# Cyrus replication

Note that Cyrus replication is still relatively young in the grand scheme of things, and if you choose to deploy you are doing so at your own risk. The core of the replication code has been used successfully in production at Cambridge University for about a year, but it has been rewritten extensively as part of the integration into the mainstream Cyrus codebase.
Also note that the replication protocol currently does not have the facility to support the IMAP CONDSTORE extension (modification sequences). It is recommended that you do not try to use both CONDSTORE and replication at this time. The deficiencies in the replication protocol will be fixed in version 2.3.9.

### Introduction & Assumptions

The Cyrus replication engine is designed to replicate the mailstore on standalone Cyrus servers or "backend" servers in a Cyrus Murder to provide a high-availability environment. It is NOT intended to replicate "frontend" servers or the "mupdate master" in a Cyrus Murder. Note that load balancing is not possible with the current replication code, but it is intended to be supported in the future.

This document assumes that you have successfully been able to setup atleast one Cyrus IMAP server. This server will become your first "master" server. It also assumes that you are familliar with the administration and day to day operations of the Cyrus IMAP server and the SASL authentication library. If you feel uncomfortable with this, please refer to the rest of the documentation first.

## Installation

You will need to build Cyrus IMAPd with the `--enable-replication` configure option. This builds the replication client/server applications and utilities.

### Requirements

- Atleast one standalone or "backend" Cyrus IMAP server.
- Atleast one machine that will become the first "replica" server.

### Configuring the "replica" server

The "replica" server is a standalone server which listens for and processes synchronization messages from a "master" server. The "replica" server needs to be configured to accept these synchonization messages.

1. Configure a standalone server as described in the rest of the documentation.

2. Add the following line to the "`/etc/services`" file.

          csync     2005/tcp

   Note that the port number is arbitrary as long as its not being used by any other services on your network.

3. Add a line similar to the following in the SERVICES section of cyrus.conf:

         syncserver       cmd="/usr/cyrus/bin/sync_server" listen="csync"

4. Start/restart `"/usr/cyrus/bin/master"`.

### Configuring the "master" server

The "master" server is the standalone or "backend" server which is actively serving mailboxes to clients. This server needs to be configured to synchronize its mailstore with a "replica" server.

1. Add the following line to the "`/etc/services`" file.

          csync     2005/tcp

   Note that the port number MUST be the same as that used on the "replica" server.

2. Specify the hostname of the "replica" server and how to authenticate to it using the following imap.conf options:
   - `sync_host`
   - `sync_authname`
   - `sync_realm`
   - `sync_password`

   Note that `sync_authname` MUST be an admin on the "replica" server. Also note that `sync_realm` and `sync_password` may not be necessary depending on the SASL mechanism used for authentication.

#### Universally Unique Identifiers (UUIDs)

An optional, but recommended step is to enable UUIDs for messages. Use of UUIDs improves efficiency by eliminating the synchronization of messages which the "replica" has already received from the "master". Note that UUIDs can be safely enabled and disabled at any time.

1. Define the `sync_machineid` option in imapd.conf. This option specifies the numeric identifier (1 - 255) of the "master" machine which is used in constructing the UUID for each message on the server. This identifier MUST be unique across all active "backend" servers in a Murder. Example:

       sync_machineid: 1

2. For each IMAP, NNTP and LMTP service in cyrus.conf, enable the `provide_uuid` argument. Valid values are 0 or 1. The UUID itself will be calculated based on the sync_machineid above. Example:

       imap	cmd="imapd" listen="imap" prefork=5 provide_uuid=1

#### "Rolling" replication

"Rolling" replication means that the "master" server continuously synchonizes itself with the replica. To configure "rolling" replication, perform the following:

1. Enable the `sync_log` option in imapd.conf. This allows the `imapd`, `pop3d`, `nntpd`, and `lmtpd` services to log synchonization actions which will be periodically serviced by `sync_client`.

2. Optionally, adjust the `sync_repeat_interval` in imapd.conf.

3. Add a line similar to the following in the STARTUP section of cyrus.conf:

         syncclient       cmd="/usr/cyrus/bin/sync_client -r"

4. Start/restart `"/usr/cyrus/bin/master"`.

## Administration

### Manual replication

To manually synchonize any part of the mailstore, simply run `sync_client(8)` with the appropriate command line options. Note that you CAN manually synchonize even if "rolling" replication has been configured.

### Failover

last modified: $Date: 2007/04/03 15:10:02 $