Re: Disconnected mode (was: Devin's effort)

--On Monday, April 20, 2015 07:08:20 -0400 Tanstaafl
<[email protected]> wrote:

> On 4/19/2015 1:20 PM, John C Klensin
> <[email protected]> wrote:
>> if anyone knows of an MUA that handles Disconnected mode well
>> and has good support for other MIME and email features, I, at
>> least, would like to know about it. FWIW, the last time I
>> looked at Thunderbird, it didn't even try.
> 
> Any chance you can elaborate on what specific features of
> disconnected mode you use/need, and or the problems you had
> with it?

I think this thread has gone on long enough, especially with the
tone of some of the comments and noting Devin's comment about
"low volume".  So I'll try to briefly answer your question but
will not post further on the subject.

Contemporary IMAP has three modes of operation: 

(1) Online.  This is inherited from the original IMAP design,
with no messages stored on the client machine between sessions.
A somewhat more extreme form in which not even configuration
files are maintained on the client machine is described in some
Mulberry and other materials as "kiosk" mode.  A proper
implementation of online mode allows looking at a TOC without
downloading messages (even into a cache) and deciding what
messages to look at.  A competent implementation of online mode
for MIME messages allows looking at message structure without
downloading all of the message and making selective decisions
about what body parts to download and read.

(2) Offline.  This one is more or less derived from POP and
POP3, and involves downloading all messages to the client
machine and then accessing them locally.   As far as I can tell
from doing a little searching and reading, Thunderbird supports
"Synchronize all messages locally regardless of age" and
"Synchronize the most recent ___ days", both of which are
consistent with offline mode.

(3) Disconnected.  Inspired by some important features of PCMail
(Cf RFC 1056) and closely related to online mode with the
potential for selective storage of TOCs, messages or parts of
them, and then working offline and synchronizing when one comes
back online.  The Mulberry implementation has a number of
disadvantages and incomplete features.  For example, it does not
allow downloading a complete folder TOC and message structure
information without any message content and selection of what
should be downloaded the next time one is online.  If one looks
at a message (or body part) while online and then decides to
synchronize it, that message must be downloaded a second time
rather than retrieved from a temporary cache.   On the other
hand, it works.  As soon as someone says "the clients has to
download all messages (or even parts of all messages) before
proceeding to do work online" or "the best way to implement
disconnected mode is to maintain a local IMAP server that can
access a complete copy of the mailstore on the local machine",
then the discussion is really about offline mode (or a peculiar
version of online) and not disconnected mode.

Note that, if I have multiple IMAP accounts, some on the same
server as others and some completely separate, proper
implementation of disconnected mode (as well as good sense)
requires being able to selectively open (or not) one mailbox and
folder at a time, select some for synchronization (upload and/or
download) and others not, etc.  Mulberry does not, IMO, get that
quite right, but it is _really_ close.

> Thunderbird does indeed have a disconnected mode, so it
> apparently does 'try', but I've never really had a need for it
> so I haven't ever really even messed with it.

When last I checked, what it did allowed far too little user
control over what was actually downloaded (whether that user
control had to be exercised while connected, as with Mulberry,
or can be also exercised while disconnected as the spec intends)
to be described as disconnected mode.  As to the need,
requirements and preferences differ, but the need for
disconnected mode becomes clear to some of us (including some of
us who pushed for its inclusion in IMAP4) if one is working on a
relatively small machine with limited local storage, memory, and
processing power and/or regular or intermittent use of Internet
connections that make every kilobyte passed over the Internet
significant.  

As as trivial example, I've used Mulberry successfully on a
Windows-based netbook with around 16Gb of SSD, organized into
two physically separate 8Gb pieces.   Works fine in online mode
although automatic downloading and caching of large attachments
would kill it.  Also works fine disconnected if one is _very_
selective about what one downloads and retains locally.  The
same features keep its network footprint small if one is being
charged by the kilobyte (or even by the megabyte).  Notions of
keeping the whole mailstore (or even the whole inbox) locally,
even using IMAP offline synchronization to get it there, are
non-starters -- I'd be better off using some flavor of webmail
even though, as webmail clients get prettier and more clever,
one doesn't want them around a restricted or expensive Internet
connection either.

Thunderbird?  Nah, although not as bad as Outlook.   On the
other hand, Outlook is a non-issue because I haven't even been
able to install Office on a machine of that size.

And, before you ask, the answer to "why bother with a machine
like that" is "small, light, rugged, and very inexpensive".

   john