Handlers, idempotency & tags

Idempotency is Ansible's core promise: running the same playbook twice produces the same end state, and the second run reports nothing changed. The apt module checks if nginx is already installed before installing it; copy checks if the file content already matches before copying. You get this for free by using proper modules instead of raw shell commands.

A handler is a task that only runs when explicitly notified by another task — the classic use is "only restart nginx if its config actually changed," instead of restarting it (and dropping active connections) on every single run.

tasks:
  - name: Deploy nginx config
    template:
      src: nginx.conf.j2
      dest: /etc/nginx/sites-available/app.conf
    notify: Restart nginx

handlers:
  - name: Restart nginx
    service:
      name: nginx
      state: restarted

Tags let you run only part of a long playbook — handy when you don't want to redo every step just to fix one thing.

tasks:
  - name: Install nginx
    apt: { name: nginx, state: present }
    tags: [install]

  - name: Deploy config
    template: { src: nginx.conf.j2, dest: /etc/nginx/sites-available/app.conf }
    tags: [config]

ansible-playbook site.yml --tags config