imapsync/FAQ.d/FAQ.Migration_Plan.txt

#!/bin/cat
$Id: FAQ.Migration_Plan.txt,v 1.7 2021/03/02 22:31:36 gilles Exp gilles $

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


=====================================================================
   Imapsync. Suggestions for a good, low impact on users, 
   well executed email migration plan.
=====================================================================

There is two main different scenarios depending on the response to the
following question:

Will the imap software tools used by the users use the exact same
credentials triplet for both imap servers, the old server host1 and
the new server host2?

The credentials triplet is hostname/username/password.

If the answer is yes, ie, clients email tools use the exact same
triplet credentials, then it is possible to perform a migration
without changing anything on the users side. This may be a very time
saving option. But it's a rare condition so I'll describe this
scenario later in this document.


=====================================================================
Classical scenario, credentials triplets are different on both sides
=====================================================================

 * Decrease the TTL of the MX, to 5 minutes (or even less).  See
   FAQ.TTL.txt to understand why it's an advantage.  If you can't
   decrease the TTL, the migration will span a little more but that's
   ok, the situation is not that bad.

 * Create the new mailboxes on the destination server host2.  If the
   users are already playing with the new mailboxes on host2, don't
   follow this scenario.

 * Presync all the mailboxes from the old server host1 to the new
   server host2. If the imap server names are going to change their IP
   resolution then don't use those names, use names that will always
   match the same imap servers, or use their IP addresses.
   
   Presyncs can usefully be done with --delete2 in order to get an
   exact sync. But never use the option --delete2 once users have
   started to play with their new account on host2, their play will be
   lost on the next presync, or when the MX is changed, since INBOX
   will start to receive new messages that are not on host1.

 * Decide a migration day/hour.

 * Repeat the presyncs (with the --delete2 options) daily until the
   migration hour. This repeated process will show how long should
   take the last sync.

 * At the migration hour, cut access to the users to the old server
   host1, if you can. Or tell them to not use it anymore.

 * Do a last presync exactly like previous ones.

 * Change the MX, the new messages should start to arrive in the new
   imap server host2.

 * Wait the TTL value, aka 5 minutes. Now, new messages should 
   not arrive to the old server host1.

 * Tell the users that the old imap server host1 is down and no 
   longer available.

 * Do a postsync. A postsync is a sync with the following options 
   --maxage 1 --delete1 --folder INBOX

   This postsync will move the last new messages arrived on host1 to
   host2 during the TTL interval and delete them on host1. Do not use
   the option --delete2 in a postsync.

 * Give access to new accounts to the users with their new credential
   triplet hostname/username/password.  If the way to contact users is
   email then you should give this long before shutting down the old
   server.

 * Migration done.

 * In case there are still messages arriving at the old imap server
   host1, you can perform more postsyncs, ie, syncs every day 
   with the options:
   --maxage 1 --delete1 --folder INBOX


=====================================================================
Lucky scenario, credentials triplets are the same on both sides
=====================================================================
 
 * Decrease the TTL of the MX, as well as the imap hostname resolution,
   to 5 minutes (or even less). See FAQ.TTL.txt to understand why.

 * Create the new mailboxes on the destination server host2.
 
 * Presync all the mailboxes from the old host1 to the new server host2, 
   using different names that the ones used by the imap software 
   clients (use their IP for example).
   Presyncs have to be done with --delete2 but never use --delete2 
   once users have started playing with their new account on host2.
 
 * Decide a migration day/hour.

 * repeat the presyncs (with the --delete2 options) daily until the 
   migration hour. This repeated process will show how long should 
   take the last sync.

 * At the migration hour, cut access to the users to the old server.
   You can do this by changing the imap host1 hostname to a non-imap 
   server for example, or by changing their password on host1.

 * Do a last sync exactly like the presyncs, not using the imap hostname.

 * Change also the MX resolution, the new messages should start 
   to arrive in the new imap server very soon.
 
 * Wait the TTL value, aka 5 minutes. Now, new messages should 
   not arrive to the old server host1.
 
 * Do a postsync. A postsync is a sync with the following options 
   --maxage 1 --delete1 --folder INBOX

   This postsync will move the last new messages arrived on host1 to
   host2 during the TTL interval and delete them on host1. Do not use
   the option --delete2 in a postsync.

 * Shutdown the old imap server.

 * Change the user imap hostname resolution from the old IP of host1
   to the IP of the new imap server host2.

 * Migration done.

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