Replication with dsync
======================

Dovecot supports master/master replication using dsync. It's recommended that
the same user always gets redirected to the same replica, but no changes get
lost even if the same user modifies mails simultaneously on both replicas, some
mails just might have to be redownloaded. The replication is done
asynchronously, so high latency between the replicas isn't a problem. The
replication is done by looking at Dovecot index files (not what exists in
filesystem), so no mails get lost due to filesystem corruption or an accidental
rm -rf, they will simply be replicated back.

NOTE: v2.2 is highly recommended for this. Earlier versions can't do
incremental metadata syncing. This means that the more mails a mailbox has, the
slower it is to sync it.

Replication works only between server pairs. If you have a large cluster, you
need multiple independently functioning Dovecot backend pairs. This means that
<director> [Director.txt] isn't currently supported with replication. The
replication in general is a bit resource intensive, so it's not recommended to
be used in multi-million user installations.

*WARNING*: Shared folder replication doesn't work correctly right now – mainly
it can generate a lot of duplicate emails. This is because there's currently a
per-user lock that prevents multiple dsyncs from working simultaneously on the
same user. But with shared folders multiple users can be syncing the same
folder. So this would need additional locks (e.g. shared folders would likely
need to lock the owner user, and public folders would likely need a per-folder
lock or a maybe a global public folder lock). Fixing this is currently low
priority for Dovecot developers.

Configuration
-------------

Make sure that user listing is configured for your userdb, this is required by
replication to find the list of users that are periodically replicated:

---%<-------------------------------------------------------------------------
doveadm user '*'
---%<-------------------------------------------------------------------------

Enable the replication plugin globally (most likely you'll need to do this in
10-mail.conf):

---%<-------------------------------------------------------------------------
mail_plugins = $mail_plugins notify replication
---%<-------------------------------------------------------------------------

Replicator process should be started at startup, so it can start replicating
users immediately:

---%<-------------------------------------------------------------------------
service replicator {
  process_min_avail = 1
}
---%<-------------------------------------------------------------------------

You need to configure how and where to replicate. Using SSH for example:

---%<-------------------------------------------------------------------------
dsync_remote_cmd = ssh -l%{login} %{host} doveadm dsync-server -u%u
plugin {
  mail_replica = remote:vmail@anotherhost.example.com
}
---%<-------------------------------------------------------------------------

The mail processes need to have access to the replication-notify fifo and
socket. If you have a single vmail UID, you can do:

---%<-------------------------------------------------------------------------
service aggregator {
  fifo_listener replication-notify-fifo {
    user = vmail
  }
  unix_listener replication-notify {
    user = vmail
  }
}
---%<-------------------------------------------------------------------------

The replication-notify only notifies the replicator processes that there is
work to be done, so it's not terribly insecure either to just set 'mode=0666'.

Enable doveadm replicator commands by setting:

---%<-------------------------------------------------------------------------
service replicator {
  unix_listener replicator-doveadm {
    mode = 0600
  }
}
---%<-------------------------------------------------------------------------

You can configure how many dsyncs can be run in parallel (10 by default):

---%<-------------------------------------------------------------------------
replication_max_conns = 10
---%<-------------------------------------------------------------------------

Normally all replication is asynchronous. You can also optionally configure new
mail saving to be synchronous, with a timeout to avoid waiting too long. This
can be configured with:

---%<-------------------------------------------------------------------------
plugin {
  # When saving a new mail via IMAP or delivering a mail via LDA/LMTP,
  # wait for the mail to be synced to the remote site. If it doesn't finish
  # in 2 seconds, return success anyway.
  replication_sync_timeout = 2s
}
---%<-------------------------------------------------------------------------

dsync over TCP connections (v2.2+)
----------------------------------

Create a listener for doveadm-server:

---%<-------------------------------------------------------------------------
service doveadm {
  inet_listener {
    port = 12345
  }
}
---%<-------------------------------------------------------------------------

And tell doveadm client to use this port by default:

---%<-------------------------------------------------------------------------
doveadm_port = 12345
---%<-------------------------------------------------------------------------

Both the client and the server also need to have a shared secret:

---%<-------------------------------------------------------------------------
doveadm_password = secret
---%<-------------------------------------------------------------------------

Now you can use 'tcp:hostname' as the dsync target. You can also override the
port with 'tcp:hostname:port'.

