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I want to get a second server for my site. The first one is currently at my office, and the second one will be at my home (so two different locations so that if the power/Internet goes out, the site is still up).

How do I keep the site files up to date across both servers? Pointing the server at home to the files at my office defeats the whole purpose. If a user adds a file to the first server, how do I get it transferred to the second server?

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From SuperUser https://superuser.com/a/31578

Unison might be a good candidate:

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.

It already does 2-way syncs. I think you can daisy-chain the 3 machines with 2-way syncs to get what you need. e.g

machines A, B, C => (A <-> B), (B <-> C), (C <-> A)

It's not ideal, but I couldn't find anything better at the moment.

I've learnt that there are very few things rsync cannot do, and it can probably provide an equal or better solution, but you'll have to wait for an rsync expert to turn up for that solution.

Unison can sync more than 2 machines. From their user manual:

Using Unison to Synchronize More Than Two Machines

Unison is designed for synchronizing pairs of replicas. However, it is possible to use it to keep larger groups of machines in sync by performing multiple pairwise synchronizations.

If you need to do this, the most reliable way to set things up is to organize the machines into a “star topology,” with one machine designated as the “hub” and the rest as “spokes,” and with each spoke machine synchronizing only with the hub. The big advantage of the star topology is that it eliminates the possibility of confusing “spurious conflicts” arising from the fact that a separate archive is maintained by Unison for every pair of hosts that it synchronizes.

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