Bidirectional replication with Unison

About UNISON

Unison is a file-synchronization tool for Unix and Windows. It 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.

For this implementation I use unison227-2.27.57-13.el6.x86_64.rpm on RHEL6.

The setup work under following scenarios:

1) Replication is for two hosts (node1 & node2)

2) The synced folder is user's home directory - /home/rsftp

3) The file transfer is done using passwordless ssh protocol

4) The synchronization is done in 1 minute interval using cron scheduler

Step 1: Install unison rpm and add user

On both nodes, install the package

rpm  -ivh  unison227-2.27.57-13.el6.x86_64.rpm

useradd rsftp 

echo  rsftp!@#  |  passwd  --stdin  rsftp

above says "set rsftp!@# as password for rsftp user.

Step 2: Create ssh public key and exchange to both nodes

As rsftp user, 

On node1:

[rsftp@node1 ~] $ ssh-keygen -t  dsa

[rsftp@node1 ~] $ ssh-copy-id  -i  /home/rsftp/.ssh/id_dsa.pub node2

On node2:

[rsftp@node2 ~] $ ssh-keygen -t  dsa

[rsftp@node2 ~] $ ssh-copy-id  -i  /home/rsftp/.ssh/id_dsa.pub node1

Once done, next is try ssh in to the partner, expected result is passwordless login.

Step 3: Modify unison configuration file

The configuration file is /home/rsftp/.unison/default.prf

On node1, put these lines

root = /home/rsftp

root = ssh://node2//home/rsftp

auto = true

batch = true

do the same on node2,

root = /home/rsftp

root = ssh://node1//home/rsftp

auto = true

batch = true

Step 4: Create a cronjob on both nodes

As rsftp user

crontab -e

put this line

*/1 * * * * /usr/bin/unison &> /dev/null

Step 5: Create a sample file using dd command to create a file with 10MB size

On node1,

[rsftp@node1 ~] $ cd  ~

[rsftp@node1 ~] $ dd if=/dev/zero of=file1.dat  count=1 bs=10M

After 1 minute pass, on node2 list file in rsftp home directory,

On node2,

[rsftp@node2 ~] $ cd  ~

[rsftp@node2 ~] $ ls -lah

You should see file1.dat with 10MB size.Repeat above steps, create a file2.dat in node2 and list it in node1. Also try other operation such deleting and modifying a file.