SYNOPSIS

 use Mail::Box::MH;
 my $folder = new Mail::Box::MH folder => $ENV{MAIL}, ...;

See SYNOPSIS in Mail::Box::Dir

DESCRIPTION

This documentation describes how MH mailboxes work, and what you can do with the MH folder object Mail::Box::MH.

See DESCRIPTION in Mail::Box::Dir

DETAILS

Different kinds of folders

See Different kinds of folders in Mail::Box

Available folder types

See Available folder types in Mail::Box

Folder class implementation

See Folder class implementation in Mail::Box

How MH folders work

MH-type folders use a directory to store the messages of one folder. Each message is stored in a separate file. This seems useful, because changes in a folder change only a few of these small files, in contrast with file-based folders where changes in a folder cause rewrites of huge folder files.

However, MH-based folders perform very bad if you need header information of all messages. For instance, if you want to have full knowledge about all message-threads (see Mail::Box::Thread::Manager) in the folder, it requires to read all header lines in all message files. And usually, reading your messages in threads is desired.

So, each message is written in a separate file. The filenames are numbers, which count from 1. Next to these message files, a directory may contain a file named .mh_sequences, storing labels which relate to the messages. Furthermore, a folder-directory may contain sub-directories, which are seen as sub-folders.

This implementation

This implementation supports the .mh-sequences file and sub-folders. Next to this, considerable effort it made to avoid reading each message-file. This should boost performance of the MailBox distribution over other Perl-modules which are able to read folders.

Folder types which store their messages each in one file, together in one directory, are bad for performance. Consider that you want to know the subjects of all messages, while browser through a folder with your mail-reading client. This would cause all message-files to be read.

Mail::Box::MH has two ways to try improve performance. You can use an index-file, and use on delay-loading. The combination performs even better. Both are explained in the next sections.

An index-file

If you specify new(keep_index), then all header-lines of all messages from the folder which have been read once, will also be written into one dedicated index-file (one file per folder). The default filename is .index

However, index-files are not supported by any other reader which supports MH (as far as I know). If you read the folders with such I client, it will not cause unrecoverable conflicts with this index-file, but at most be bad for performance.

If you do not (want to) use an index-file, then delay-loading may save your day.