[Dovecot] Replication protocol design

[Timo Sirainen]

On May 1, 2008, at 3:23 PM, Ed W wrote:

>> 2) Sending updated flag/keyword changes can't be done in a standard  
>> way,
>> because it only shows the last flag+keyword state, not the changes  
>> that
>> were done (e.g. "\Seen" vs. "+\Seen -\Flagged").
>>
>
> Hmm... I guess that can only be done anyway by storing the state  
> before and after and figuring out the changes based on a comparison?

Dovecot stores flag changes as "added flags" and "removed flags" in  
transaction file, so it doesn't need to do any comparing to figure out  
what had changed. This makes the flag changes also more reliable. For  
example if a message originally had flags (\Flagged) and then two  
servers changed them:

S1: STORE 1 +FLAGS \Answered
S2: STORE 1 +FLAGS \Seen
S2: STORE 1 -FLAGS \Flagged

If replication protocol sent the changes as +flags -flags, it would be  
unambiguous what the final flags are: (\Answered \Seen).

If replication protocol instead sent the flags as their currently  
known flag states (as IMAP protocol does):

S1: * 1 FLAGS (\Answered \Flagged)
S2: * 1 FLAGS (\Seen)

There aren't any good ways to figure out what the wanted final flags  
are supposed to be.

> I do like the idea of making this more generic and hence hackable  
> than writing all the code into dovecot itself.  Perhaps we could  
> start with an external proxy app at each end of the link which is  
> external to the imap server, ie basically start with IMAP sync.

That would work for the mailbox synchronization part, but I'm more  
interested in the incremental synchronization part which replicates  
all changes in all mailboxes immediately. That's not really possible  
to base on an external proxy. Mostly because the IMAP protocol  
supports seeing changes only in a single mailbox at a time, and trying  
to change that would most likely make the protocol different enough  
from IMAP that there's not much point in using IMAP as a base anymore.
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