Date: Mon, 25 Jan 2010 13:46:59 +1300
From: martin f krafft <madduck@madduck.net>
To: Asheesh Laroia <asheesh@asheesh.org>
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Cc: notmuch <notmuch@notmuchmail.org>
Subject: [notmuch] Git as notmuch object store (was: Potential problem using
 Git for mail)
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also sprach Asheesh Laroia <asheesh@asheesh.org> [2010.01.21.1928 +1300]:
> >I suppose that I never actually considered merges on the IMAP
> >server side, but obviously the IMAP server has to work off a clone,
> >and that means it needs to merge.
>=20
> It's not "merge" that's unsafe; that just builds a tree in the git
> index (assuming no conflicts). It's the ensuing process of git
> writing a tree to the filesystem that is problematic.

There is no way to make that atomic, I am afraid. As you say.

> I could probably actually write a wrapper that locks the Maildir
> while git is operating. It would probably be specific to each IMAP
> server.

Ouch! I'd really rather not go there.

> Note that this mean git is fundamentally incompatible with
> Maildir, not just IMAP servers.

We had an idea about using Git to replace IMAP altogether, along
with making notmuch use a bare Git repository as object store. The
idea is that notmuch uses low-level Git commands to access the .git
repository (from which you can still checkout a tree tying the blobs
into a Maildir). The benefit would be compression, lower inode count
(due to packs), and backups using clones/merges.

You could either have the MDA write to a Git repo on the server side
and use git packs to download mail to a local clone, or one could
have e.g. offlineimap grow a Git storage backend. The interface to
notmuch would be the same.

If we used this, all the rename and delete code would be refactored
into Git and could be removed from notmuch. In addition, notmuch
could actually use Git tree objects to represent the results of
searches, and you could checkout these trees. However, deleting
messages from search results would not have any effect on the
message or its existence in other search results, much like what
happens with mairix nowadays.

I think we all kinda agreed that the Maildir flags should not be
used by notmuch and that things like Sebastian's notmuchsync should
be used if people wanted flags represented in Maildir filenames.

Instead of a Maildir checkout, notmuch could provide an interface to
browse the store contents in a way that could make it accessible to
mutt. The argument is that with 'notmuch {ls,cat,rm,=E2=80=A6}', a mutt
backend could be trivially written. I am not sure about that, but
it's worth a try.

But there are still good reasons why you'd want to have IMAP
capability too, e.g. Webmail. Given the atomicity problems that come
=66rom Git, maybe an IMAP server reading from the Git store would make
sense.

However, this all sounds like a lot of NIH and reinvention. It's
a bit like the marriage between the hypothetical Maildir2 and Git,
which is definitely worth pursuing. Before we embark on any of this,
however, we'd need to define the way in which Git stores mail.

Stewart, you've worked most on this so far. Would you like to share
your thoughts?

--=20
martin | http://madduck.net/ | http://two.sentenc.es/
=20
"reife des mannes, das ist es,
 den ernst wiedergefunden zu haben, den
 man hatte als kind beim spiel."
                                                -- friedrich nietzsche
=20
spamtraps: madduck.bogus@madduck.net

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