IMAPsync/FAQ.d/FAQ.Two_Ways_Sync.txt

$Id: FAQ.Two_Ways_Sync.txt,v 1.8 2022/07/25 14:03:49 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


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================  Two ways sync with Imapsync =========================
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Q. Can Imapsync do a good "two ways" sync? 
   Short answer: no, not a good one. 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 the current. 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 with moves, messages moves across folders, folders
moves, and also folders renamings. Deletions and moves are ambiguous
changes when combined with creations on the opposite side.
 
For example, if a message is deleted from A by a user, then imapsync
cannot know whether it is a message deleted from A that has to be
deleted in B (what the user actually did) or a missing message on A
that has to be copied from B.

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. This assumes there 
is no new messages on B because --delete2 will delete them.

If you know that the missing messages on A are 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, that there are some deletions/moves
on A, and there are some deletions/moves on B, but not the same, then
you are in trouble and it ends up with a not very good "two ways"
sync.  In that case, two runs of imapsync, one from A to B, one from B
to A, put the two accounts in a synchronized state. The deletions of
messages on either side get cancelled, deleted messages come back. A
folder renaming on either side brings back the old folder.  It's not a
good two ways synchronization.


My suggestion is:

a) Use a single active imap account that fits all your needs,
   available from anywhere, at any time.  Access it from both your
   laptop and your iphone.

b) Feed this active account with foreign and passive accounts with any
   tool that can handle the transfer, imapsync being one of them.

c) Have a slave imap backup account of the active one, available also
   from anywhere, at any time. Use it only as a failsafe account, when
   the active one is broken or to bring back misguidely deleted
   messages to the active account. Use any tool that can handle the
   synchronization, imapsync being a good one of them.


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Q. What can I do instead of having two ways synchronizations?

R. My suggestion is:

a) Use only one single active imap account that fits all your needs,
   available from anywhere, at any time.  Access it from all your 
   devices, laptops, phones, tablets.

b) Feed this active account with foreign and passive accounts with any
   tool that can handle the transfer, imapsync being one of them.

c) Have a slave imap backup account of the active one, available also
   from anywhere, at any time. Use it only as a failsafe account, when
   the active one is broken or to bring back misguidely deleted
   messages to the active account. Use any tool that can handle the
   synchronization, imapsync being a good one of them.


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Q. What tools can I use to handle two ways synchronizations?

R. There are offlineimap and mbsync/isync tools:

offlineimap  is located at http://www.offlineimap.org/
mbsync/isync is located at https://isync.sourceforge.io/


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