---%<-------------------------------------------------------------------------
plugin {
  mail_replica = tcp:anotherhost.example.com # use doveadm_port
  #mail_replica = tcp:anotherhost.example.com:12345 # use port 12345 explicitly
}
---%<-------------------------------------------------------------------------

SSL
---

You can also use SSL for the connection:

---%<-------------------------------------------------------------------------
service doveadm {
  inet_listener {
    port = 12345
    ssl = yes
  }
}
---%<-------------------------------------------------------------------------

The doveadm listener will use the SSL certificate that is configured globally
for all SSL listeners, i.e. via the following settings at the top level of the
configuration file:

---%<-------------------------------------------------------------------------
ssl_cert = </etc/ssl/dovecot.pem
ssl_key = </etc/ssl/dovecot.pem
---%<-------------------------------------------------------------------------

'ssl_cert' is not a valid setting inside 'service' or 'inet_listener' blocks,
so you can't use a separate SSL certificate for the doveadm listener.You can,
however, use separate SSL certificates for the /other/ protocols, like so:

---%<-------------------------------------------------------------------------
protocol imap {
  ssl_cert = </etc/ssl/certs/imap.pem
  ssl_key = </etc/ssl/private/imap.pem
}
protocol pop3 {
  ssl_cert = </etc/ssl/certs/pop3.pem
  ssl_key = </etc/ssl/private/pop3.pem
}
---%<-------------------------------------------------------------------------

When one Dovecot instance connects to the other one in the replication pair, it
has to verify that the partner's SSL certificate is valid, so you need to
specify a directory or file containing valid SSL CA roots:

---%<-------------------------------------------------------------------------
ssl_client_ca_dir = /etc/ssl/certs # Debian/Ubuntu
ssl_client_ca_file = /etc/pki/tls/cert.pem # RedHat
---%<-------------------------------------------------------------------------

Now you can use 'tcps:hostname' or 'tcps:hostname:port' as the dsync target.

Note that the SSL certificate must be signed by one of the CAs in the
'ssl_client_ca_dir' or 'ssl_client_ca_file'. You can't use a self-signed
certificate or a private CA, unless you correctly set them up into the CA
file/directory (see openssl documentation for details).

You could point 'ssl_client_ca_file' to your private CA, but keep in mind that
'ssl_client_ca_file' and 'ssl_client_ca_dir' also affect other services where
Dovecot acts as an SSL client (e.g. the imapc feature), so be careful not to
break SSL for those services.

dsync wrapper script for root SSH login (v2.2+)
-----------------------------------------------

If you're using multiple UIDs, dsync needs to be started as root, which means
you need to log in as root with ssh (or use sudo). Another possibility is to
allow root to run only a wrapper script. There is some built-in support for
this in v2.2+ to make it easier:

dovecot.conf:

---%<-------------------------------------------------------------------------
dsync_remote_cmd = /usr/bin/ssh -i /root/.ssh/id_dsa.dsync %{host}
/usr/local/bin/dsync-in-wrapper.sh
plugin {
  mail_replica = remoteprefix:vmail@anotherhost.example.com
}
---%<-------------------------------------------------------------------------

/root/.ssh/authorized_keys:

---%<-------------------------------------------------------------------------
command="/usr/local/bin/dsync-in-wrapper.sh",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty
<ssh key>
---%<-------------------------------------------------------------------------

/usr/local/bin/dsync-in-wrapper.sh:

---%<-------------------------------------------------------------------------
read username
ulimit -c unlimited # for debugging any crashes
/usr/local/bin/doveadm dsync-server -u $username
---%<-------------------------------------------------------------------------

dsync parameters
----------------

With v2.2.9+ you can configure what parameters replicator uses for the 'doveadm
sync' command:

---%<-------------------------------------------------------------------------
replication_dsync_parameters = -d -N -l 30 -U
---%<-------------------------------------------------------------------------

The '-f' and '-s' parameters are added automatically when needed.

Usually the only change you may want to do is replace '-N' (= sync all
namespaces) with '-n <namespace>' or maybe just add '-x <exclude>'
parameter(s).

