IMAP Email
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IMAP Email Summary
As I may have mentioned before, I use Courier, and its worked really well for quite some time.
I've been thinking about how the heck to get IMAP email dual-homed for redundancy, but I really couldn't think of much. A few possibilities do exist however:
- failover to active backup
- failover to read-only backup
- other ideas
IMAP is great because IMAP clients can synchronize to a primary server. However, synchronizing two IMAP servers isn't so easy. There are some IMAP mirror tools, some maildir mirror and sync tools, but they run into the same problems as most synchronization systems. No doubt about it, synchronization is a tough nut to crack.
IMAP Servers
IMAP Failover to active backup
This one is tough because the two servers could easily find their way into a "split brain" scenario, where there are conflicting changes on both servers, and neither knows which one is primary.
IMAP Failover with read-only access
This one is good, because it provides access while the primary server is down, but that isn't too great, most people want to access their email to get new mail.
Courier IMAP has pretty smart acl capabilities with the maildiracl command. My plan is to use it this way:
maildiracl -set .maildir INBOX owner arl
And then:
find .maildir -name 'courierimapacl' -exec cp .maildir/courierimapacl {} \;
I've only tried this out once, so I have no idea if it really works or if it will cause major problems. UPDATE - Just tried it out a little bit more, and it actually works OK. With Mozilla Thunderbird and Roundcube, you can access the IMAP server without any problems. If you try to make any changes, you get a reasonably decent error message. If I setup a redundant or failover SMTP server as well, the mail client can still send outgoing mail, but will get an error about the email not being able to be copied to the sent messages folder.
And while I was doing that, I remembered one of the problems with this setup which pretty much requires it to be read only. If I use rsync to maintain the failover backup along with the scripts above to set the folders as read only, I would probably want to do a "full" (meaning destructive) rsync. In other words, if the user deleted a message off the main server, it would get deleted off the failover server as well.
In the case of the failover, it might make sense to keep the "Sent" folder writable, continue to do destructive synchronizations on it, but attempt to keep track of any messages that we sent out during the outage for uploading to the primary server once it is back online. And since I would have control of the backup mx / failover smtp server, it might be doable. I had thought of trying to get the failover smtp server to automatically forward the outgoing messages back to the source address so that it would automatically queue for delivery to the final destination, but I can't figure out how to do that, and its not really optimal since it would end up in the inbox, not the sent messages folder.
IMAP Redundancy
Domain Aliases
Google suggested a cool idea for email redundancy: to create a domain alias, and forward all incoming mail to the domain alias, on another server. That is helpful for archival and transitioning purposes, but it doesn't do much for synchronization. You could presumably extend upon this concept, and with the help of rsync, do a decent job of faking it.
OfflineIMAP
I've been manually experimenting with IMAP synchronization and so far I'm happy with the results, though I'm far from automating the process. In concept, I'd compare it to Subversion synchronization and MySQL Replication, where there is one primary storage volume, and one or more secondary storage volumes. With regular, non-sync'ed Subversion, the checkouts can commit their changes back to the primary repository, and this is what I'd like to achieve with IMAP synchronization. While I'm currently doing this manually, I believe that OfflineIMAP can run as a daemon monitoring for any changes. This idea leads me to wonder about using the IMAP IDLE extension for triggering synchronization.
IMAP IDLE
What is IMAP IDLE? It is a notification protocol. I might compare it to the notify kernel module, as opposed to the polling technique sometimes used for monitoring.