At 12:48 -0400 2003/06/29, Greg A. Woods wrote:
>[ On Sunday, June 29, 2003 at 17:03:21 (+0100), Giuliano Gavazzi wrote: ]
>> Subject: [Exim] mirror MX
>>
>> I am thinking of implementing a mirror MX setup where two servers,
>> spatially separated, will both handle and have a copy of the email
>> for their domains (all other services will be also handled this way,
>> but this is not an exim problem and it is anyway easier to implement).
>
>How do you propose they keep their mailboxes in sync?
that is the question. The mailboxes are not going to be in sync, and
this is because users are going to access only one of the two servers
to retrieve their email. Sync-ing could only be done safely at a time
when no mail is waiting to be transferred from one server to the
other. So the sync-ing should first stop the mail servers, then check
the queues and finally synchronise the secondary server to the main
one (that also offers pop/imap)
My main concern is to be able to keep all incoming mail in duplicate,
so that if a server has a hard failure new mail is not lost (except
for what was being delivered at the time of failure) or even if the
main server goes off-line for several hours, one could still access
email logging in the other one.
I suppose that when one of the two server goes off line, the other
will keep the copy emails in the queue for delivery. A potential
problem would be if the main server is off-line for so long that the
secondary will give up delivering the copies, as the corresponding
emails would be lost at the next sync with the primary. On could set
it so that a server that has been off-line over four days (or
whatever) would lose its primary status.
Clearly a RAID would be a simpler solution, but not necessarily as
flexible, in particular when the two servers are 1500 Km apart and at
one end there are not really technically minded people..
Once in place I would calle such a system "paranoia".
Giuliano
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