An idea I just had: Director basically works by assigning the backend IP address
by:
ip = vhosts[ md5(username) mod vhosts_count ].ip
The rest of director is about what happens when vhosts[] or vhosts_count
changes. What about instead doing this on IP address level?
ip = ip_pool[ md5(username) mod ip_pool_size ]
When a backend dies, you'll reassign the backend's IPs to other
backends. Each backend should have many IPs. The main restriction here is that
the IP pool cannot change without stopping the entire Dovecot. But if you
initially allocate enough IPs, that shouldn't be a problem.
And the advantage of this over the current director? To guarantee that one
director can't break others, because they don't need to communicate with
each others.
The disadvantage of course is that it's a little less flexible and requires
more planning ahead. The IP address reassignment would also need some
distro-specific scripts.
This could be implemented as an alternative director-lite or something. The
doveadm director status-related commands could still work with it.
