Safe service upgrades using system.stateVersion

One of the most important features for system administrators who operate NixOS systems are atomic upgrades which means that a deployment won’t reach an inconsistent state: if building a new system’s configuration succeeds, it will be activated in a single step by replacing the /run/current-system-symlink. If a build fails, e.g. due to broken packages, the configuration won’t be activated. This also means that downgrades are fairly simple since a previous configuration can be reactivated in a so-called rollback by changing the symlink to /run/current-system back to the previous store-path.

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