idempotence

The property of an operation whose second run changes nothing, because the first already left things as described. mkdir -p is idempotent; mkdir is not, and fails the second time. The misconception is that it means "does nothing": an idempotent step does whatever is needed to reach the state, and nothing when the state is already there. That is what makes a script safe to rerun after a crash halfway through, and what a configuration tool promises for every module: apt with state: present installs the package once and reports ok afterwards. It is a property of the step, not of the tool: a command task that runs useradd is not idempotent because Ansible ran it, and it fails on the second run exactly as the shell line would. The changed=0 of a second run is idempotence measured, task by task.