dovecot - Apr 2013 - Replication -- multiple users, three or more servers?

I'm running Dovecot 2.2.1 on an Ubuntu 12.04.2 server, with half a dozen
accounts for various family members.  I want to set up replication
involving at least three Ubuntu servers, with different users replicated
on different sets of servers.

For example, I might have mail for "user1" replicated on
"server1",
"server2", and "server3"...

while mail for "user2" would be on "server1" and
"server2"...

and mail for "user3" would be on "server1" and
"server3".

I've read the wiki page (http://wiki2.dovecot.org/Replication), but I'm
still confused.  I'd love to see an example that clearly shows how to
set up specific individual mail users to be replicated on a different
set of servers for each user, like what I described above.

Rich Wales
richw at richw.org

On Mon, 2013-04-22 at 11:23 -0700, Rich Wales wrote:
> I'm running Dovecot 2.2.1 on an Ubuntu 12.04.2 server, with half a
dozen
> accounts for various family members.  I want to set up replication
> involving at least three Ubuntu servers, with different users replicated
> on different sets of servers.
> 
> For example, I might have mail for "user1" replicated on
"server1",
> "server2", and "server3"...
> 
> while mail for "user2" would be on "server1" and
"server2"...
> 
> and mail for "user3" would be on "server1" and
"server3".
> 
> I've read the wiki page (http://wiki2.dovecot.org/Replication), but
I'm
> still confused.  I'd love to see an example that clearly shows how to
> set up specific individual mail users to be replicated on a different
> set of servers for each user, like what I described above.
Everything is the same as in that wiki page, except you need to have
userdb field override the mail_replica setting. Or I guess you wouldn't
want to have a default mail_replica at all, so users won't accidentally
get replicated to wrong place. See
http://wiki2.dovecot.org/UserDatabase/ExtraFields

For example with SQL something like:

user_query = SELECT home, uid, gid, \
  concat('tcp:', replicahost) as mail_replica \
  FROM users WHERE userid = '%u'