[Imap-use] replication

[Joel Reicher]

> > I think you need to be more specific about the problems you are trying to
> > solve. The suitability of a solution depends on details.
> 
> Basically a failover mechanism with 1 or more mirrors of the email 
> server, possibly on 2 different locations. One could seamlessly switch 
> from one to the other and have no loss of emails (where emails are on 
> one but not on the other).

There's no such thing as an "email server". If you mean IMAP server, then
the problem you describe above does not exist for a setup of one IMAP
server for a user. It is only a "solution" like replication which would
introduce the problem.

If, on the other hand, you mean a mail exchanger, then the "problem" you
describe above is actually a solution. If the ultimate destination of
an email is unavailable, then the exchanger attempting to deliver the
email will hold it and retry later. There could be any number of
exchangers waiting to retry, anywhere in the world.

But perhaps you're talking about potentially out-of-sync multiple
instances of the ultimate destination. Again, this is a problem that would
be introduced by replication, and is absent without it.

> > I think so, yes. Firstly, if you can't make this kind of failover work,
> > why do you think IMAP replication would have anything to offer?
> 
> Because in the case of replication the failover machine could take over 
> all email activities, including storing it in mailboxes.

What makes you think you need replication for this, as opposed to all of
the other mechanisms that are available?

> > Secondly, while I'm not entirely sure what you mean by "smarthost", I can't
> > see why you think the main machine needs to be available. The backup
> > machine(s) can just poll it.
> 
> By smarthost (perhaps it is a Debian term) I mean one mailserver which 
> can receive email for another (primary) email server and sends it over. 
> It generally does not store email locally. You have a point that the 
> backup server can queue the email until the main one is up again.

Smarthost is not a Debian term, I just don't see what mail transfer has
to do with any of this. I thought we were only talking about IMAP servers.

When it comes to mail transfer, failover is built into the protocol. That's
why MX records have priorities, and why there can be multiple records.

> > that needs to be kept in sync, and if the storage mirroring is being
> > done at the storage unit, as is the case with hardware RAID, it won't
> > matter which machine does the IO.
> 
> I guess you refer to something like NAS. I would think those use 
> protocals like NFS or SMB, which do not work well with imap. If you 
> refer to SAN (annoying abbreviations) that would work but I guess that's 
> overkill. I figure it also costs too much.

Yes, I'm referring to SAN, but not to NAS. IMAP already does NAS; that's
the point of it.

The reason not to use NFS or SMB is not so much that they don't work well
with IMAP, but because an NFS server has nothing to offer over an IMAP
server. Any potential failure for an IMAP server is a potential failure for
an NFS server, so there is no point trying to cover the former by using
the latter. If anything, it makes the problem much worse.

Cheers,

	- Joel