Administration
--------------

'doveadm replicator status' provides a summary. For example:

---%<-------------------------------------------------------------------------
Queued 'sync' requests        0
Queued 'high' requests        0
Queued 'low' requests         0
Queued 'failed' requests      0
Queued 'full resync' requests 90
Waiting 'failed' requests     10
Total number of known users   100
---%<-------------------------------------------------------------------------

The first 3 fields describe users who have a replication pending with a
specific priority. The same user can only be in one (or none) of these queues:

 * Queued 'sync' requests: This priority is used only for mail saves if
   'replication_sync_timeout' setting is used.
 * Queued 'high' requests: This priority is used only for mail saves if
   'replication_sync_timeout' setting is not used, or if the sync request timed
   out.
 * Queued 'low' requests: This priority is used for everything else except mail
   saves.

The following fields are:

 * Queued 'failed' requests: Number of users who have a replication pending and
   where the last sync attempt failed. These users are retried as soon as
   higher priority users' replication has finished.
 * Queued 'full resync' requests: Number of users who don't specifically have
   any replication pending, but who are currently waiting for a periodic "full
   sync". This is controlled by the 'replication_full_sync_interval' setting.
 * Waiting 'failed' requests: Number of users whose last replication attempt
   failed, and we're now waiting for the retry interval (5 mins) to pass before
   another attempt.
 * Total number of known users: Number of users that replicator knows about.
   The users can be listed with:'doveadm replicator status '*''

The per-user replication status can be shown with 'doveadm replicator status
<username pattern>'. The username pattern can contain '*' and '?' wildcards.
The response contains for example:

---%<-------------------------------------------------------------------------
username           priority fast sync  full sync  success sync failed
test100            none     02:03:52   02:08:52   02:03:52     -
test1              none     00:00:01   00:43:33   03:20:46     y
test2              none     02:03:51   02:03:51   02:03:51     -
---%<-------------------------------------------------------------------------

These fields mean:

 * priority: none, low, high, sync
 * fast sync: How long time ago the last "fast sync" (non-full sync) attempt
   was performed. Ideally this is close to the time when the user was last
   modified. This doesn't mean that the sync succeeded necessarily.
 * full sync: How long time ago the last "full sync" attempt was performed.
   This should happen once per 'replication_full_sync_interval'. This doesn't
   mean that the sync succeeded necessarily.
 * success sync: Time when the last successful sync was performed. If the last
   sync succeeded, this is the same as the "fast sync" or the "full sync"
   timestap.
 * failed: "y" if the last sync failed, "-" if not.

The current dsync replication status can be looked up with 'doveadm replicator
dsync-status'. This shows the dsync replicator status for each potential dsync
connection, as configured by 'replication_max_conns'. An example output is:

---%<-------------------------------------------------------------------------
username                   type   status
test100                    full   Waiting for dsync to finish
test1                      normal Waiting for handshake
                           -      Not connected
                           -      Not connected
---%<-------------------------------------------------------------------------

Here there are 4 lines, meaning 'replication_max_conns=4'. Only two of the
dsync-connections are being used currently.

The fields mean:

 * username: User currently being replicated.
 * type: incremental, normal or full. Most of the replications are
   "incremental", while full syncs are "full". A "normal" sync is done when
   incremental syncing state isn't available currently. The "incremental"
   matches doveadm sync's -s parameter, "full" is -f parameter and "normal" is
   the default.
 * status: Human-readable status of the connection. These are the current
   values:
    * Not connected
    * Failed to connect to '%s' - last attempt %ld secs ago
    * Idle
    * Waiting for handshake
    * Waiting for dsync to finish

Failed replication attempts are always automatically retried, so any temporary
problems should get fixed automatically. In case of bugs it may be necessary to
fix something manually. These should be visible in the error logs. So if a user
is marked as failed, try to find any errors logged for the user and see if the
same error keeps repeating in the logs. If you want to debug the dsync, you can
manually trigger it with:'doveadm -D sync -u user@domain -d -N -l 30 -U' (the
parameters after "sync" should be the same as in 'replication_dsync_parameters'
setting).

Notes
-----

Random things to remember:

 * The replicas can't share the same quota database, since both will always
   update it
 * With mdbox format "doveadm purge" won't be replicated
 * "doveadm force-resync", "doveadm quota recalc" and other similar fixing
   commands don't get replicated
 * The servers must have different hostnames or the locking doesn't work and
   can cause replication problems.
    * v2.2.6+: If you're having trouble, verify that 'dovecot --hostdomain'
      returns different values for the servers.

(This file was created from the wiki on 2017-10-10 04:42)