Interview › Infrastructure as Code (Terraform, Ansible)
What is the when clause, and how do you write conditional tasks?
Infrastructure as Code (Terraform, Ansible) · Advanced level
Answer
The when clause makes a task conditional. It evaluates a Jinja2 expression without wrapping the entire expression in {{ }}. I use when for OS-specific tasks, feature flags, registered results, and rollout conditions.
Technical explanation
when expressions should be readable and based on clear variables or facts.
Use boolean variables instead of complex string comparisons where possible.
Combine conditions with and, or, in, is defined, and filters.
Prefer idempotent modules over shell so repeated runs are safe and change reporting is meaningful.
Separate reusable role logic from inventory-specific variables so the same automation works across environments.
Run lint, syntax checks, check mode where useful, and staged rollouts before production-wide changes.
Hands-on example
1. Use register, when, and loop for: What is the when clause, and how do you write conditional tasks?
2. Example:
- name: Check app health
ansible.builtin.uri:
url: http://localhost:8080/health
status_code: 200
register: health
failed_when: false
- name: Restart app only when health check failed
ansible.builtin.service:
name: app
state: restarted
when: health.status | default(0) != 200
- name: Install required packages
ansible.builtin.package:
name: "{{ item }}"
state: present
loop:
- nginx
- curl
- jq
3. Run once with the service healthy and once after stopping it; confirm the conditional task changes behavior based on the registered result.
4. Use loop_control.label when iterating over dictionaries to keep output readable.
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