easy with postfix: DNS round robin (solution inside DNS Server, nearly all
MTAs are aware of this)
So DNS round robins between the two mail servers. A new mail comes into
server1, how does this mail make it to server2.
Yes this is definitely the difficultest part. It must be solved somewhere
in the LDA (cyrus). The MTA just pass mails to the LDA. So round robin
should work.
I don't see that Postgres supports multiple masters either. Circular
I'm not an expert but what about this?
http://www.postgresql.org/about/news.289
The storage has to reside somewhere. If that site goes down, both servers go
down. You either need both servers in the same site with shared storage or
figure out how to do a shared nothing backend.
It depends on how both sites are connected. Just like redundancy in
Servers/Network you can have redundant storage. I.E. with (a)synchronous
replication over IP or FC-networks.
BTW: I don't know a cluster-solution with cyrus. We use a cluster aware
commercial one.
Sascha.
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