Magic Variables and Hostvars¶

Why This Exists¶

Ansible provides a handful of variables automatically, without you ever defining them — they're how one host's tasks can reference another host's facts or variables, which is essential for anything involving load balancers, clusters, or cross-host coordination.

What It Will Cover¶

• hostvars — the full dictionary of every host's variables, keyed by inventory name; the mechanism behind hostvars['db01']['ansible_default_ipv4']['address']
• inventory_hostname — the current host's name, as Ansible knows it
• groups — a dictionary of every group name to its member hosts
• group_names — the list of groups the current host belongs to
• ansible_play_hosts — the hosts targeted by the current play
• A worked example: a web server task templating a database connection string using hostvars[groups['db'][0]]['ansible_default_ipv4']['address']

Common Mistakes¶

• Trying to reference another host's facts without hostvars, and getting an "undefined variable" error — a task only has direct access to the current host's own variables by default.
• Confusing groups (all groups, everywhere) with group_names (only the current host's groups).

Interview Questions¶

• How would a web server task read a fact from a database host in the same inventory?
• What's the difference between groups and group_names?

Next¶

Continue to set_fact and combine.