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In order to run backups, the following components act in concert:
In the most simple use case those roles are all implemented on the same machine. Also, a central controller is supported but not required - it is up to you if you prefer to run a controller on each client or one single central one.
You will need to install the backup software at least on the backup controller and the backup repository. The backup client can be backed up without installing the backup software (but requires SSH and rsync).