How is check mode implemented in Ansible?

Check mode (--check) runs a playbook in a "dry run" fashion: Ansible connects to real hosts and asks each module to report what change would be made, without actually making that change on the target system.

ansible-playbook site.yml --check --diff

This works because well-written modules implement supports_check_mode logic internally: when check mode is active, the module performs its normal "what's the current state versus desired state" comparison, but stops short of the step that actually writes the change, instead reporting whether it would have reported changed. Adding --diff alongside --check shows a line-level diff for file-based changes, like what a config file would look like after templating. Not every module supports check mode equally well, particularly custom or older modules and raw shell/command tasks, which may report inaccurate results or need explicit check_mode: false/changed_when handling to behave sensibly during a dry run.

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