$Id: FAQ.Two_Ways_Sync.txt,v 1.4 2021/02/01 15:43:33 gilles Exp gilles $

This documentation is also available online at
https://imapsync.lamiral.info/FAQ.d/
https://imapsync.lamiral.info/FAQ.d/FAQ.Two_Ways_Sync.txt


=======================================================================
             Two ways sync with Imapsync 
=======================================================================


Questions answered in this FAQ are:

Q. Can Imapsync do a good "two ways" sync? 
   No. Why?


Now the questions again with their answers.

=======================================================================
Q. Can Imapsync do a good "two ways" sync? 
   No. Why?

R. Imapsync can't do good two ways syncs.

A good "two ways" sync is impossible with imapsync because imapsync is
stateless.

Each time imapsync runs, it considers messages and folders as if it
were the first time it encounters them. Imapsync looks at messages,
flags, and folders as they are now, not considering what they were
before. Imapsync has no memory outside the current running sync.

So now, why a stateless behavior cannot handle well a two ways sync
between an account A and an account B?

The problem arises with deletions, messages deletions, folders
deletions, or movings, messages movings across folders, folders
movings, and also folders renamings. Deletions and moves are ambiguous
changes when combined with creations on the opposite side.
 
For example, when a message is deleted from A by a user, imapsync
cannot know whether it is a message deleted from A that has to be
deleted in B (what the user did) or a missing message from B that has
to be copied to A.

But if you know the answer yourself, that missing messages on one side
A are deleted messages that have to be deleted on the other side then
run a sync with the --delete2 option from A to B.

If you know that the missing messages on A are missing messages from B
that has to be copied to A then run a sync from B to A.

If you know it's a mixed scenario then you are in trouble and so you
end up with a not very good "two ways" sync. I suggest avoiding
deletions in that case, which is the default imapsync behavior.

With a two ways sync, the mailbox user is very surprised and
disapointed when his actions (deletions, renamings, or movings) come
back.

=======================================================================
=======================================================================