How to keep multiple servers in sync file wise?

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http://serverfault.com – I'm currently managing a cluster of PHP-FPM servers, all of which tend to get out of sync with each other. The application that I'm using on top of the app servers (Magento) allows for admins to modify various files on the system, but now that the site is in a clustered set up modifying a file only modifies it on a single instance (on one of the app servers) of the various machines in the cluster. Is there an open-source application for Linux that may allow me to keep all of these servers in sync? I have no problem with creating a small VM instance that can listen for changes from machines to (HowTos)