Since Unison can also be used to sychronise more than two hosts, it’s perfect for big amounts of data that has to be shared in a team.

A scenario like this is possible and works for me: UserA <-> Server <-> UserB.
But of course, also other users could sync with the server. Unison rocks!

Today, after reinstalling his OS, my friend got the following error message:

Warning: inconsistent state.  
The archive file is missing on some hosts.
For safety, the remaining copies should be deleted.
  Archive are96968da50882488164ef52510703a8e on 
     host <UserAs-local-hostname> is MISSING
  Archive ar664775fc717afcf6cc46edbc47d25641 on ...

The easy solution

Unison keeps archive-files for each pair it synchronises with. One on each host, i.e. one on your local PC (UserA) and one on the server, both stored in a folder called ~/.unison and looking like the files above.

The easiest solution would be to just delete all archive-files on the server as unison suggests. But that would result in a situation where all the other users that sync their files with the server would have to do a slow sync which is extremely slow and sometimes takes more than 20 minutes for large folders (e.g. > 10 GB).

The best solution …

… is to just delete the required archive-files on the server. Pretty easy, that’s how it works:

Log on to the server (assuming that it’s a Linux machine, but it certainly works similar for any other system) and find the relevant archive files:

$ ssh mysyncuser@mysyncserver.com
$ cd ~/.unison
$ fgrep '//UserAs-local-hostname/' ar*

You should get something like this:

Binary file ar664775fc717afcf6cc46edbc47d25641 matches

Delete the listed files and it should work like a charm.

Well, the bad thing about Unison on the other hand is, that its backwards compatibility is anything but great, so that you have to make sure that everybody in the team uses the same version. And this can be tricky depending on what system you are using.

My home system is Ubuntu Hardy, the remote server system is Debian Etch. Both come with Unison 2.16.13 which would be great if not Apple’s new Leopard brings the newest version 2.27.57. Long story short, I needed the newest version on Hardy and Etch.

Unison on Hardy

To install the newest version of Unison on Hardy, it’s the best to just use the existing Backport package.

First, add the backports to the apt-sources, using your favorite editor, e.g. vi or nano:

$ sudo editor /etc/apt/sources.list

Look for backports and uncomment/add the following lines:

deb http://de.archive.ubuntu.com/ubuntu/ hardy-backports main restricted universe multiverse
deb-src http://de.archive.ubuntu.com/ubuntu/ hardy-backports main restricted universe multiverse

Then just install the backport using apt-get:

$ apt-get update
$ apt-get -t hardy-backports install unison

After installing you probably should remove the added lines from the sources.list-file. Otherwise the update-manager wants to install all available backports which is probably not what you want.

Debian Etch

Update 30/07/08: Before reading my post, please have a look at the much easier method described by Andrew in the user comments. Only if this doesn’t work, you might want to have a look at my post.

Unison 2.27.57 binary for Debian Etch:
If you don’t want to compile the binary on your own (as described in the following paragraphs), you can download the Unison 2.27.57 binary for Debian Etch here.

Update 30/11/08:
~ 300 downloads so far, no known problems/malfunctions!

It’s much more difficult on an Etch system because no Backport package is available. Therefore you either have to create such a package, which can be annoying and frustrating, or you just compile it on your own and overwrite the binary.

Add the unstable sources and the etch backports to your apt-get sources by editing the file /etc/apt/sources.list (as root!):

# Unstable source packages
deb-src ftp://ftp.de.debian.org/debian/ unstable main contrib non-free
 
# Backports for Etch
deb http://www.backports.org/debian etch-backports main contrib non-free

Now update apt and install the backport package of ocaml compiler. You also might want to install the debian backports keyring to avoid warning and error messages:

$ apt-get update
$ apt-get install debian-backports-keyring
$ apt-get update
$ apt-get -t etch-backports install ocaml

Then get the source package of unison 2.27.57 and make the binary:

$ apt-get update
$ apt-get source unison
$ dpkg-source -x unison_2.27.57-1.dsc
$ cd unison-2.27.57/
$ make

Note: For my Etch installation, the make progress failed with some error, but the binary was created successfully anyway. Just have a look whether the unison-binary exists and test if it is working.

$ ./unison -version
unison version 2.27.57

Now install the old version of Unison and replace the binary with the new one. Also you might want to remove the lines from /etc/apt/sources.list and uninstall the ocaml compiler.

$ apt-get install unison
$ cp unison /usr/bin/
$ unison -version
unison version 2.27.57

That’s it! I know that this is not the best solution and that it might be better to create the backports package. But as there are many dependencies to other packages it’s maybe one of the fastest solutions.

I decided to publish the binary here (for the lazy ones): Unison 2.27.57 binary for Debian Etch.