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Using Unison with Atmail
Unison allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other. In other words, it can be used for two-way synchronization between your servers.
For illustrative purposes in this example, the Unison Master hostname will be called "master-foo-01"; the Client hostname will be called "slave-foo-02".
To setup Unison on your machine, do the following for both machines:
1.) Download the dependency, OCaml from: http://caml.inria.fr/download.en.html
atmail@master-foo-01# wget "http://caml.inria.fr/pub/distrib/ocaml-3.12/ocaml-3.12.0.tar.gz"
2.) Install emacs (a dependency):
atmail@master-foo-01# yum install emacs
3.) Unpack, install OCaml:
atmail@master-foo-01# tar xvfz ocaml-3.12.0.tar.gz
atmail@master-foo-01# cd ocaml-3.12.0
atmail@master-foo-01# ./configure && make world
atmail@master-foo-01# make opt
atmail@master-foo-01# make install
4.) Download Unison from: http://www.cis.upenn.edu/~bcpierce/unison/download.html
atmail@master-foo-01# wget "http://www.seas.upenn.edu/~bcpierce/unison//download/releases/stable/unison-2.40.63.tar.gz"
5.) Untar, install:
atmail@master-foo-01# make world opt
atmail@master-foo-01# sudo make install
6.) Move the Unison binary to your preferred prefix. We suggest /usr/bin/:
atmail@master-foo-01# mv unison /usr/bin/unison
atmail@master-foo-01# chmod 755 /usr/bin/unison
7.) You will then need to set the keys. Create a SSH public key on the Master server:
atmail@master-foo-01# ssh-keygen -t rsa
8.) This will produce a public key in /home/atmail/.ssh/id_rsa.pub. Copy the .pub file to the Slave machine:
atmail@master-foo-01# scp /home/atmail/.ssh/id_rsa.pub root@slave-foo-02:/home/atmail/.ssh/id_rsa.pub
9.) Pipe the public key into a file called authorized_keys, in your .ssh directory.
atmail@slave-foo-02# cat /home/atmail/.ssh/id_rsa.pub >> authorized_keys
10.) Make sure that the authorized_keys file, and all of the contents of the .ssh directory have 600 permissions:
atmail@slave-foo-02# chmod 600 /home/atmail/.ssh/*
11.) Try to login, sans password, from the Master machine to the Client machine.
atmail@master-foo-01# ssh atmail@slave-foo-02
12.) This should allow you to login. Now, you can try to sync the differences between the users/ directory of the master and slave machines. From any of the machines, execute:
atmail@master-foo-01# unison -batch -auto /usr/local/atmail/users ssh://slave-foo-01//usr/local/atmail/users
13.) The output should be similar to:
UNISON 2.40.63 finished propagating changes at 01:25:25.57 on 18 Apr 2011
Saving synchronizer state
Synchronization complete at 01:25:25 (xx items transferred, 0 skipped, 0 failed)
14.) You will need to set this in cron. Create a file called /home/atmail/unison.sh. In the file:
#!/bin/sh
/usr/bin/unison -batch -auto /usr/local/atmail/users ssh://slave-foo-01//usr/local/atmail/users
15.) Set permissions:
% chmod 755 /home/atmail/unison.sh
16.) Add an entry to your /etc/crontab. It will look like:
01,10,20,30,40,50 * * * * atmail /home/atmail/unison.sh
This will synchronize your /usr/local/atmail/users directory for both machines.