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Shared Mailboxes in Dovecot Cluster
===================================

As mentioned in <Director.txt>, you can't have multiple servers accessing the
same user at the same time or it will lead into trouble. This becomes
problematic with shared mailboxes, because two users who are sharing a folder
may run in different servers. The solution here is to access the shared folders
via IMAP protocol, which passes through the Dovecot proxies/directors so the
actual filesystem access is done only by one server.

There are a couple of things still missing for this kind of use case:

 * imapc_* settings are global. You can't have two different namespaces with
   different imapc settings yet.
 * The imapc code doesn't support many IMAP features. Most importantly SEARCH
   isn't supported, which may result in lower performance.

Setting up user-shared folders
------------------------------

You'll need to setup master user logins to work for all the users. The logged
in user becomes the master user. This way the ACLs are applied correctly.

---%<-------------------------------------------------------------------------
namespace {
 type = shared
 prefix = shared/%%u/
 location = imapc:~/shared/%%u/ # cache for shared indexes
}
imapc_host = director-ip
imapc_master_user = %u
#imapc_user = # leave this empty. It'll be automatically filled with the
destination username.
imapc_password = master-secret

plugin {
  acl_shared_dict = fs:posix:prefix=/nfs/shared-acls/
}
---%<-------------------------------------------------------------------------

The shared dictionary needs to be accessible from all the backends. The
possibilities for it are:

 * file: A single shared file in filesystem. This becomes a performance
   bottleneck easily if there are many users sharing folders.
 * fs posix: Shared directory in filesystem. This will create many small files
   to the filesystem.
 * sql: Shared SQL server
 * Any other <shared dictionary> [Dictionary.txt]

(This file was created from the wiki on 2019-06-19 12:42)