[Dovecot] dsync redesign

[Michescu Andrei]

Hello Timo,

Thank you very much for planning a redesign of the dsycn and for opening
this discussion.

As I can see from the replies that came until now everybody misses the
main point of IMAP: IMAP has been designed to work as a disconnected,
high-latency data store.

To make this more clear: once and IMAP client finishes the synchronization
with the server, both have client and server have a consistent state of
the mailbox. After this both the "client" and the "server" act like master
for their own local copy (on the "server" new emails get created etc, on
the "client" existing emails get changed (flags) and moved, and new emails
appear (sent items)).

So the protocol is designed, originally, to handle the master-master
replication. And as this it make sense a deployment global-wide, where
servers work independently and from time to time they "merge" the changes.

This being said and acknowledged here are my 2 cents:

I think that the current '1 brain / 2 workers' seems to be the correct
model. The "the client" connects to the "server" and pushes the local
changes and after retrieves the updated/new items from the "server". "The
brain" considers first server as the "local storage" and the second server
as "server storage".

For the split design, "come to the same conclusion of the state" is very
race-condition prone.

As long as the algorithm is kept as you described it in the original
document then the backups should really be incremental (because you only
do the changes since last sync).

As the most changes are "metadata-only" the sync can be pretty fast by
merging indexes.

Thank you,
Andrei


> In case anyone is interested in reading (and maybe helping!) with a dsync
> redesign that's intended to fix all of its current problems, here are some
> possibly incoherent ramblings about it:
>
> http://dovecot.org/tmp/dsync-redesign.txt
>
> and even if you don't understand that, here's another document disguising
> as an algorithm class problem :) If anyone has thoughts on how to solve
> it, would be great:
>
> http://dovecot.org/tmp/dsync-redesign-problem.txt
>
> It only deals with saving new messages, not expunges/flag changes/etc, but
> those should be much simpler.
>
>
> !DSPAM:4f6cea4c260302917022693!
>
>