ............. Mail::Box::Message
                                         ^
                                         | <isa>
                                         |
                                   Mail::Message
                                        / \
                                       <has-a>  
                                      /     \
                           Mail::Message   Mail::Message 
                             ::Body            ::Head

The situation for MH and Maildir folders is a little more complicated,
because they have an extra intermediate level of abstraction: Mail::Box::Dir.
The POP3 folder has an intermediate Mail::Box::Net.

In the future, when more Mbox-like folder types get implemented, there
may be a Mail::Box::File level too.  The following is also true
for the mail boxes

 MB::MH::Message                 MB::POP3::Message
       \  MB::Maildir::Message            /
        \         /                      /
         \       /   MB::Mbox::Message  /
          \     /         |            /
        MB::Dir::Message  |   MB::Net::Message
                     \    |    /
                      \   |   /
                      MB::Message
                          |
                          |
                     Mail::Message


=head2 The Manager:  Mail::Box::Manager

The mailbox manager encapsulates folder management issues. It maintains a
set of open mail folders (mailboxes), and provides methods for opening and
closing them, efficiently moving messages between folders, and
efficiently appending messages to folders.  It contains Mail::Box objects
which may be of different types.  Most folder types can be detected
automatically.

The main manager also manages message-thread detector objects, and informs them
when the contents of a folder have changed. This manager class is the
only one you instantiate yourself: objects of all other classes will be
provided by your folder manager.

You are strongly advised to use this object, but you can often do without it
and open a specific folder-type directly.

=head2 The Messages

=over 4

=item Mail::Message

A base class that defines an interface for manipulating the head and
body of a message.  There are various header object types
(Mail::Message::Head's) and a bunch of body object types
(Mail::Message::Body's).  

The Mail::Message::Construct package is loaded when more complex tasks
have to be performed on messages, like creating replies, bounces, or a
forward message.  These
functionalities are described and implemented in the ::Construct
file, but are automatically added to the Mail::Message namespace when used.

Message types which are foreign to MailBox can be used in the
MailBox environment: there are some converters implemented via
Mail::Message::Convert.  Particularly the popular Mail::Internet
and MIME::Entity are supported.

=item Mail::Box::Message

An abstract base class which defines an interface for mail messages
which are stored in any folder.  It inherits from Mail::Message, and
adds the basic idea of I<location> to a message.

=item Mail::Message::Body

This is the base class for all message bodies.  It describes what you
can do with any kind of body.  The body types differ on the way how the
keep the body content during the run of your program.

One special case of the body types is the Mail::Message::Body::Multipart, which
contains a set of Mail::Message::Part objects.  These are just like
normal messages, except that they are contained in an other message.
The Mail::Message::Body::Nested body type is comparible, but contains only
one message: they are used for C<message/rfc822> message encodings.

When needed, the functionality of the body objects is extended with
Mail::Message::Body::Construct and Mail::Message::Body::Encode.  The
former package implements things like concatenation, the later controls message
encoding and decoding.  In the current implementation this is limited to
transfer encodings (implemented in the Mail::Message::TransferEnc packages).
Automatic character and mime recodings are on the wish-list.

=item Mail::Message::Head

The header for a single message. Maintains a set of Mail::Message::Field
objects, each containing one header line.  Fields are the only
objects which have no logging and tracing facilities, purely for reasons
of performance.

The header object has three sub-classes: the Mail::Message::Head::Complete
version knows all lines for sure, Mail::Message::Head::Subset maintains
an unknown subset of lines, and the Mail::Message::Head::Delayed has no
lines yet but knows where to get them.

The latter two will automatically get the missing header lines from the
mailbox files when needed, and so transform into a C<::Complete> header.
It is fully transparent to the user of MailBox in which shape the
header really is on the moment.

=back

=head2 The Folder types

=over 4

=item Mail::Box

A base class that defines a standard interface for mail boxes which
is independent of mailbox type. Objects of this class contain
a Mail::Box::Locker and a list of Mail::Box::Message objects.

=item Mail::Box::Dir

The base class for all folders which use a directory organization: each
message is a separate entity (file) grouped in a directory.  Each
Mail::Box::Dir::Message represents one message, one such entity.

