[Dovecot] Replication protocol design

[Asheesh Laroia]

On Tue, 29 Apr 2008, Timo Sirainen wrote:

> I'll probably be implementing multi-master replication this summer. My
> previous thoughts about it are here:
> http://dovecot.org/list/dovecot/2007-December/027284.html
>
> Below is a description of how the replication protocol will probably
> work. It should work just as well for master-slave and multi-master
> setups. Comments welcome. I'll write later a separate mail about how
> it'll be implemented to Dovecot.

(I'm CC:ing tv since he and I once chatted about IMAP replication.)

Multi-master will be very interesting.  I'm really curious what you will 
do as far as this scenario:

1. A mail arrives in server1
2. User reads it (therefore the mail has a UID assigned)
3. server2 gets isolated from server1
4. A mail arrives at server2
5. User logs in to server2 and reads it (therefore the mail has a UID 
assigned)
6. server2 and server1 are connected again, and are told to sync

Questions:

a. Does the client get told two different messages have the same UID in 
the folder?

b. What is the graceful sync proposal?

Oh, look - you mention that right at the end of your protocol:

> A special case is when two servers have been saving messages 
> independently from each others. In this case it's possible that the 
> servers have used the same UIDs for different messages (different global 
> UIDs). These need to be resolved by giving both conflicting UIDs new 
> unused UIDs, otherwise IMAP clients may show them as wrong messages from 
> their caches.
>
> FIXME: If the other server had expunged a conflicting UID it still 
> should be given a new UID. How do we find out this has happened?

I guess this doesn't address my Question (a), though.

The above scenario may be handled by your root-based election process - 
but what prevents two roots from simultaneously existing, and therefore 
two masters existing, and then the above scenario happening?

I'm really interested to see where this goes - I think this would be 
really swell as a replacement for my use of offlineimap.  I also want to 
have you consider letting non-root users use Dovecot replication features; 
that way, I could just have a cron job that asks my laptop's Dovecot to 
replicate from my server's Dovecot, without giving it any special 
permissions.

Thanks for all the great work Dovecot represents!

-- Asheesh.

-- 
I joined scientology at a garage sale!!