[Dovecot] [ Re: best practises for mail systems]

[Matthias-Christian Ott]

On 2012-06-05 17:33, Michescu Andrei wrote:
> The fear of lossing the imap session does not make sense (at least to me)
> as the client will reconnect automatically in the background.

I agree, in practice this is not an issue compared to the unavailability
of the service, but on longer IMAP sessions (e.g. transferring a big
file) the connection loss is noticeable.

> Like this you have no SPOF and no split-brain and you get the flexibility
> (if needed) to geographically distribute your servers in the the future.
> 
> Keep each server with its own ip, connect to them via DNS (round robin etc
> etc).

This depends on the resolver, operating systems and clients you want to
support, because I read that not all networks generate proper
ICMP/ICMPv6 Destination Unreachable messages and instead simple drop the
packets, so that the clients first try to connect to the failed server
until timeout and then connects to the second server. Since IMAP is a
stateful protocol the latency of the initial connect to the failed
server can be ignored, but if you want to eliminate this, you can use
dynamic DNS to automatically remove the corresponding RRs (depending on
your situation you need an external monitoring server for this to avoid
problems in case of net splits).

> We are currently experimenting with a setup similar to this one, but with
> geographically distributed servers (trans-continental) (bandwidth limited
> and high cost).

I also have some plans for a similar setup in the near future. Can you
share your results on the mailing list? I'm especially interested if
failover via DNS works in practice (I did some searches, but I'm not
fully convinced of it, but it seems quite simple compared to other
solutions).

Regards,
Matthias-Christian