=item Mail::Box::Net

The base class for all folders which have the messages outside direct
reach of the MailBox library, for instance on a remote system, or in
a database.

=item Mail::Box::Mbox

This class derives from Mail::Box, and implements its interface for
mbox-style folders. It maintains a set of Mail::Box::Mbox::Message
objects, which are derived from a Mail::Box::Message.

Mbox-style folders have one file containing multiple messages per
folder.  When folders get large, access tends to get slow.

=item Mail::Box::MH

This class derives from Mail::Box::Dir, and implements its interface for
MH-style folders. It maintains a set of Mail::Box::MH::Message
objects, which are derived from a Mail::Box::Dir::Message.

MH-style folders are represented by a directory, where each message is
stored in a separate file.  The message files are sequentially numbered.
It is fast to open one single message, but hard to get an overview.

=item Mail::Box::MH::Index

The base class for MH mailbox indexes which provides methods for reading,
writing, and managing message indexes.  These indexes are used to
speed-up access to directory based folders.

=item Mail::Box::MH::Labels

Also for efficiency reasons, a separate file is maintained which contains
flags about the messages.  This file for instance lists new files.  This
way, the MH message files do not have to be opened to find that out.

=item Mail::Box::Maildir

Like the MH folder type, this class derives from Mail::Box::Dir.  It
implements its interface for Maildir-style folders. It maintains a set
of Mail::Box::Maildir::Message objects, which are derived from a
Mail::Box::Dir::Message.

=item Mail::Box::POP3

Implements the POP3 protocol based on Mail::Box::Net.  The
Mail::Transport::POP3 implementation handles the protocol details.  In this
kind of folders, you can only read and delete messages.

=back

=head2 Various Other Classes

=over 4

=item Mail::Box::Thread::Manager

Maintains a set of message-threads over one or more folders.  A
message-tread is a start message with all the replies on it.  And the
replies on replies, and so on.  This object is used to construct the
thread for a set of open folders.

This object maintains linked lists of Mail::Box::Thread::Node objects.
Mail::Message::Dummy's fill-up some holes.

=item Mail::Box::Locker

Provides a folder locking interface which is inherited by the Mail::Box
class.  Currently it supports dot-file locking (C<filename.lock>),
flock filehandle locking, and locking over NFS.  Each is implemented in a
separate class.  A multi-locker, using a set of lock-methods at the
same time is also available.

=item Mail::Box::Search

The set of search packages implement various search techniques in an uniformal
way.  Although implementing your own search algorithm is simple in general,
in practice multiparts, encodings, and mime-types complicate things.

=item Mail::Box::Parser

The parser reads messages, and transforms them into data-structures such
that the content of header and body can be used within the program.  The
first parser is implemented in pure Perl.  A second parser is under development,
and will written in C, to gain speed.

=item Mail::Box::Tie

Provides hash (Mail::Box::Tie::HASH>) or array tied (Mail::Box::Tie::ARRAY)
access to any mail folder derived from Mail::Box.  This beautifies your code
in some applications.

=item Mail::Transport

Various ways of sending and receiving messages are implemented.  Sending
is possible via external programs, like C<mail>, C<Mailx>, C<sendmail>, or
autonomously with direct SMTP.  Receiving is currently only implemented
via POP3.

=item Mail::Reporter

A debugging and logging class which is inherited by most of the Mail::*
modules.  For each object, you can say what log and error reports must be
kept or directly presented to the user.  This way you can decide to have
Mail::Box report about problems, or do it all yourself.

=back

All classes are written to be extensible.  

=head1 SEE ALSO

A good start to read is Mail::Box-Overview.
More documentation and a mailinglist are available from the project's
website at L<http://perl.overmeer.net/mailbox/>.

=head1 AUTHOR

Written by Mark Overmeer (L<mark@overmeer.net|mailto:mark@overmeer.net>)
with the help of many.  See the ChangeLog for details.

=head1 VERSION

This code is beta, version 2.037.

Copyright (c) 2001-2003 by the authors. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
